如何在不调用复制构造函数的情况下向向量添加元素？

Question

出于某种原因，我的临时本地对象在添加到向量时总是 copy-constructed/destroyed，由于嵌套 std::reference_wrapper 而导致问题，嵌套 std::reference_wrapper 由于复制构造和之后的破坏而变得无效（ std::reference_wrapper 目标位于被销毁的对象内部 -> 因此如果源对象被销毁，它们在复制构造的对象中无效）。但如果可能的话，我想完全避免额外的复制/销毁——这似乎是不可能的，因为无论我尝试什么，它总是想要调用复制构造函数（即使使用 std::vector::emplace_back）。

考虑到这个简单的示例（为了在不涉及 std::reference_wrapper 的情况下更容易理解），它总是尝试调用复制构造函数 - 我不明白为什么。

#include <vector>

class A{
public:
  A(int a) : a(a){ }
  int getInt() const{ return a; }
  A(const A&) = delete; /* to deny copy-construction */
private:
  int a;
};

int main(int argc, char* argv[]){
  std::vector<A> vec;
  vec.emplace_back(3);              /* tries to call copy constructor */
  vec.push_back(A(3));              /* tries to call copy constructor */
  vec.push_back(std::move(A(3)));   /* tries to call copy constructor */
  return 0;
}

知道我在这里遗漏了什么吗？

Answer 1

根据 Visual Studio 2013 documentation，强调我的：

"Rvalue references v3.0" adds new rules to automatically generate move constructors and move assignment operators under certain conditions. However, this is not implemented in Visual C++ in Visual Studio 2013, due to time and resource constraints.

Visual Studio 2013 指定为使用右值引用 v2.1.

注：如T.C. notes in the comments there is also an issue with explicitly disabling the copy constructor in your example. Per cppreference.com.

If no user-defined move constructors are provided for a class type (struct, class, or union), and all of the following is true:

there are no user-declared copy constructors

there are no user-declared copy assignment operators

there are no user-declared move assignment operators

there are no user-declared destructors

(until C++14) the implicitly-declared move constructor is not defined as deleted due to conditions detailed in the next section

then the compiler will declare a move constructor as an inline public member of its class with the signature T::T(T&&).

A class can have multiple move constructors, e.g. both T::T(const T&&) and T::T(T&&). If some user-defined move constructors are present, the user may still force the generation of the implicitly declared move constructor with the keyword default.

这意味着您的示例代码还会阻止自动生成移动构造函数（即，它具有用户声明的复制构造函数）。

您需要显式声明您的移动构造函数 and/or 移动赋值运算符。以下适用于您的示例。

class A
{
public:
    A(int a) : a(a) {}

    A(const A&) = delete;

    A(A&& other) : a(other.a) {}

    int getInt() const { return a; }

private:
    int a;
};

int main(int argc, char* argv[])
{
    std::vector<A> vec;
    vec.emplace_back(3);
    vec.push_back(A(3));
    vec.push_back(std::move(A(3)));

    return 0;
}

如何在不调用复制构造函数的情况下向向量添加元素？

How to add elements to vector without invoking copy-constructor?

c++

vector

copy-constructor

move-semantics

c++11