爲什麼emplace_back需要移動的構造

我有以下代碼：爲什麼emplace_back需要移動的構造

#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"); 
}

當移動構造函數是允許的，會發生以下情況：

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

但是，如果移動構造函數被移除：

[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emplace_back不使用移動構造函數。爲什麼初始化在這個實例中需要移動構造函數？

來源

2013-03-25 Matt Clarkson

用'GCC 4.7.2' – 2013-03-25 13:22:17

也這樣做，你也defing了拷貝構造函數，佈設回需要或者是禁止複製或移動，構造函數。 – inf 2013-03-25 13:24:28

錯誤的，它不應該爲侵入本身。然而，對於再生，'T'需要至少'MoveConstructible'，這是IIRC，也是'std :: vector'的一般要求。 – Xeo 2013-03-25 13:26:59

正如在問題後的評論中指定的那樣。 emplace_back操作員可能需要重新分配容器內存，因此vector模板類型需要是複製或移動可構造的。

這不是參數的轉發問題，它是爲新對象分配內存的問題。

來源

2013-03-25 13:41:24

還記得移動構造函數的'noexcept'，因爲它是不安全的移動多個元素不是原子的。 – 2013-03-26 07:52:59

好點。請記住添加'noexcept' – 2013-03-26 17:26:20

僅供參考，這在C++ 11標準的表101中稱爲「對於向量，T也應該是MoveInsertable into X」。 – 2013-03-26 19:39:02

如果在載體中沒有空間，那麼它應該分配新的空間，有移動的一切，避免了資源的副本中包含或對象擁有的（內部矢量）移動需要

對於類型的矢量Test

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

來源

2013-03-25 13:42:13

爲什麼emplace_back需要移動的構造

回答

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