ambiguous call to function using initializer_list in VS2013

Question

I have the following example code. It compiles on clang but it doesn't on Visual Studio 2013.

#include <iostream>
#include <utility>
#include <string>
using namespace std;

void f(const pair<string, string>& p)
{
    cout << p.first << ", " << p.second << endl;
}

void f(initializer_list<pair<string, string> > ps) {
    for (auto p : ps) f(p);
}

int main()
{
    f({ "2", "3" });
    f({ { "2", "3" }, { "3", "4" } });
}

The second call to f fails to compile with:

1error C2668: 'f' : ambiguous call to overloaded function
could be 'void f(std::initializer_list<std::pair<std::string,std::string>>)'
or       'void f(const std::pair<std::string,std::string> &)'
1>          while trying to match the argument list '(initializer-list)'

If I use pairs of int instead of pairs of string it does work fine.

Is anybody aware of a problem like this in Visual Studio? Or am I doing something wrong?

Thank you.

I'm almost sure I've seen something like this in a bug report.. — Marco A.
– Marco A., Commented Oct 2, 2014 at 19:37
I suspect the second call is being interpreted as f(pair(string("2", "3"), string("3, 4"))), using the constructor of std::string that takes a pair of iterators (const char* pointers are valid iterators), in addition to "initializer list of pairs" interpretation. Off the top of my head, I can't say whether such an interpretation is legal. — Igor Tandetnik
– Igor Tandetnik, Commented Oct 2, 2014 at 19:39
@IgorTandetnik is there an initializer list ctor for pair? — Yakk - Adam Nevraumont
– Yakk - Adam Nevraumont, Commented Oct 2, 2014 at 20:35
@Yakk: std::pair is an aggregate; this is why brace-initializer works for it. — Igor Tandetnik
– Igor Tandetnik, Commented Oct 3, 2014 at 0:54
@IgorTandetnik std::pair is not an aggregate - it's got plenty of user-provided constructors. — T.C.
– T.C., Commented Oct 3, 2014 at 16:41

T.C. · Accepted Answer · 2014-10-03 00:13:21Z

2

{ { "2", "3" }, { "3", "4" } } may be used to initialize either a initializer_list<pair<string, string> > or a pair<string, string>, in the second case by calling the two-iterator constructor of std::string (and causing UB in the process). Both are list-initialization sequences. Both invoke a user-defined conversion.

§13.3.3.2 [over.ics.rank]/p3:

List-initialization sequence L1 is a better conversion sequence than list-initialization sequence L2 if:

L1 converts to std::initializer_list<X> for some X and L2 does not [...]

There's no ambiguity. void f(initializer_list<pair<string, string> > ps); should be unambiguously selected.

answered Oct 3, 2014 at 0:13

T.C.

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