c++ - std::set 没有成员 emplace

Question

g++ 4.7.2 是否实现std::set::emplace，由C++11 标准定义并记录here ?

我写了以下小测试用例:

#include <set>
#include <string>

struct Foo
{
    std::string mBar;
    bool operator<(const Foo& rhs) const
    {
        return mBar < rhs.mBar;
    }
    Foo(const std::string bar) : mBar(bar) {};
};

typedef std::set<Foo> Foos;

int main()
{
    Foos foos;
    foos.emplace(std::string("Hello"));
}

在 G++ 4.7.2 下，编译失败:

[john.dibling@somewhere hacks]$ g++ -o main.o -std=c++0x -c main.cpp
main.cpp: In function ‘int main()’:
main.cpp:19:10: error: ‘Foos’ has no member named ‘emplace’

在IDEOne下也编译失败，但是它确实在 MSVC 2012 Update 1 下编译。

Answer 1

It is not implemented in gcc 4.7.2.

有一些解释:

Just to clarify a bit: this wasn't an oversight. We had the nasty problem in the Draft C++0x Standard with std::pair, which essentially made impossible adding the emplace_* members to std::map, std::multimap, etc, without breaking existing user code. Thus we waited on that, until things got clarified in this whole area. Now it's actually possible to work on those facilities.

您的代码在 gcc 4.8.0 上编译得很好，参见 LWS .