为什么emplace_back需要移动构造函数

问题描述

我有以下代码:

#include <string>
#include <vector>
#include <iostream>

class Test final {
public:
  Test(const std::string& s)
    : s_(s) {
    std::cout << "constructing: " << s_ << std::endl;
  }
#ifdef NO_MOVE
private:
  Test(const Test& t) = delete;
  Test(Test&& t) = delete;
#else
public:
  Test(const Test& t)
    : s_(t.s_) {
    std::cout << "copying: " << s_ << std::endl;
  };
  Test(Test&& t)
    : s_(std::move(t.s_)) {
    std::cout << "moving: " << s_ << std::endl;
  };
#endif
private:
  std::string s_;
};

int main() {
  std::vector<Test> v;
  v.emplace_back("emplace_back");
}

允许使用move构造函数时，会发生以下情况:

When a move constructor is allowed, the following occurs:

[matt test] g++ -std=c++11 main.cpp && ./a.out
constructing: emplace_back

但是，如果删除了移动构造函数:

However, if the move constructor is removed:

[matt test] g++ -std=c++11 main.cpp -DNO_MOVE && ./a.out
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = Test; _Args = {Test}]’:
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_uninitialized.h:77:3:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<Test*>; _ForwardIterator = Test*; bool _TrivialValueTypes = false]’
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_uninitialized.h:119:41:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<Test*>; _ForwardIterator = Test*]’
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_uninitialized.h:260:63:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = std::move_iterator<Test*>; _ForwardIterator = Test*; _Tp = Test]’
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_uninitialized.h:283:69:   required from ‘_ForwardIterator std::__uninitialized_move_if_noexcept_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&) [with _InputIterator = Test*; _ForwardIterator = Test*; _Allocator = std::allocator<Test>]’
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/vector.tcc:410:6:   required from ‘void std::vector<_Tp, _Alloc>::_M_emplace_back_aux(_Args&& ...) [with _Args = {const char (&)[13]}; _Tp = Test; _Alloc = std::allocator<Test>]’
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/vector.tcc:102:4:   required from ‘void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {const char (&)[13]}; _Tp = Test; _Alloc = std::allocator<Test>]’
main.cpp:32:32:   required from here
main.cpp:14:3: error: ‘Test::Test(Test&&)’ is private
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/vector:63:0,
                 from main.cpp:2:
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_construct.h:77:7: error: within this context
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_construct.h:77:7: error: use of deleted function ‘Test::Test(Test&&)’
main.cpp:14:3: error: declared here

但是 emplace_back 不使用move构造函数.为什么在这种情况下初始化需要移动构造函数?

But the emplace_back doesn't use the move constructor. Why does the initialization require a move constructor in this instance?

推荐答案

问题后面的注释中指定. emplace_back 操作员可能需要重新分配容器内存，因此， vector 模板类型必须是可复制的或可移动构造的.

As specified in the comment after the question. The emplace_back operator may need to reallocate the containers memory and as such the vector template type needs to be either copy or move constructable.

这不是问题的转发，而是新对象的内存分配问题.

It's not the forwarding of the arguments that is the issue it is the allocating of memory for the new object.

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