C++ 为什么emplace_back需要移动构造函数

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");
}

但是，如果删除了移动构造函数：

[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

[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:在“void std:：_construct（_T1*，_Args&&&…[带_T1=Test；_Args={Test}”）的实例化中：
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../include/c++/4.7.2/bits/stl_uninitialized.h:77:3:static_ForwardIterator std:：_uninitialized_copy:_uninit_copy（_inputierator，_inputierator，_ForwardIterator，_ForwardIterator）[带(inputierator=std:：move)移动(迭代器；(转发器=Test*；bool)bool)值类型=false]中需要
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../../../../include/c++/4.7.2/bits/stl_uninitialized.h:119:41:从“_forwarditeratorstd:：uninitialized_copy（_inputierator，_inputierator，_ForwardIterator，_ForwardIterator）[with_inputierator=std:：move_iterator；_ForwardIterator=Test*]中需要”
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../include/c++/4.7.2/bits/stl_uninitialized.h:260:63:从“_forwarditeratorstd:：_uninitialized_copy_a（_inputierator，_inputierator，_ForwardIterator，std:：allocator&]）[带_inputierator=std:：move_迭代器；_ForwardIterator=Test*)
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../../../include/c++/4.7.2/bits/stl_未初始化。h:283:69:从“_forwarditeratorstd:：_uninitialized_move_if_noexcept_a（_InputIterator，_InputIterator，_ForwardIterator，_ForwardIterator，_分配器，)分配器&）[带_InputIterator=Test*)(转发器=Test*)分配器]，)分配器]中必须输入
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../../../include/c++/4.7.2/bits/vector.tcc:410:6:void std:：vector:：_M_emplace_back_aux（_Args&&…[with _Args={const char（&）[13]}；_tptest；_Alloc=std:：allocator
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../../../../include/c++/4.7.2/bits/vector.tcc:102:4:从“void std:：vector:：emplace_back（_Args&&&…）[with _Args={const char（&）[13]}；_tptest；_Alloc=std:：Alloc:：alloctor]中需要
main.cpp:32:32:此处为必填项
main.cpp:14:3:错误：“Test:：Test（Test&&）”是私有的
在/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../../include/c++/4.7.2/vector:63:0中包含的文件中，
来自main.cpp:2：
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../../../include/c++/4.7.2/bits/stl_construct.h:77:7:错误：在此上下文中
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../../../../../include/c++/4.7.2/bits/stl_construct.h:77:7:错误：使用删除的函数“Test:：Test（Test&）”
main.cpp:14:3:错误：在此处声明

---

但是

emplace\u back

不使用移动构造函数。为什么初始化在此实例中需要移动构造函数？

如问题后的注释所述。

emplace\u back

操作员可能需要重新分配容器内存，因此

vector

模板类型需要是可复制或可移动的

---

问题不在于参数的转发，而在于为新对象分配内存。

如果向量中没有空间，那么它应该分配新空间并将所有内容移动到那里，为了避免对象包含或拥有的资源的副本（向量内），需要移动

对于

测试类型的向量

Test object(original)--->resource on heap
Test object(relocated with move constructor)------>resource on heap
Test object(relocated with copy constructor)------>copy of resource on heap

---

如果在

gcc 4.7.2

错误的情况下执行此操作，则炮位本身不需要这两种情况。但是，对于重增长，

T

至少需要是可移动的

MoveConstructible

，这也是

std:：vector

@Xeo的一般要求，有什么不对？重新定位会导致向量重新生长，这需要复制/移动构造函数。因此，重新生长需要复制/移动构造函数。伙计们，请添加这些注释作为答案。当我给出我的答案时，我不想复制这些注释：（还要记住关于移动构造函数的

noexcept

，因为不原子地移动多个元素是不安全的。仅供参考，这在C++11标准的表101中被称为“对于向量，t也应该可以移动插入X”。

Test object(original)--->resource on heap
Test object(relocated with move constructor)------>resource on heap
Test object(relocated with copy constructor)------>copy of resource on heap