c++ - emplace_back 调用移动构造函数和析构函数

Question

我尝试将类 cBar 的两个实例放置到具有 emplace_back 函数的 vector 中。

根据reference调用 emplace_back 仅保留 vector 中的位置，然后“就地”创建新实例。

现在，我试着用它做实验:

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


class cBar
{
public:

    cBar(const int index);
    cBar(cBar&& other); //needed for emplace_back?
    ~cBar();

private:
    cBar(const cBar& other) = delete;
    cBar& operator=(const cBar& other) = delete;

public:
    int                         mIndex;

};




cBar::cBar(const int index) : mIndex(index)
{
    std::cout << "cBar being created ..." << std::endl;
}
cBar::cBar(cBar&& other) : mIndex(other.mIndex)
{
    std::cout << "cBar being moved ..." << std::endl;
}
cBar::~cBar()
{
    std::cout << "cBar being destroyed ..." << std::endl;
}

int main()
{
    std::vector<cBar> mBars;

    std::cout << "Begin to create 2 cBar instance, I am expecting 2 \"cBar being created ...\" messages here" << std::endl;
    mBars.emplace_back(0);//this supposed to call only cBar(const int index) constructor, and nothing more
    mBars.emplace_back(1);//this works as expected, only one constructor call

    //destroy all
    std::cout << "Destroy the 2 isntances (this works, I see the two expected \"cBar being destroyed ...\" messages here" << std::endl;
    mBars.clear();

    std::cin.get();
    return 0;
}

输出:

Begin to create 2 cBar instance, I am expecting 2 "cBar being created ..." messages here
cBar being created ...
cBar being moved ...
cBar being destroyed ...
cBar being created ...
Destroy the 2 isntances (this works, I see the two expected "cBar being destroyed ..." messages here
cBar being destroyed ...
cBar being destroyed ...

如果你运行上面的那个，你会看到第一个 emplace_back “就地”创建实例，但随后立即调用移动构造函数，然后调用析构函数。

更奇怪的是，在第二个位置的情况下，我看到了预期的行为:只有一个构造函数调用。

我有两个问题:

1. 如果我只想 emplace_back 项目而不使用 push_back，为什么我需要在我的类中定义移动构造函数。

2. 在第一个实例创建的情况下，为什么移动构造函数，然后调用析构函数？如果我访问第一个实例的数据，一切似乎都是正确的，所以我不知道为什么调用移动构造函数和析构函数。

我使用 Visual Studio 2015。

每一步输出 vector 大小:

Begin to create 2 cBar instance, I am expecting 2 "cBar being created ..." messages here
Vector size:0
cBar being created ...
Vector size:1
cBar being moved ...
cBar being destroyed ...
cBar being created ...
Vector size:2
Destroy the 2 isntances (this works, I see the two expected "cBar being destroyed ..." messages here
cBar being destroyed ...
cBar being destroyed ...

Answer 1

2.In case of the first instance creation, why the move constructor, then the destructor is called?

因为 emplace_back 插入了第二个元素导致重新分配； vector 的内部存储需要扩展，旧存储中的元素必须复制/移动到新存储，然后销毁。

您可以使用 reserve提前避免重新分配。

1.Why do I need to define a move constructor in my class if I just want to emplace_back items, and never use push_back.

正如上面的解释所说，vector需要通过复制/移动操作来移动元素。所以你必须为类定义复制或移动构造函数。 emplace_back 和 push_back 都是如此，因为它们都将元素添加到 vector 并可能导致重新分配。