我在下面有这个示例代码。我对 RVO(返回值优化)以及如何在优化过程中跳过复制构造函数和赋值运算符以及将值的返回直接放在左侧的内存中了解一点。那么如果共享指针是 RVO,共享指针如何知道何时增加它的计数器呢?因为出于某种原因,我认为共享指针类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 计数 = 1
pass_shared_ptr_by_val:计数 sp = 2
pass_shared_ptr_by_val:计数 sp = 3
pass_shared_ptr_by_val:计数 sp1 = 3
pass_shared_ptr_by_ref:计数 sp = 1
pass_shared_ptr_by_ref:计数 sp = 2
pass_shared_ptr_by_ref:计数 sp1 = 2
〜一个