C++ 拷贝构造和赋值构造示例

一开始，对这些拷贝构造，赋值构造也是很迷，然后就把这些常见的总结到一起，进行对比。希望对各位有些帮助。

#include <iostream>
using namespace std;
class A
{
  public:
    A() { cout << "A default constructor" << endl; }

    A(const A &obj) { cout << "A copy constructor" << endl; }

    ~A() { cout << "A destroy" << endl; }

    A &operator=(const A &s)
    {
        if (this == &s)
            return *this;
        cout << "operator left = " << endl;
        return *this;
    }
    A &operator=(A &&s)
    {
        cout << "operator right = " << endl;
        return *this;
    }
};
class B
{
  public:
    B()
    {
        cout << "B construcor" << endl;
    }
    A btemp;
    A get() { return btemp; }
};
A fun()
{
    A temp;
    return temp;
}

int main()
{

    cout << "--------------part 1 ------------" << endl;
    A a1;
    a1 = fun(); //  1.对temp对象进行拷贝一个t1拷贝完成后 2.析构temp,然后调用a1的 "=" 运算符，t1作为参数传入
                //  gcc中是不这样干的,不拷贝生成一个t1，直接调用a1的 "=" 运算符，用temp做参数
    cout << "--------------part 2 ------------" << endl;
    A a2 = fun(); //微软的编译器:1.直接用fun()里面的temp,调用拷贝构造函数创建a2 2.a2生成后才析构fun()里面的temp
                  //gcc中不是这样干的，直接把fun()里面的temp作用域提升，扶正成为a2,两个内存地址是一样的
    cout << "--------------part 3 ------------" << endl;
    cout << "--" << endl;
    B b;
    cout << "--" << endl;
    A a3 = b.get(); // 1.直接用b.get()里面的btemp对象,进行copy构造一个a3，和上面a2一样。
    cout << "--------------part 4 ------------" << endl;
    A a4;
    a4 = b.get(); // 1.通过b.get()里面的btemp对象,copy构造一个对象t,3.构造然后调用 "=" 运算符 3.调用完再析构这个t
    cout << "--------------part 5 ------------" << endl;
    a3 = a4; // 直接调用a3的 = 运算符
    cout << "--------------part 6 ------------" << endl;
    A a5;
    a5 = A(); // 1.创建匿名对象 2.调用 "=" 运算符 3.再析构匿名对象
    cout << "--------------part 7 ------------" << endl;
    A a6 = A(); // 等价 A a6();
}

gcc 和微软家的编译器有些不同，上面代码注释有说明。

gcc环境下输出：

--------------part 1 ------------
A default constructor
A default constructor
operator right = 
A destroy
--------------part 2 ------------
A default constructor
--------------part 3 ------------
--
A default constructor
B construcor
--
A copy constructor
--------------part 4 ------------
A default constructor
A copy constructor
operator right = 
A destroy
--------------part 5 ------------
operator left = 
--------------part 6 ------------
A default constructor
A default constructor
operator right = 
A destroy
--------------part 7 ------------
A default constructor
A destroy
A destroy
A destroy
A destroy
A destroy
A destroy
A destroy

visual studio 输出：

--------------part 1 ------------
A default constructor
A default constructor
A copy constructor
A destroy
operator right =
A destroy
--------------part 2 ------------
A default constructor
A copy constructor
A destroy
--------------part 3 ------------
--
A default constructor
B construcor
--
A copy constructor
--------------part 4 ------------
A default constructor
A copy constructor
operator right =
A destroy
--------------part 5 ------------
operator left =
--------------part 6 ------------
A default constructor
A default constructor
operator right =
A destroy
--------------part 7 ------------
A default constructor
A destroy
A destroy
A destroy
A destroy
A destroy
A destroy
A destroy

对比，左边是gcc右边是vs：