C++ 右值

什么是左值，右值？一个形象的表述：出现在“=”左边的就是左值，出现在“=”右边的就是右值。形象，但不严谨。
在C++中被广泛认同的:

左值：可以取地址，有名字的；
右值： 不可以取地址，没有名字的。

C++ 11中的右值由两个概念组成：将亡值（expire value），纯右值（pure Rvalue）:

将亡值
C++11新增加跟右值引用相关的表达式。这样的表达式通常是将要被移动的对象。例如：返回右值引用T&& 的函数返回值，std::move()的返回值，转换为T&&的类型转换函数的返回值。

纯右值
临时变量和一些跟对象不相关的值。例如：函数返回的非引用的临时变量；一些运算表达式（1+2）产生临时变量值；跟对象不相关的字面值：2， true；此外类型转换函数返回值，lambda表达式也是纯右值。

C++ 容器处理左值右值

C++ 的容器会根据传入的参数类型，进行左值右值的处理。

从效率上面看 emplace(123) > push_back(move(Bjarne(123)) > push_back(Bjarne(123))

测试代码如下：

#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <shared_mutex>
#include <string>
#include <memory>
#include <vector>


using namespace std;

class Bjarne
{
public:
    Bjarne(int num) :data(new int(num))
    { 
        std::cout << "Construct "  << std::endl; 
    }

    Bjarne(const Bjarne &bjarne) :data(new int(*bjarne.data))
    {
        std::cout << "Copy Construct " << std::endl;
    }

    Bjarne(Bjarne &&bjarne):data(bjarne.data)
    {
        bjarne.data = nullptr;
        std::cout << "Move Construct " << std::endl;
    }

    int getValue() {
        return *data;
    }

    ~Bjarne() {
        delete data; 
        std::cout << "Destruct "  << std::endl;
    }
    int *data;
};

Bjarne getBjarne(){
    return Bjarne(1);
}


int main(int argc, char*argv[])
{
    // emplace(1,2,3) > insert(move(AAA(1,2,3)) > insert(AAA(1,2,3))

    {
        std::cout << "1.0 push_back receive lValue will call Copy Constructor.  " << std::endl;
        std::cout << " " << std::endl;

        std::vector<Bjarne> jackyVec;
        Bjarne bjarne(10);
        jackyVec.push_back(bjarne);

        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }

    {
        std::cout << " " << std::endl;
        std::cout << "1.1 emplace_back receive lValue will call Copy Constructor.  " << std::endl;
        std::cout << " " << std::endl;

        std::vector<Bjarne> jackyVec;
        Bjarne bjarne(10);
        jackyVec.emplace_back(bjarne);

        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }

    {
        std::cout << " " << std::endl;
        std::cout << "1.2 emplace_back receive rValue will call Move Constructor.  " << std::endl;
        std::cout << " " << std::endl;

        std::vector<Bjarne> jackyVec;
        Bjarne bjarne(10);
        jackyVec.emplace_back(std::move(bjarne));

        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }

    {
        std::cout << " " << std::endl;
        std::cout << "1.3 push_back receive rValue will call Move Constructor.  " << std::endl;
        std::cout << " " << std::endl;

        std::vector<Bjarne> jackyVec;
        Bjarne bjarne(10);
        jackyVec.push_back(std::move(bjarne));

        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }

    {
        std::cout << " " << std::endl;
        std::cout << "2.0 Construct object in parameter treated as rValue. push_back" << std::endl;
        std::cout << " " << std::endl;

        std::vector<Bjarne> jackyVec;
        jackyVec.push_back(Bjarne(21));
        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }

    {
        std::cout << "2.1 Construct object in parameter treated as rValue. emplace_back" << std::endl;
        std::cout << " " << std::endl;
        std::vector<Bjarne> jackyVec;

        jackyVec.emplace_back(Bjarne(21));
        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }

    {
        std::cout << " " << std::endl;
        std::cout << "3.0 emplace_back in-place call Constructor  .  " << std::endl;
        std::vector<Bjarne> jackyVec;
        std::cout << " " << std::endl;
        jackyVec.emplace_back(20);   //Bjarne(int bjarne)
        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }

    {
        std::cout << " " << std::endl;
        std::cout << "4.0 Return value treated as rValue.  emplace_back " << std::endl;
        std::vector<Bjarne> jackyVec;
        std::cout << " " << std::endl;
        jackyVec.emplace_back(getBjarne());
        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }

    {
        std::cout << " " << std::endl;
        std::cout << "4.1 Return value treated as rValue.  push_back " << std::endl;
        std::vector<Bjarne> jackyVec;
        std::cout << " " << std::endl;
        jackyVec.push_back(getBjarne());
        std::cout << " " << std::endl;
        std::cout << " " << std::endl;
    }


    return 0;
}

测试结果：

1.0 push_back receive lValue will call Copy Constructor.  
 
Construct 
Copy Construct 
 
 
Destruct 
Destruct 
 
1.1 emplace_back receive lValue will call Copy Constructor.  
 
Construct 
Copy Construct 
 
 
Destruct 
Destruct 
 
1.2 emplace_back receive rValue will call Move Constructor.  
 
Construct 
Move Construct 
 
 
Destruct 
Destruct 
 
1.3 push_back receive rValue will call Move Constructor.  
 
Construct 
Move Construct 
 
 
Destruct 
Destruct 
 
2.0 Construct object in parameter treated as rValue. push_back
 
Construct 
Move Construct 
Destruct 
 
 
Destruct 
2.1 Construct object in parameter treated as rValue. emplace_back
 
Construct 
Move Construct 
Destruct 
 
 
Destruct 
 
3.0 emplace_back in-place call Constructor  .  
 
Construct 
 
 
Destruct 
 
4.0 Return value treated as rValue.  emplace_back 
 
Construct 
Move Construct 
Destruct 
 
 
Destruct 
 
4.1 Return value treated as rValue.  push_back 
 
Construct 
Move Construct 
Destruct 
 
 
Destruct 

总结

1. emplace() 只有在传入对象的构造参数的时候才会进行“就地构造”。
2. emplace() 和push_back()在传入右值的时候，会调用移动构造，效率比拷贝构造更高一些。
3. emplace() 和push_back()在传入左值的时候，会调用拷贝构造。

写在最后： 这个实验，纠正了我之前的一些错误理解。

引用： (2条消息) C++拷贝构造、移动拷贝构造、左值、右值码灵薯的博客-CSDN博客移动拷贝