先举个栗子

class C {
public:
    C(){
        cout << "none argument constructor called" << endl;
    }
    
    C(int a):i(a) {
        cout << "constructor called" << endl;
    }
    
    C(const C& c) : i(c.i) {
        cout << "copy constructor called" << endl;
    }
    
    C& operator=(const C& c) {
        
        cout << "operator = called" << endl;
        
        if(this == &c){
            return *this;
        }
        
        i = c.i;
        return *this;
    }
    
    ~C(){
       cout << "destructor called" << endl;
    }
    
private:
    int i;
};

int main(){
    C c1(1);
    cout << "=======" << endl;
    
    C c2(c1);
    cout << "=======" << endl;
    
    C c3;
    c3 = c2;
    cout << "=======" << endl;
    
    C c4 = c3;
    
    return 0;
}

执行代码输出

constructor called
=======
copy constructor called
=======
none argument constructor called
operator = called
=======
copy constructor called
destructor called
destructor called
destructor called
destructor called
Program ended with exit code: 0

总结就是：

1. 对象不存在，且没用别的对象来初始化，就调用了构造函数
2. 对象不存在，且用别的对象来初始化，就是拷贝构造函数
3. 对象存在，用别的对象来给它赋值，就是调用赋值函数