对std::move的一点理解

#include <iostream>
#include <utility>
#include <vector>
#include <string>
int main()
{
    std::string str = "Hello";
    std::vector<std::string> v;
    //调用常规的拷贝构造函数，新建字符数组，拷贝数据
    v.push_back(str);
    std::cout << "After copy, str is \"" << str << "\"\n";
    //调用移动构造函数，掏空str，掏空后，最好不要使用str
    v.push_back(std::move(str));
    std::cout << "After move, str is \"" << str << "\"\n";
    std::cout << "The contents of the vector are \"" << v[0]
                                         << "\", \"" << v[1] << "\"\n";
}

输出：

After copy, str is "Hello"

After move, str is ""

The contents of the vector are "Hello", "Hello"

std::move(str)将str从左值转为右值引用，然后在push_back的时候，根据类型是右值引用，因此会调用string类的移动构造函数，从而大大提升了效率。

代码地址：blog.csdn.net/p942005405/…

class MyString {
public:
    MyString(){
        std::cout << "default constructor." << std::endl;
    }
    MyString(const MyString& rhs) {
        std::cout << "copy constructor." << std::endl;
    }
    MyString& operator=(const MyString& rhs) {
        std::cout << "assignment constructor." << std::endl;
    }
    MyString(MyString&& rhs) {
        std::cout << "move constructor" << std::endl;
    }

private:
    char* data_;
    int32_t data_size_;
};

MyString func_mystr()
{
    MyString x;
    //return x;
    return std::move(x);
}

int main()
{
    // 0. std::move
    MyString a;
    MyString c;
    MyString d = c; // copy constructor
    MyString b = std::move(a); // move constructor
    std::cout << "###############" << std::endl;
    MyString e = func_mystr();
}

输出：

default constructor.
default constructor.
copy constructor.
move constructor
###############
default constructor.
move constructor

而如果func_mystr()里写成return x，则输出为：

###############
default constructor.

具体解释参考：thbecker.net/articles/rv…
简单说就是编译器会自动优化，用了std::move，反而使得编译器不好优化了.