1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
|
.. index:: ! contract
.. _contracts:
##########
Contracts
##########
Contracts in Solidity are similar to classes in object-oriented languages. They
contain persistent data in state variables and functions that can modify these
variables. Calling a function on a different contract (instance) will perform
an EVM function call and thus switch the context such that state variables are
inaccessible.
.. index:: ! contract;creation, constructor
******************
Creating Contracts
******************
Contracts can be created "from outside" via Ethereum transactions or from within Solidity contracts.
IDEs, such as `Remix <https://remix.ethereum.org/>`_, make the creation process seamless using UI elements.
Creating contracts programmatically on Ethereum is best done via using the JavaScript API `web3.js <https://github.com/ethereum/web3.js>`_.
It has a function called `web3.eth.Contract <https://web3js.readthedocs.io/en/1.0/web3-eth-contract.html#new-contract>`_
to facilitate contract creation.
When a contract is created, its constructor_ (a function declared with the ``constructor`` keyword) is executed once.
A constructor is optional. Only one constructor is allowed, which means
overloading is not supported.
After the constructor has executed, the final code of the contract is deployed to the
blockchain. This code includes all public and external functions and all functions
that are reachable from there through function calls. The deployed code does not
include the constructor code or internal functions only called from the constructor.
.. index:: constructor;arguments
Internally, constructor arguments are passed :ref:`ABI encoded <ABI>` after the code of
the contract itself, but you do not have to care about this if you use ``web3.js``.
If a contract wants to create another contract, the source code
(and the binary) of the created contract has to be known to the creator.
This means that cyclic creation dependencies are impossible.
::
pragma solidity >=0.4.22 <0.6.0;
contract OwnedToken {
// `TokenCreator` is a contract type that is defined below.
// It is fine to reference it as long as it is not used
// to create a new contract.
TokenCreator creator;
address owner;
bytes32 name;
// This is the constructor which registers the
// creator and the assigned name.
constructor(bytes32 _name) public {
// State variables are accessed via their name
// and not via e.g. `this.owner`. Functions can
// be accessed directly or through `this.f`,
// but the latter provides an external view
// to the function. Especially in the constructor,
// you should not access functions externally,
// because the function does not exist yet.
// See the next section for details.
owner = msg.sender;
// We do an explicit type conversion from `address`
// to `TokenCreator` and assume that the type of
// the calling contract is `TokenCreator`, there is
// no real way to check that.
creator = TokenCreator(msg.sender);
name = _name;
}
function changeName(bytes32 newName) public {
// Only the creator can alter the name --
// the comparison is possible since contracts
// are explicitly convertible to addresses.
if (msg.sender == address(creator))
name = newName;
}
function transfer(address newOwner) public {
// Only the current owner can transfer the token.
if (msg.sender != owner) return;
// We ask the creator contract if the transfer
// should proceed by using a function of the
// `TokenCreator` contract defined below. If
// the call fails (e.g. due to out-of-gas),
// the execution also fails here.
if (creator.isTokenTransferOK(owner, newOwner))
owner = newOwner;
}
}
contract TokenCreator {
function createToken(bytes32 name)
public
returns (OwnedToken tokenAddress)
{
// Create a new `Token` contract and return its address.
// From the JavaScript side, the return type is
// `address`, as this is the closest type available in
// the ABI.
return new OwnedToken(name);
}
function changeName(OwnedToken tokenAddress, bytes32 name) public {
// Again, the external type of `tokenAddress` is
// simply `address`.
tokenAddress.changeName(name);
}
// Perform checks to determine if transferring a token to the
// `OwnedToken` contract should proceed
function isTokenTransferOK(address currentOwner, address newOwner)
public
pure
returns (bool ok)
{
// Check an arbitrary condition to see if transfer should proceed
return keccak256(abi.encodePacked(currentOwner, newOwner))[0] == 0x7f;
}
}
.. index:: ! visibility, external, public, private, internal
.. _visibility-and-getters:
**********************
Visibility and Getters
**********************
Since Solidity knows two kinds of function calls (internal
ones that do not create an actual EVM call (also called
a "message call") and external
ones that do), there are four types of visibilities for
functions and state variables.
Functions have to be specified as being ``external``,
``public``, ``internal`` or ``private``.
For state variables, ``external`` is not possible.
``external``:
External functions are part of the contract interface,
which means they can be called from other contracts and
via transactions. An external function ``f`` cannot be called
internally (i.e. ``f()`` does not work, but ``this.f()`` works).
External functions are sometimes more efficient when
they receive large arrays of data.
``public``:
Public functions are part of the contract interface
and can be either called internally or via
messages. For public state variables, an automatic getter
function (see below) is generated.
``internal``:
Those functions and state variables can only be
accessed internally (i.e. from within the current contract
or contracts deriving from it), without using ``this``.
``private``:
Private functions and state variables are only
visible for the contract they are defined in and not in
derived contracts.
.. note::
Everything that is inside a contract is visible to
all observers external to the blockchain. Making something ``private``
only prevents other contracts from accessing and modifying
the information, but it will still be visible to the
whole world outside of the blockchain.
The visibility specifier is given after the type for
state variables and between parameter list and
return parameter list for functions.
::
pragma solidity >=0.4.16 <0.6.0;
contract C {
function f(uint a) private pure returns (uint b) { return a + 1; }
function setData(uint a) internal { data = a; }
uint public data;
}
In the following example, ``D``, can call ``c.getData()`` to retrieve the value of
``data`` in state storage, but is not able to call ``f``. Contract ``E`` is derived from
``C`` and, thus, can call ``compute``.
::
pragma solidity >=0.4.0 <0.6.0;
contract C {
uint private data;
function f(uint a) private pure returns(uint b) { return a + 1; }
function setData(uint a) public { data = a; }
function getData() public view returns(uint) { return data; }
function compute(uint a, uint b) internal pure returns (uint) { return a + b; }
}
// This will not compile
contract D {
function readData() public {
C c = new C();
uint local = c.f(7); // error: member `f` is not visible
c.setData(3);
local = c.getData();
local = c.compute(3, 5); // error: member `compute` is not visible
}
}
contract E is C {
function g() public {
C c = new C();
uint val = compute(3, 5); // access to internal member (from derived to parent contract)
}
}
.. index:: ! getter;function, ! function;getter
.. _getter-functions:
Getter Functions
================
The compiler automatically creates getter functions for
all **public** state variables. For the contract given below, the compiler will
generate a function called ``data`` that does not take any
arguments and returns a ``uint``, the value of the state
variable ``data``. State variables can be initialized
when they are declared.
::
pragma solidity >=0.4.0 <0.6.0;
contract C {
uint public data = 42;
}
contract Caller {
C c = new C();
function f() public view returns (uint) {
return c.data();
}
}
The getter functions have external visibility. If the
symbol is accessed internally (i.e. without ``this.``),
it evaluates to a state variable. If it is accessed externally
(i.e. with ``this.``), it evaluates to a function.
::
pragma solidity >=0.4.0 <0.6.0;
contract C {
uint public data;
function x() public returns (uint) {
data = 3; // internal access
return this.data(); // external access
}
}
If you have a ``public`` state variable of array type, then you can only retrieve
single elements of the array via the generated getter function. This mechanism
exists to avoid high gas costs when returning an entire array. You can use
arguments to specify which individual element to return, for example
``data(0)``. If you want to return an entire array in one call, then you need
to write a function, for example:
::
pragma solidity >=0.4.0 <0.6.0;
contract arrayExample {
// public state variable
uint[] public myArray;
// Getter function generated by the compiler
/*
function myArray(uint i) returns (uint) {
return myArray[i];
}
*/
// function that returns entire array
function getArray() returns (uint[] memory) {
return myArray;
}
}
Now you can use ``getArray()`` to retrieve the entire array, instead of
``myArray(i)``, which returns a single element per call.
The next example is more complex:
::
pragma solidity >=0.4.0 <0.6.0;
contract Complex {
struct Data {
uint a;
bytes3 b;
mapping (uint => uint) map;
}
mapping (uint => mapping(bool => Data[])) public data;
}
It generates a function of the following form. The mapping in the struct is omitted
because there is no good way to provide the key for the mapping:
::
function data(uint arg1, bool arg2, uint arg3) public returns (uint a, bytes3 b) {
a = data[arg1][arg2][arg3].a;
b = data[arg1][arg2][arg3].b;
}
.. index:: ! function;modifier
.. _modifiers:
******************
Function Modifiers
******************
Modifiers can be used to easily change the behaviour of functions. For example,
they can automatically check a condition prior to executing the function. Modifiers are
inheritable properties of contracts and may be overridden by derived contracts.
::
pragma solidity >0.4.99 <0.6.0;
contract owned {
constructor() public { owner = msg.sender; }
address payable owner;
// This contract only defines a modifier but does not use
// it: it will be used in derived contracts.
// The function body is inserted where the special symbol
// `_;` in the definition of a modifier appears.
// This means that if the owner calls this function, the
// function is executed and otherwise, an exception is
// thrown.
modifier onlyOwner {
require(
msg.sender == owner,
"Only owner can call this function."
);
_;
}
}
contract mortal is owned {
// This contract inherits the `onlyOwner` modifier from
// `owned` and applies it to the `close` function, which
// causes that calls to `close` only have an effect if
// they are made by the stored owner.
function close() public onlyOwner {
selfdestruct(owner);
}
}
contract priced {
// Modifiers can receive arguments:
modifier costs(uint price) {
if (msg.value >= price) {
_;
}
}
}
contract Register is priced, owned {
mapping (address => bool) registeredAddresses;
uint price;
constructor(uint initialPrice) public { price = initialPrice; }
// It is important to also provide the
// `payable` keyword here, otherwise the function will
// automatically reject all Ether sent to it.
function register() public payable costs(price) {
registeredAddresses[msg.sender] = true;
}
function changePrice(uint _price) public onlyOwner {
price = _price;
}
}
contract Mutex {
bool locked;
modifier noReentrancy() {
require(
!locked,
"Reentrant call."
);
locked = true;
_;
locked = false;
}
/// This function is protected by a mutex, which means that
/// reentrant calls from within `msg.sender.call` cannot call `f` again.
/// The `return 7` statement assigns 7 to the return value but still
/// executes the statement `locked = false` in the modifier.
function f() public noReentrancy returns (uint) {
(bool success,) = msg.sender.call("");
require(success);
return 7;
}
}
Multiple modifiers are applied to a function by specifying them in a
whitespace-separated list and are evaluated in the order presented.
.. warning::
In an earlier version of Solidity, ``return`` statements in functions
having modifiers behaved differently.
Explicit returns from a modifier or function body only leave the current
modifier or function body. Return variables are assigned and
control flow continues after the "_" in the preceding modifier.
Arbitrary expressions are allowed for modifier arguments and in this context,
all symbols visible from the function are visible in the modifier. Symbols
introduced in the modifier are not visible in the function (as they might
change by overriding).
.. index:: ! constant
************************
Constant State Variables
************************
State variables can be declared as ``constant``. In this case, they have to be
assigned from an expression which is a constant at compile time. Any expression
that accesses storage, blockchain data (e.g. ``now``, ``address(this).balance`` or
``block.number``) or
execution data (``msg.value`` or ``gasleft()``) or makes calls to external contracts is disallowed. Expressions
that might have a side-effect on memory allocation are allowed, but those that
might have a side-effect on other memory objects are not. The built-in functions
``keccak256``, ``sha256``, ``ripemd160``, ``ecrecover``, ``addmod`` and ``mulmod``
are allowed (even though, with the exception of ``keccak256``, they do call external contracts).
The reason behind allowing side-effects on the memory allocator is that it
should be possible to construct complex objects like e.g. lookup-tables.
This feature is not yet fully usable.
The compiler does not reserve a storage slot for these variables, and every occurrence is
replaced by the respective constant expression (which might be computed to a single value by the optimizer).
Not all types for constants are implemented at this time. The only supported types are
value types and strings.
::
pragma solidity >=0.4.0 <0.6.0;
contract C {
uint constant x = 32**22 + 8;
string constant text = "abc";
bytes32 constant myHash = keccak256("abc");
}
.. index:: ! functions
.. _functions:
*********
Functions
*********
Function Parameters and Return Variables
========================================
As in JavaScript, functions may take parameters as input. Unlike in JavaScript
and C, functions may also return an arbitrary number of values as output.
Function Parameters
-------------------
Function parameters are declared the same way as variables, and the name of
unused parameters can be omitted.
For example, if you want your contract to accept one kind of external call
with two integers, you would use something like::
pragma solidity >=0.4.16 <0.6.0;
contract Simple {
uint sum;
function taker(uint _a, uint _b) public {
sum = _a + _b;
}
}
Function parameters can be used as any other local variable and they can also be assigned to.
.. note::
An :ref:`external function<external-function-calls>` cannot accept a
multi-dimensional array as an input
parameter. This functionality is possible if you enable the new
experimental ``ABIEncoderV2`` feature by adding ``pragma experimental ABIEncoderV2;`` to your source file.
An :ref:`internal function<external-function-calls>` can accept a
multi-dimensional array without enabling the feature.
.. index:: return array, return string, array, string, array of strings, dynamic array, variably sized array, return struct, struct
Return Variables
----------------
Function return variables are declared with the same syntax after the
``returns`` keyword.
For example, suppose you want to return two results: the sum and the product of
two integers passed as function parameters, then you use something like::
pragma solidity >=0.4.16 <0.6.0;
contract Simple {
function arithmetic(uint _a, uint _b)
public
pure
returns (uint o_sum, uint o_product)
{
o_sum = _a + _b;
o_product = _a * _b;
}
}
The names of return variables can be omitted.
Return variables can be used as any other local variable and they
are initialized with their :ref:`default value <default-value>` and have that value unless explicitly set.
You can either explicitly assign to return variables and
then leave the function using ``return;``,
or you can provide return values
(either a single or :ref:`multiple ones<multi-return>`) directly with the ``return``
statement::
pragma solidity >=0.4.16 <0.6.0;
contract Simple {
function arithmetic(uint _a, uint _b)
public
pure
returns (uint o_sum, uint o_product)
{
return (_a + _b, _a * _b);
}
}
This form is equivalent to first assigning values to the
return variables and then using ``return;`` to leave the function.
.. note::
You cannot return some types from non-internal functions, notably
multi-dimensional dynamic arrays and structs. If you enable the
new experimental ``ABIEncoderV2`` feature by adding ``pragma experimental
ABIEncoderV2;`` to your source file then more types are available, but
``mapping`` types are still limited to inside a single contract and you
cannot transfer them.
.. _multi-return:
Returning Multiple Values
-------------------------
When a function has multiple return types, the statement ``return (v0, v1, ..., vn) can be used to return multiple values.
vn)`` can return multiple values. The number of components must be
the same as the number of return types.
.. index:: ! view function, function;view
.. _view-functions:
View Functions
==============
Functions can be declared ``view`` in which case they promise not to modify the state.
.. note::
If the compiler's EVM target is Byzantium or newer (default) the opcode
``STATICCALL`` is used for ``view`` functions which enforces the state
to stay unmodified as part of the EVM execution. For library ``view`` functions
``DELEGATECALL`` is used, because there is no combined ``DELEGATECALL`` and ``STATICCALL``.
This means library ``view`` functions do not have run-time checks that prevent state
modifications. This should not impact security negatively because library code is
usually known at compile-time and the static checker performs compile-time checks.
The following statements are considered modifying the state:
#. Writing to state variables.
#. :ref:`Emitting events <events>`.
#. :ref:`Creating other contracts <creating-contracts>`.
#. Using ``selfdestruct``.
#. Sending Ether via calls.
#. Calling any function not marked ``view`` or ``pure``.
#. Using low-level calls.
#. Using inline assembly that contains certain opcodes.
::
pragma solidity >0.4.99 <0.6.0;
contract C {
function f(uint a, uint b) public view returns (uint) {
return a * (b + 42) + now;
}
}
.. note::
``constant`` on functions used to be an alias to ``view``, but this was dropped in version 0.5.0.
.. note::
Getter methods are automatically marked ``view``.
.. note::
Prior to version 0.5.0, the compiler did not use the ``STATICCALL`` opcode
for ``view`` functions.
This enabled state modifications in ``view`` functions through the use of
invalid explicit type conversions.
By using ``STATICCALL`` for ``view`` functions, modifications to the
state are prevented on the level of the EVM.
.. index:: ! pure function, function;pure
.. _pure-functions:
Pure Functions
==============
Functions can be declared ``pure`` in which case they promise not to read from or modify the state.
.. note::
If the compiler's EVM target is Byzantium or newer (default) the opcode ``STATICCALL`` is used,
which does not guarantee that the state is not read, but at least that it is not modified.
In addition to the list of state modifying statements explained above, the following are considered reading from the state:
#. Reading from state variables.
#. Accessing ``address(this).balance`` or ``<address>.balance``.
#. Accessing any of the members of ``block``, ``tx``, ``msg`` (with the exception of ``msg.sig`` and ``msg.data``).
#. Calling any function not marked ``pure``.
#. Using inline assembly that contains certain opcodes.
::
pragma solidity >0.4.99 <0.6.0;
contract C {
function f(uint a, uint b) public pure returns (uint) {
return a * (b + 42);
}
}
.. note::
Prior to version 0.5.0, the compiler did not use the ``STATICCALL`` opcode
for ``pure`` functions.
This enabled state modifications in ``pure`` functions through the use of
invalid explicit type conversions.
By using ``STATICCALL`` for ``pure`` functions, modifications to the
state are prevented on the level of the EVM.
.. warning::
It is not possible to prevent functions from reading the state at the level
of the EVM, it is only possible to prevent them from writing to the state
(i.e. only ``view`` can be enforced at the EVM level, ``pure`` can not).
.. warning::
Before version 0.4.17 the compiler did not enforce that ``pure`` is not reading the state.
It is a compile-time type check, which can be circumvented doing invalid explicit conversions
between contract types, because the compiler can verify that the type of the contract does
not do state-changing operations, but it cannot check that the contract that will be called
at runtime is actually of that type.
.. index:: ! fallback function, function;fallback
.. _fallback-function:
Fallback Function
=================
A contract can have exactly one unnamed function. This function cannot have
arguments, cannot return anything and has to have ``external`` visibility.
It is executed on a call to the contract if none of the other
functions match the given function identifier (or if no data was supplied at
all).
Furthermore, this function is executed whenever the contract receives plain
Ether (without data). Additionally, in order to receive Ether, the fallback function
must be marked ``payable``. If no such function exists, the contract cannot receive
Ether through regular transactions.
In the worst case, the fallback function can only rely on 2300 gas being
available (for example when `send` or `transfer` is used), leaving little
room to perform other operations except basic logging. The following operations
will consume more gas than the 2300 gas stipend:
- Writing to storage
- Creating a contract
- Calling an external function which consumes a large amount of gas
- Sending Ether
Like any function, the fallback function can execute complex operations as long as there is enough gas passed on to it.
.. note::
Even though the fallback function cannot have arguments, one can still use ``msg.data`` to retrieve
any payload supplied with the call.
.. warning::
The fallback function is also executed if the caller meant to call
a function that is not available. If you want to implement the fallback
function only to receive ether, you should add a check
like ``require(msg.data.length == 0)`` to prevent invalid calls.
.. warning::
Contracts that receive Ether directly (without a function call, i.e. using ``send`` or ``transfer``)
but do not define a fallback function
throw an exception, sending back the Ether (this was different
before Solidity v0.4.0). So if you want your contract to receive Ether,
you have to implement a payable fallback function.
.. warning::
A contract without a payable fallback function can receive Ether as a recipient of a `coinbase transaction` (aka `miner block reward`)
or as a destination of a ``selfdestruct``.
A contract cannot react to such Ether transfers and thus also cannot reject them. This is a design choice of the EVM and Solidity cannot work around it.
It also means that ``address(this).balance`` can be higher than the sum of some manual accounting implemented in a contract (i.e. having a counter updated in the fallback function).
::
pragma solidity >0.4.99 <0.6.0;
contract Test {
// This function is called for all messages sent to
// this contract (there is no other function).
// Sending Ether to this contract will cause an exception,
// because the fallback function does not have the `payable`
// modifier.
function() external { x = 1; }
uint x;
}
// This contract keeps all Ether sent to it with no way
// to get it back.
contract Sink {
function() external payable { }
}
contract Caller {
function callTest(Test test) public returns (bool) {
(bool success,) = address(test).call(abi.encodeWithSignature("nonExistingFunction()"));
require(success);
// results in test.x becoming == 1.
// address(test) will not allow to call ``send`` directly, since ``test`` has no payable
// fallback function. It has to be converted to the ``address payable`` type via an
// intermediate conversion to ``uint160`` to even allow calling ``send`` on it.
address payable testPayable = address(uint160(address(test)));
// If someone sends ether to that contract,
// the transfer will fail, i.e. this returns false here.
return testPayable.send(2 ether);
}
}
.. index:: ! overload
.. _overload-function:
Function Overloading
====================
A contract can have multiple functions of the same name but with different parameter
types.
This process is called "overloading" and also applies to inherited functions.
The following example shows overloading of the function
``f`` in the scope of contract ``A``.
::
pragma solidity >=0.4.16 <0.6.0;
contract A {
function f(uint _in) public pure returns (uint out) {
out = _in;
}
function f(uint _in, bool _really) public pure returns (uint out) {
if (_really)
out = _in;
}
}
Overloaded functions are also present in the external interface. It is an error if two
externally visible functions differ by their Solidity types but not by their external types.
::
pragma solidity >=0.4.16 <0.6.0;
// This will not compile
contract A {
function f(B _in) public pure returns (B out) {
out = _in;
}
function f(address _in) public pure returns (address out) {
out = _in;
}
}
contract B {
}
Both ``f`` function overloads above end up accepting the address type for the ABI although
they are considered different inside Solidity.
Overload resolution and Argument matching
-----------------------------------------
Overloaded functions are selected by matching the function declarations in the current scope
to the arguments supplied in the function call. Functions are selected as overload candidates
if all arguments can be implicitly converted to the expected types. If there is not exactly one
candidate, resolution fails.
.. note::
Return parameters are not taken into account for overload resolution.
::
pragma solidity >=0.4.16 <0.6.0;
contract A {
function f(uint8 _in) public pure returns (uint8 out) {
out = _in;
}
function f(uint256 _in) public pure returns (uint256 out) {
out = _in;
}
}
Calling ``f(50)`` would create a type error since ``50`` can be implicitly converted both to ``uint8``
and ``uint256`` types. On another hand ``f(256)`` would resolve to ``f(uint256)`` overload as ``256`` cannot be implicitly
converted to ``uint8``.
.. index:: ! event
.. _events:
******
Events
******
Solidity events give an abstraction on top of the EVM's logging functionality.
Applications can subscribe and listen to these events through the RPC interface of an Ethereum client.
Events are inheritable members of contracts. When you call them, they cause the
arguments to be stored in the transaction's log - a special data structure
in the blockchain. These logs are associated with the address of the contract,
are incorporated into the blockchain, and stay there as long as a block is
accessible (forever as of the Frontier and Homestead releases, but this might
change with Serenity). The Log and its event data is not accessible from within
contracts (not even from the contract that created them).
It is possible to request a simple payment verification (SPV) for logs, so if
an external entity supplies a contract with such a verification, it can check
that the log actually exists inside the blockchain. You have to supply block headers
because the contract can only see the last 256 block hashes.
You can add the attribute ``indexed`` to up to three parameters which adds them
to a special data structure known as :ref:`"topics" <abi_events>` instead of
the data part of the log. If you use arrays (including ``string`` and ``bytes``)
as indexed arguments, its Keccak-256 hash is stored as a topic instead, this is
because a topic can only hold a single word (32 bytes).
All parameters without the ``indexed`` attribute are :ref:`ABI-encoded <ABI>`
into the data part of the log.
Topics allow you to search for events, for example when filtering a sequence of
blocks for certain events. You can also filter events by the address of the
contract that emitted the event.
For example, the code below uses the web3.js ``subscribe("logs")``
`method <https://web3js.readthedocs.io/en/1.0/web3-eth-subscribe.html#subscribe-logs>`_ to filter
logs that match a topic with a certain address value:
.. code-block:: javascript
var options = {
fromBlock: 0,
address: web3.eth.defaultAccount,
topics: ["0x0000000000000000000000000000000000000000000000000000000000000000", null, null]
};
web3.eth.subscribe('logs', options, function (error, result) {
if (!error)
console.log(result);
})
.on("data", function (log) {
console.log(log);
})
.on("changed", function (log) {
});
The hash of the signature of the event is one of the topics, except if you
declared the event with the ``anonymous`` specifier. This means that it is
not possible to filter for specific anonymous events by name.
::
pragma solidity >=0.4.21 <0.6.0;
contract ClientReceipt {
event Deposit(
address indexed _from,
bytes32 indexed _id,
uint _value
);
function deposit(bytes32 _id) public payable {
// Events are emitted using `emit`, followed by
// the name of the event and the arguments
// (if any) in parentheses. Any such invocation
// (even deeply nested) can be detected from
// the JavaScript API by filtering for `Deposit`.
emit Deposit(msg.sender, _id, msg.value);
}
}
The use in the JavaScript API is as follows:
::
var abi = /* abi as generated by the compiler */;
var ClientReceipt = web3.eth.contract(abi);
var clientReceipt = ClientReceipt.at("0x1234...ab67" /* address */);
var event = clientReceipt.Deposit();
// watch for changes
event.watch(function(error, result){
// result contains non-indexed arguments and topics
// given to the `Deposit` call.
if (!error)
console.log(result);
});
// Or pass a callback to start watching immediately
var event = clientReceipt.Deposit(function(error, result) {
if (!error)
console.log(result);
});
The output of the above looks like the following (trimmed):
.. code-block:: json
{
"returnValues": {
"_from": "0x1111…FFFFCCCC",
"_id": "0x50…sd5adb20",
"_value": "0x420042"
},
"raw": {
"data": "0x7f…91385",
"topics": ["0xfd4…b4ead7", "0x7f…1a91385"]
}
}
.. index:: ! log
Low-Level Interface to Logs
===========================
It is also possible to access the low-level interface to the logging
mechanism via the functions ``log0``, ``log1``, ``log2``, ``log3`` and ``log4``.
``logi`` takes ``i + 1`` parameter of type ``bytes32``, where the first
argument will be used for the data part of the log and the others
as topics. The event call above can be performed in the same way as
::
pragma solidity >=0.4.10 <0.6.0;
contract C {
function f() public payable {
uint256 _id = 0x420042;
log3(
bytes32(msg.value),
bytes32(0x50cb9fe53daa9737b786ab3646f04d0150dc50ef4e75f59509d83667ad5adb20),
bytes32(uint256(msg.sender)),
bytes32(_id)
);
}
}
where the long hexadecimal number is equal to
``keccak256("Deposit(address,bytes32,uint256)")``, the signature of the event.
Additional Resources for Understanding Events
==============================================
- `Javascript documentation <https://github.com/ethereum/wiki/wiki/JavaScript-API#contract-events>`_
- `Example usage of events <https://github.com/debris/smart-exchange/blob/master/lib/contracts/SmartExchange.sol>`_
- `How to access them in js <https://github.com/debris/smart-exchange/blob/master/lib/exchange_transactions.js>`_
.. index:: ! inheritance, ! base class, ! contract;base, ! deriving
***********
Inheritance
***********
Solidity supports multiple inheritance by copying code including polymorphism.
All function calls are virtual, which means that the most derived function
is called, except when the contract name is explicitly given.
When a contract inherits from other contracts, only a single
contract is created on the blockchain, and the code from all the base contracts
is copied into the created contract.
The general inheritance system is very similar to
`Python's <https://docs.python.org/3/tutorial/classes.html#inheritance>`_,
especially concerning multiple inheritance, but there are also
some :ref:`differences <multi-inheritance>`.
Details are given in the following example.
::
pragma solidity >0.4.99 <0.6.0;
contract owned {
constructor() public { owner = msg.sender; }
address payable owner;
}
// Use `is` to derive from another contract. Derived
// contracts can access all non-private members including
// internal functions and state variables. These cannot be
// accessed externally via `this`, though.
contract mortal is owned {
function kill() public {
if (msg.sender == owner) selfdestruct(owner);
}
}
// These abstract contracts are only provided to make the
// interface known to the compiler. Note the function
// without body. If a contract does not implement all
// functions it can only be used as an interface.
contract Config {
function lookup(uint id) public returns (address adr);
}
contract NameReg {
function register(bytes32 name) public;
function unregister() public;
}
// Multiple inheritance is possible. Note that `owned` is
// also a base class of `mortal`, yet there is only a single
// instance of `owned` (as for virtual inheritance in C++).
contract named is owned, mortal {
constructor(bytes32 name) public {
Config config = Config(0xD5f9D8D94886E70b06E474c3fB14Fd43E2f23970);
NameReg(config.lookup(1)).register(name);
}
// Functions can be overridden by another function with the same name and
// the same number/types of inputs. If the overriding function has different
// types of output parameters, that causes an error.
// Both local and message-based function calls take these overrides
// into account.
function kill() public {
if (msg.sender == owner) {
Config config = Config(0xD5f9D8D94886E70b06E474c3fB14Fd43E2f23970);
NameReg(config.lookup(1)).unregister();
// It is still possible to call a specific
// overridden function.
mortal.kill();
}
}
}
// If a constructor takes an argument, it needs to be
// provided in the header (or modifier-invocation-style at
// the constructor of the derived contract (see below)).
contract PriceFeed is owned, mortal, named("GoldFeed") {
function updateInfo(uint newInfo) public {
if (msg.sender == owner) info = newInfo;
}
function get() public view returns(uint r) { return info; }
uint info;
}
Note that above, we call ``mortal.kill()`` to "forward" the
destruction request. The way this is done is problematic, as
seen in the following example::
pragma solidity >=0.4.22 <0.6.0;
contract owned {
constructor() public { owner = msg.sender; }
address payable owner;
}
contract mortal is owned {
function kill() public {
if (msg.sender == owner) selfdestruct(owner);
}
}
contract Base1 is mortal {
function kill() public { /* do cleanup 1 */ mortal.kill(); }
}
contract Base2 is mortal {
function kill() public { /* do cleanup 2 */ mortal.kill(); }
}
contract Final is Base1, Base2 {
}
A call to ``Final.kill()`` will call ``Base2.kill`` as the most
derived override, but this function will bypass
``Base1.kill``, basically because it does not even know about
``Base1``. The way around this is to use ``super``::
pragma solidity >=0.4.22 <0.6.0;
contract owned {
constructor() public { owner = msg.sender; }
address payable owner;
}
contract mortal is owned {
function kill() public {
if (msg.sender == owner) selfdestruct(owner);
}
}
contract Base1 is mortal {
function kill() public { /* do cleanup 1 */ super.kill(); }
}
contract Base2 is mortal {
function kill() public { /* do cleanup 2 */ super.kill(); }
}
contract Final is Base1, Base2 {
}
If ``Base2`` calls a function of ``super``, it does not simply
call this function on one of its base contracts. Rather, it
calls this function on the next base contract in the final
inheritance graph, so it will call ``Base1.kill()`` (note that
the final inheritance sequence is -- starting with the most
derived contract: Final, Base2, Base1, mortal, owned).
The actual function that is called when using super is
not known in the context of the class where it is used,
although its type is known. This is similar for ordinary
virtual method lookup.
.. index:: ! constructor
.. _constructor:
Constructors
============
A constructor is an optional function declared with the ``constructor`` keyword
which is executed upon contract creation, and where you can run contract
initialisation code.
Before the constructor code is executed, state variables are initialised to
their specified value if you initialise them inline, or zero if you do not.
After the constructor has run, the final code of the contract is deployed
to the blockchain. The deployment of
the code costs additional gas linear to the length of the code.
This code includes all functions that are part of the public interface
and all functions that are reachable from there through function calls.
It does not include the constructor code or internal functions that are
only called from the constructor.
Constructor functions can be either ``public`` or ``internal``. If there is no
constructor, the contract will assume the default constructor, which is
equivalent to ``constructor() public {}``. For example:
::
pragma solidity >0.4.99 <0.6.0;
contract A {
uint public a;
constructor(uint _a) internal {
a = _a;
}
}
contract B is A(1) {
constructor() public {}
}
A constructor set as ``internal`` causes the contract to be marked as :ref:`abstract <abstract-contract>`.
.. warning ::
Prior to version 0.4.22, constructors were defined as functions with the same name as the contract.
This syntax was deprecated and is not allowed anymore in version 0.5.0.
.. index:: ! base;constructor
Arguments for Base Constructors
===============================
The constructors of all the base contracts will be called following the
linearization rules explained below. If the base constructors have arguments,
derived contracts need to specify all of them. This can be done in two ways::
pragma solidity >=0.4.22 <0.6.0;
contract Base {
uint x;
constructor(uint _x) public { x = _x; }
}
// Either directly specify in the inheritance list...
contract Derived1 is Base(7) {
constructor() public {}
}
// or through a "modifier" of the derived constructor.
contract Derived2 is Base {
constructor(uint _y) Base(_y * _y) public {}
}
One way is directly in the inheritance list (``is Base(7)``). The other is in
the way a modifier is invoked as part of
the derived constructor (``Base(_y * _y)``). The first way to
do it is more convenient if the constructor argument is a
constant and defines the behaviour of the contract or
describes it. The second way has to be used if the
constructor arguments of the base depend on those of the
derived contract. Arguments have to be given either in the
inheritance list or in modifier-style in the derived constructor.
Specifying arguments in both places is an error.
If a derived contract does not specify the arguments to all of its base
contracts' constructors, it will be abstract.
.. index:: ! inheritance;multiple, ! linearization, ! C3 linearization
.. _multi-inheritance:
Multiple Inheritance and Linearization
======================================
Languages that allow multiple inheritance have to deal with
several problems. One is the `Diamond Problem <https://en.wikipedia.org/wiki/Multiple_inheritance#The_diamond_problem>`_.
Solidity is similar to Python in that it uses "`C3 Linearization <https://en.wikipedia.org/wiki/C3_linearization>`_"
to force a specific order in the directed acyclic graph (DAG) of base classes. This
results in the desirable property of monotonicity but
disallows some inheritance graphs. Especially, the order in
which the base classes are given in the ``is`` directive is
important: You have to list the direct base contracts
in the order from "most base-like" to "most derived".
Note that this order is the reverse of the one used in Python.
Another simplifying way to explain this is that when a function is called that
is defined multiple times in different contracts, the given bases
are searched from right to left (left to right in Python) in a depth-first manner,
stopping at the first match. If a base contract has already been searched, it is skipped.
In the following code, Solidity will give the
error "Linearization of inheritance graph impossible".
::
pragma solidity >=0.4.0 <0.6.0;
contract X {}
contract A is X {}
// This will not compile
contract C is A, X {}
The reason for this is that ``C`` requests ``X`` to override ``A``
(by specifying ``A, X`` in this order), but ``A`` itself
requests to override ``X``, which is a contradiction that
cannot be resolved.
Inheriting Different Kinds of Members of the Same Name
======================================================
When the inheritance results in a contract with a function and a modifier of the same name, it is considered as an error.
This error is produced also by an event and a modifier of the same name, and a function and an event of the same name.
As an exception, a state variable getter can override a public function.
.. index:: ! contract;abstract, ! abstract contract
.. _abstract-contract:
******************
Abstract Contracts
******************
Contracts are marked as abstract when at least one of their functions lacks an implementation as in the following example (note that the function declaration header is terminated by ``;``)::
pragma solidity >=0.4.0 <0.6.0;
contract Feline {
function utterance() public returns (bytes32);
}
Such contracts cannot be compiled (even if they contain implemented functions alongside non-implemented functions), but they can be used as base contracts::
pragma solidity >=0.4.0 <0.6.0;
contract Feline {
function utterance() public returns (bytes32);
}
contract Cat is Feline {
function utterance() public returns (bytes32) { return "miaow"; }
}
If a contract inherits from an abstract contract and does not implement all non-implemented functions by overriding, it will itself be abstract.
Note that a function without implementation is different from a :ref:`Function Type <function_types>` even though their syntax looks very similar.
Example of function without implementation (a function declaration)::
function foo(address) external returns (address);
Example of a Function Type (a variable declaration, where the variable is of type ``function``)::
function(address) external returns (address) foo;
Abstract contracts decouple the definition of a contract from its implementation providing better extensibility and self-documentation and
facilitating patterns like the `Template method <https://en.wikipedia.org/wiki/Template_method_pattern>`_ and removing code duplication.
Abstract contracts are useful in the same way that defining methods in an interface is useful. It is a way for the designer of the abstract contract to say "any child of mine must implement this method".
.. index:: ! contract;interface, ! interface contract
.. _interfaces:
**********
Interfaces
**********
Interfaces are similar to abstract contracts, but they cannot have any functions implemented. There are further restrictions:
- They cannot inherit other contracts or interfaces.
- All declared functions must be external.
- They cannot declare a constructor.
- They cannot declare state variables.
Some of these restrictions might be lifted in the future.
Interfaces are basically limited to what the Contract ABI can represent, and the conversion between the ABI and
an interface should be possible without any information loss.
Interfaces are denoted by their own keyword:
::
pragma solidity >=0.4.11 <0.6.0;
interface Token {
enum TokenType { Fungible, NonFungible }
struct Coin { string obverse; string reverse; }
function transfer(address recipient, uint amount) external;
}
Contracts can inherit interfaces as they would inherit other contracts.
Types defined inside interfaces and other contract-like structures
can be accessed from other contracts: ``Token.TokenType`` or ``Token.Coin``.
.. index:: ! library, callcode, delegatecall
.. _libraries:
*********
Libraries
*********
Libraries are similar to contracts, but their purpose is that they are deployed
only once at a specific address and their code is reused using the ``DELEGATECALL``
(``CALLCODE`` until Homestead)
feature of the EVM. This means that if library functions are called, their code
is executed in the context of the calling contract, i.e. ``this`` points to the
calling contract, and especially the storage from the calling contract can be
accessed. As a library is an isolated piece of source code, it can only access
state variables of the calling contract if they are explicitly supplied (it
would have no way to name them, otherwise). Library functions can only be
called directly (i.e. without the use of ``DELEGATECALL``) if they do not modify
the state (i.e. if they are ``view`` or ``pure`` functions),
because libraries are assumed to be stateless. In particular, it is
not possible to destroy a library.
.. note::
Until version 0.4.20, it was possible to destroy libraries by
circumventing Solidity's type system. Starting from that version,
libraries contain a :ref:`mechanism<call-protection>` that
disallows state-modifying functions
to be called directly (i.e. without ``DELEGATECALL``).
Libraries can be seen as implicit base contracts of the contracts that use them.
They will not be explicitly visible in the inheritance hierarchy, but calls
to library functions look just like calls to functions of explicit base
contracts (``L.f()`` if ``L`` is the name of the library). Furthermore,
``internal`` functions of libraries are visible in all contracts, just as
if the library were a base contract. Of course, calls to internal functions
use the internal calling convention, which means that all internal types
can be passed and types :ref:`stored in memory <data-location>` will be passed by reference and not copied.
To realize this in the EVM, code of internal library functions
and all functions called from therein will at compile time be pulled into the calling
contract, and a regular ``JUMP`` call will be used instead of a ``DELEGATECALL``.
.. index:: using for, set
The following example illustrates how to use libraries (but manual method
be sure to check out :ref:`using for <using-for>` for a
more advanced example to implement a set).
::
pragma solidity >=0.4.22 <0.6.0;
library Set {
// We define a new struct datatype that will be used to
// hold its data in the calling contract.
struct Data { mapping(uint => bool) flags; }
// Note that the first parameter is of type "storage
// reference" and thus only its storage address and not
// its contents is passed as part of the call. This is a
// special feature of library functions. It is idiomatic
// to call the first parameter `self`, if the function can
// be seen as a method of that object.
function insert(Data storage self, uint value)
public
returns (bool)
{
if (self.flags[value])
return false; // already there
self.flags[value] = true;
return true;
}
function remove(Data storage self, uint value)
public
returns (bool)
{
if (!self.flags[value])
return false; // not there
self.flags[value] = false;
return true;
}
function contains(Data storage self, uint value)
public
view
returns (bool)
{
return self.flags[value];
}
}
contract C {
Set.Data knownValues;
function register(uint value) public {
// The library functions can be called without a
// specific instance of the library, since the
// "instance" will be the current contract.
require(Set.insert(knownValues, value));
}
// In this contract, we can also directly access knownValues.flags, if we want.
}
Of course, you do not have to follow this way to use
libraries: they can also be used without defining struct
data types. Functions also work without any storage
reference parameters, and they can have multiple storage reference
parameters and in any position.
The calls to ``Set.contains``, ``Set.insert`` and ``Set.remove``
are all compiled as calls (``DELEGATECALL``) to an external
contract/library. If you use libraries, be aware that an
actual external function call is performed.
``msg.sender``, ``msg.value`` and ``this`` will retain their values
in this call, though (prior to Homestead, because of the use of ``CALLCODE``, ``msg.sender`` and
``msg.value`` changed, though).
The following example shows how to use :ref:`types stored in memory <data-location>` and
internal functions in libraries in order to implement
custom types without the overhead of external function calls:
::
pragma solidity >=0.4.16 <0.6.0;
library BigInt {
struct bigint {
uint[] limbs;
}
function fromUint(uint x) internal pure returns (bigint memory r) {
r.limbs = new uint[](1);
r.limbs[0] = x;
}
function add(bigint memory _a, bigint memory _b) internal pure returns (bigint memory r) {
r.limbs = new uint[](max(_a.limbs.length, _b.limbs.length));
uint carry = 0;
for (uint i = 0; i < r.limbs.length; ++i) {
uint a = limb(_a, i);
uint b = limb(_b, i);
r.limbs[i] = a + b + carry;
if (a + b < a || (a + b == uint(-1) && carry > 0))
carry = 1;
else
carry = 0;
}
if (carry > 0) {
// too bad, we have to add a limb
uint[] memory newLimbs = new uint[](r.limbs.length + 1);
uint i;
for (i = 0; i < r.limbs.length; ++i)
newLimbs[i] = r.limbs[i];
newLimbs[i] = carry;
r.limbs = newLimbs;
}
}
function limb(bigint memory _a, uint _limb) internal pure returns (uint) {
return _limb < _a.limbs.length ? _a.limbs[_limb] : 0;
}
function max(uint a, uint b) private pure returns (uint) {
return a > b ? a : b;
}
}
contract C {
using BigInt for BigInt.bigint;
function f() public pure {
BigInt.bigint memory x = BigInt.fromUint(7);
BigInt.bigint memory y = BigInt.fromUint(uint(-1));
BigInt.bigint memory z = x.add(y);
assert(z.limb(1) > 0);
}
}
As the compiler cannot know where the library will be
deployed at, these addresses have to be filled into the
final bytecode by a linker
(see :ref:`commandline-compiler` for how to use the
commandline compiler for linking). If the addresses are not
given as arguments to the compiler, the compiled hex code
will contain placeholders of the form ``__Set______`` (where
``Set`` is the name of the library). The address can be filled
manually by replacing all those 40 symbols by the hex
encoding of the address of the library contract.
.. note::
Manually linking libraries on the generated bytecode is discouraged, because
it is restricted to 36 characters.
You should ask the compiler to link the libraries at the time
a contract is compiled by either using
the ``--libraries`` option of ``solc`` or the ``libraries`` key if you use
the standard-JSON interface to the compiler.
Restrictions for libraries in comparison to contracts:
- No state variables
- Cannot inherit nor be inherited
- Cannot receive Ether
(These might be lifted at a later point.)
.. _call-protection:
Call Protection For Libraries
=============================
As mentioned in the introduction, if a library's code is executed
using a ``CALL`` instead of a ``DELEGATECALL`` or ``CALLCODE``,
it will revert unless a ``view`` or ``pure`` function is called.
The EVM does not provide a direct way for a contract to detect
whether it was called using ``CALL`` or not, but a contract
can use the ``ADDRESS`` opcode to find out "where" it is
currently running. The generated code compares this address
to the address used at construction time to determine the mode
of calling.
More specifically, the runtime code of a library always starts
with a push instruction, which is a zero of 20 bytes at
compilation time. When the deploy code runs, this constant
is replaced in memory by the current address and this
modified code is stored in the contract. At runtime,
this causes the deploy time address to be the first
constant to be pushed onto the stack and the dispatcher
code compares the current address against this constant
for any non-view and non-pure function.
.. index:: ! using for, library
.. _using-for:
*********
Using For
*********
The directive ``using A for B;`` can be used to attach library
functions (from the library ``A``) to any type (``B``).
These functions will receive the object they are called on
as their first parameter (like the ``self`` variable in Python).
The effect of ``using A for *;`` is that the functions from
the library ``A`` are attached to *any* type.
In both situations, *all* functions in the library are attached,
even those where the type of the first parameter does not
match the type of the object. The type is checked at the
point the function is called and function overload
resolution is performed.
The ``using A for B;`` directive is active only within the current
contract, including within all of its functions, and has no effect
outside of the contract in which it is used. The directive
may only be used inside a contract, not inside any of its functions.
By including a library, its data types including library functions are
available without having to add further code.
Let us rewrite the set example from the
:ref:`libraries` in this way::
pragma solidity >=0.4.16 <0.6.0;
// This is the same code as before, just without comments
library Set {
struct Data { mapping(uint => bool) flags; }
function insert(Data storage self, uint value)
public
returns (bool)
{
if (self.flags[value])
return false; // already there
self.flags[value] = true;
return true;
}
function remove(Data storage self, uint value)
public
returns (bool)
{
if (!self.flags[value])
return false; // not there
self.flags[value] = false;
return true;
}
function contains(Data storage self, uint value)
public
view
returns (bool)
{
return self.flags[value];
}
}
contract C {
using Set for Set.Data; // this is the crucial change
Set.Data knownValues;
function register(uint value) public {
// Here, all variables of type Set.Data have
// corresponding member functions.
// The following function call is identical to
// `Set.insert(knownValues, value)`
require(knownValues.insert(value));
}
}
It is also possible to extend elementary types in that way::
pragma solidity >=0.4.16 <0.6.0;
library Search {
function indexOf(uint[] storage self, uint value)
public
view
returns (uint)
{
for (uint i = 0; i < self.length; i++)
if (self[i] == value) return i;
return uint(-1);
}
}
contract C {
using Search for uint[];
uint[] data;
function append(uint value) public {
data.push(value);
}
function replace(uint _old, uint _new) public {
// This performs the library function call
uint index = data.indexOf(_old);
if (index == uint(-1))
data.push(_new);
else
data[index] = _new;
}
}
Note that all library calls are actual EVM function calls. This means that
if you pass memory or value types, a copy will be performed, even of the
``self`` variable. The only situation where no copy will be performed
is when storage reference variables are used.
|