c++ - 为什么 boost::make_shared 使用复制语义

Question

我有一个小程序，我无法理解它的工作原理。

class A {
     public:
     A() {
        cout<<"Costructor called"<<endl;
    }

    ~A() {
        cout<<"Destructor called"<<endl;
    }
};

A foo() {
    return A();
}


int main() {
    A obj = foo();
    cout<<"A initialized "<<endl;
    boost::shared_ptr<A> ptr1 = boost::make_shared<A>(foo());
    cout<<"ptr1 initialized "<<endl;
return 0;
}

输出是:-

./a.out
Costructor called
A initialized 
Costructor called
Destructor called
ptr1 initialized 
Destructor called
Destructor called

为什么 A 在创建 ptr 时被初始化和销毁​​，而在创建 obj 时却没有？

Answer 1

让我们通过添加一个复制构造函数并在每个特殊成员中打印 this 让您的类更嘈杂

class A {
     public:
     A() {
        cout<<"Constructor called "<< this << endl;
    }

    A(A const&) {
        cout<<"Copy Constructor called "<< this << endl;
    }

    ~A() {
        cout<<"Destructor called "<< this << endl;
    }
};

Live demo

这会产生输出

Constructor called 0x7fff0ed4883e   // obj is constructed
A initialized 
Constructor called 0x7fff0ed4883f   // return value of foo() is constructed in make_shared call
Copy Constructor called 0x1430c39   // shared_ptr copy constructs A from return value of foo()
Destructor called 0x7fff0ed4883f    // return value of foo() is destroyed
ptr1 initialized 
Destructor called 0x1430c39         // shared_ptr deletes the A it owns
Destructor called 0x7fff0ed4883e    // obj is destroyed