emplacing a POD

Question

Possible Duplicate:
C++11 emplace_back on vector<struct>?

Is emplacement possible with PODs? It does not seem to work in Visual Studio 2012:

struct X
{
    int a;
    int b;
};

void whatever()
{
    std::vector<X> xs;
    X x = {1, 2};

    // okay
    xs.push_back(x);

    // okay
    xs.emplace_back(x);

    //error C2661: 'X::X': error C2661: no overloaded function takes 2 arguments
    xs.emplace_back(1, 2);
}

Is this just a shortcoming of Visual Studio 2012, or does emplacing a POD simply not work in C++11?

It constructs it in place. There's no constructor like that. — chris, Dec 19 '12 at 22:08
I wouldn't consider it definitive, but g++ (4.7.1) doesn't accept it either. — Jerry Coffin, Dec 19 '12 at 22:08
I think you probably have to have a constructor for `X` that takes two arguments (as the error message suggests). — Timo Geusch, Dec 19 '12 at 22:08
Related: [Why doesn't emplace_back() use uniform initialization?](http://stackoverflow.com/q/8782895/636019) — ildjarn, Dec 19 '12 at 22:09

Joseph Mansfield · Accepted Answer · 2012-12-19T23:12:33.477

14

There is no constructor X::X(int,int), which is what your call to emplace_back would use to construct the X object. Containers use allocator_traits<A>::construct(allocator, p, args) to construct objects, where p is a pointer to some allocated space and args are the arguments passed to the constructor. This is used by emplace_back. This construct function calls ::new((void*)p) T(std::forward<Args>(args)...), so it doesn't use uniform initialization.

xs.emplace_back({1, 2}) will also be an error, despite the fact that an aggregate can be constructed with list-initialization. That's because a brace-enclosed initializer list cannot be forwarded.

edited Dec 19 '12 at 23:12

answered Dec 19 '12 at 22:13

Joseph Mansfield

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Will work fine with a compliant compiler, but not with VC++ 2012 (yet). :-] – ildjarn Dec 19 '12 at 22:14
5

`emplace_back({1, 2})` is an error, `push_back({1, 2})` will work however. List initialization cannot take place in the former as there's nothing to initialize (informally speaking), it's a necessary evil with template argument deduction. – Luc Danton Dec 19 '12 at 23:06

emplacing a POD

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