Merge pull request #461 from ethereum/mappingStructDocumentation

Inner struct mapping copy into a mapping question

1 files changed, 17 insertions, 0 deletions

diff --git a/docs/frequently-asked-questions.rst b/docs/frequently-asked-questions.rst
index 6ac5a9e9..fc7d7b7f 100644
--- a/docs/frequently-asked-questions.rst
+++ b/docs/frequently-asked-questions.rst
@@ -666,6 +666,23 @@ gas and return your 20 Wei).
 In the above example, the low-level function `call` is used to invoke another
 contract with `p.data` as payload and `p.amount` Wei is sent with that call.
 
+What happens to a struct's mapping when copying over a struct?
+==============================================================
+
+This is a very interesting question. Suppose that we have a contract field set up like such::
+
+    struct user{
+        mapping(string => address) usedContracts;
+    }
+    function somefunction{
+       user user1;
+       user1.usedContracts["Hello"] = "World";
+       user user2 = user1;
+    }
+
+In this case, the mapping of the struct being copied over into the userList is ignored as there is no "list of mapped keys".
+Therefore it is not possible to find out which values should be copied over.
+
 How do I initialize a contract with only a specific amount of wei?
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