shared_ptr 如何在按值传递时增加计数器？

Question

我在下面有这个示例代码。我对RVO（return值优化）知之甚少，在优化过程中如何跳过复制构造函数和赋值运算符，并将值的return直接放在左侧的内存中。那么，如果共享指针执行 RVO，共享指针如何知道何时增加其计数器？因为出于某种原因，我认为共享指针 class 会根据它所做的副本数或分配知道何时增加计数器。

#include <iostream>
#include <memory>
using namespace std;
class A{
public:
    A(){}
    A(const A& other){ std::cout << " Copy Constructor " << std::endl; }
    A& operator=(const A&other){
        std::cout << "Assingment operator " <<  std::endl;        
        return *this;
    }    
    ~A(){
        std::cout << "~A" <<  std::endl;
    } 
};

std::shared_ptr<A> give_me_A(){
    std::shared_ptr<A> sp(new A);
    return sp;
}

void pass_shared_ptr_by_val(std::shared_ptr<A> sp){

    std::cout << __func__ << ": count  sp = " << sp.use_count() << std::endl;
    std::shared_ptr<A> sp1 = sp;
    std::cout << __func__ << ": count  sp = " << sp.use_count() << std::endl;
    std::cout << __func__ << ": count sp1 = " << sp1.use_count() << std::endl;
}

void pass_shared_ptr_by_ref(std::shared_ptr<A>& sp){
    std::cout << __func__ << ": count  sp = " << sp.use_count() << std::endl;  
    std::shared_ptr<A> sp1 = sp;
    std::cout << __func__ << ": count  sp = " << sp.use_count() << std::endl;
    std::cout << __func__ << ": count sp1 = " << sp1.use_count() << std::endl;
}

int main(){

    {
        shared_ptr<A> sp3 = give_me_A();

        std::cout << "sp3 count = " << sp3.use_count() << std::endl;
        pass_shared_ptr_by_val(sp3);
        pass_shared_ptr_by_ref(sp3);
    }
return 0;
}

输出：

sp3 count = 1

pass_shared_ptr_by_val: count sp = 2

pass_shared_ptr_by_val: count sp = 3

pass_shared_ptr_by_val: count sp1 = 3

pass_shared_ptr_by_ref: count sp = 1

pass_shared_ptr_by_ref: count sp = 2

pass_shared_ptr_by_ref: count sp1 = 2

~A

Answer 1

如果没有副本，则无需统计

如果 RVO 正在运行，则不会进行任何复制，那么为什么需要增加引用计数？没有额外的对象来销毁和减少引用计数。

shared_ptr 如何在按值传递时增加计数器？

How does shared_ptr increase counter when passed by value?

c++

shared-ptr

smart-pointers

rvo