插入具有没有复制构造函数的对象的向量 [英] Insert into vector having objects without copy constructor

问题描述

我有一个类的拷贝构造函数被显式删除(因为 A 在内部使用指针，我不想陷入浅拷贝陷阱):

class A {上市:A(const A&) = 删除；A&运算符=(const A&) = 删除；A(const B& b, const C& c);}

现在我有一个 vector 类型的向量.aVector; 并且我想在其中插入元素 - 所以我使用 emplace_back:

aVector.emplace_back(b, c);

但是，这无法使用 gcc 进行编译，并且出现错误 -

third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_construct.h:在'的实例化中void std::_Construct(_T1*, _Args&&...)第三方/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:77:3: 需要来自'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator)第三方/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:119:41:来自'_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator)第三方/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:260:63: 需要来自'_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&)第三方/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:283:67:来自'_ForwardIterator std::__uninitialized_move_if_noexcept_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&)第三方/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/vector.tcc:410:6: 'void std 要求::vector<_Tp, _Alloc>::_M_emplace_back_aux(_Args&& ...)第三方/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/vector.tcc:102:4: 'void std: 要求::vector<_Tp, _Alloc>::emplace_back(_Args&& ...)

这个错误的原因是什么，如何在不删除复制构造函数的情况下修复它?我是否需要移动构造函数 - 是否需要明确定义?

解决方案

您应该添加移动构造函数 - 因为 std::vector::emplace_back 可能会执行需要复制/移动构造函数的重定位.或者只是使用 std::deque.

现场演示

#include <向量>#include <deque>使用命名空间标准；结构 NoCopyNoMove{NoCopyNoMove(const NoCopyNoMove&) = 删除；NoCopyNoMove&运算符=(const NoCopyNoMove&)=删除；NoCopyNoMove(NoCopyNoMove&&) = 删除；NoCopyNoMove&运算符=(NoCopyNoMove&&)=删除；NoCopyNoMove(int){};};结构仅移动{OnlyMove(const OnlyMove&) = delete;OnlyMove&运算符=(const OnlyMove&)=删除；OnlyMove(OnlyMove&&) noexcept {}OnlyMove&operator=(OnlyMove&&) noexcept {}OnlyMove(int){};};int main(){双端队列X;x.emplace_back(1);矢量y;y.emplace_back(1);}

<小时><块引用>

§ 23.2.3 表 101 — 可选的序列容器操作

a.emplace_back(args) [...]

要求:T 应该是 EmplaceConstructible 从 args 到 X 的.对于vector，T 也应该是MoveInsertable 到 X.

I have a class whose copy constructors are explicitly deleted (because A uses pointers internally and I don't want to fall into shallow copy pitfalls):

class A {
  public:
    A(const A&) = delete;
    A& operator=(const A&) = delete;

    A(const B& b, const C& c);
}

Now I have a vector of type vector<A> aVector; and I want to insert elements into it - so I use emplace_back:

aVector.emplace_back(b, c);

However, this fails to compile using gcc and I get the error -

third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...)
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:77:3:   required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator)  
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:119:41:   required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) 
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:260:63:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) 
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/stl_uninitialized.h:283:67:   required from '_ForwardIterator std::__uninitialized_move_if_noexcept_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&)
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/vector.tcc:410:6:   required from 'void std::vector<_Tp, _Alloc>::_M_emplace_back_aux(_Args&& ...) 
third-party/gcc-4.7.1-glibc-2.14.1/libgcc/libgcc-4.7.1/afc21dc/include/c++/4.7.1/bits/vector.tcc:102:4:   required from 'void std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...)

What is the reason for this error and how can it be fixed without removing the deletion of the copy constructors? Do I need a move constructor - does it need to be explicitly defined?

解决方案

You should add move constructor - because std::vector::emplace_back may do relocation which requires copy/move constructor. Or just use std::deque.

LIVE DEMO

#include <vector>
#include <deque>
using namespace std;

struct NoCopyNoMove
{
    NoCopyNoMove(const NoCopyNoMove&) = delete;
    NoCopyNoMove& operator=(const NoCopyNoMove&) = delete;
    NoCopyNoMove(NoCopyNoMove&&) = delete;
    NoCopyNoMove& operator=(NoCopyNoMove&&) = delete;

    NoCopyNoMove(int){};
};

struct OnlyMove
{
    OnlyMove(const OnlyMove&) = delete;
    OnlyMove& operator=(const OnlyMove&) = delete;
    OnlyMove(OnlyMove&&) noexcept {}
    OnlyMove& operator=(OnlyMove&&) noexcept {}

    OnlyMove(int){};
};

int main()
{
    deque<NoCopyNoMove> x;
    x.emplace_back(1);

    vector<OnlyMove> y;
    y.emplace_back(1);
}

---

§ 23.2.3 Table 101 — Optional sequence container operations

a.emplace_back(args) [...]

Requires: T shall be EmplaceConstructible into X from args. For vector, T shall also be MoveInsertable into X.

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