我尝试使用emplace_back函数将两个cBar类的实例添加到一个向量中。
根据参考文献,调用emplace_back只是在向量中预留位置,然后“就地”创建新实例。
现在,我想进行一些实验:
根据参考文献,调用emplace_back只是在向量中预留位置,然后“就地”创建新实例。
现在,我想进行一些实验:
#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 ...
如果您运行上面的代码,您将看到第一个emplace_back在“原地”创建实例,然后立即调用移动构造函数,之后会调用析构函数。
更奇怪的是,在第二个emplace的情况下,我看到了期望的行为:只有一个构造函数调用。
我有两个问题:
如果我只想emplace_back项目,并且从未使用push_back,为什么需要在我的类中定义移动构造函数?
在第一个实例创建的情况下,为什么会调用移动构造函数,然后调用析构函数?如果我访问第一个实例的数据,所有的东西都看起来正确,所以我不知道为什么会调用移动构造函数和析构函数。
我使用Visual Studio 2015。
每一步向量尺寸的输出:
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 ...
mBars.reserve(9);
。 - Kerrek SB