What is the difference between these two forms of list initialization for std::map?

Question

I have tested the following two forms with clang and they are both accepted:

using IntMap = std::map<int, int>;

IntMap map1 {{
  {1, 1},
  {2, 2},
  {3, 3},
  {4, 4}
}};

IntMap map2 {
  {1, 1},
  {2, 2},
  {3, 3},
  {4, 4}
};

On Visual Studio 2013, the latter example fails to compile stating there is no constructor in map that takes 4 arguments.

I'm assuming both are actually valid. What is the difference between the two? Why doesn't the second example work on Visual Studio 2013?

Both are legal. The first style may occasionally cause [weird things to happen](http://stackoverflow.com/questions/24112281/c11-initializer-list-fails-but-only-on-lists-of-length-2/24112395#24112395). If the second doesn't work on VS2013 it's a compiler bug. — T.C., Dec 30 '14 at 15:40
@T.C. Great read; thanks for the link. Unfortunately I think I need to "risk" it here since I need this code working on MSVC *and* Clang (unless you can suggest a reasonable alternative). If you post an answer with this I would be more than happy to give you a green checkmark! — void.pointer, Dec 30 '14 at 15:47
@T.C.Could you please post your comment as an answer to my question? If you do, I'll mark yours as the answer. Thanks. — void.pointer, May 26 '19 at 01:09

score 1 · Accepted Answer · edited May 23 '17 at 10:33

1

As T.C. pointed out in comments, both are legal. The first style can cause weirdness, but if the second fails for you in VS2013 then that's a compiler bug )or partial implementation).

edited May 23 '17 at 10:33

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answered Jan 03 '15 at 17:52

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What is the difference between these two forms of list initialization for std::map?

1 Answers1