1. Points to note when transferring unique_ptr to shared_ptr
#include <iostream>
#include <memory>
using namespace std;
std::unique_ptr<std::string> foo()
{
return std::make_unique<std::string>("foo");
}
int main()
{
std::shared_ptr<std::string> sp1 = foo();//发生了移动语义，不是拷贝
printf("%s\n",sp1->c_str());
auto up = std::make_unique<std::string>("Hello World");
std::shared_ptr<std::string> sp2 = std::move(up);
//std::shared_ptr<std::string> sp3 = up; 错误不能直接赋值，需要转移所有权 std::move(up)
if (sp2.unique())//判断是不是只有一个在占用
cout << "only 1 count" << endl;
system("pause");
return 0;
}
2. Circular references and solutions
#include <iostream>
#include <memory>
using namespace std;
struct BClass;
struct AClass
{
shared_ptr<BClass> pb;
~AClass() {
std::cout << "~AClass()\n"; }
};
struct CClass;
struct BClass
{
shared_ptr<AClass> pa;
~BClass() {
std::cout << "~BClass()\n"; }
};
struct CClass
{
shared_ptr<AClass> pa;
~CClass() {
std::cout << "~CClass()\n"; }
};
int main()
{
{
auto a = std::make_shared<AClass>();
auto b = std::make_shared<BClass>();
auto c = std::make_shared<CClass>();
// 循环引用
a->pb = b;//b ref 2
b->pa = a;//a ref 2
c->pa = a;//a ref 3,c ref 1
// c 释放
//c.reset();
std::cout << "计数: " << a.use_count() << "\n";
std::cout << "计数: " << b.use_count() << "\n";
std::cout << "计数: " << c.use_count() << "\n";
}
system("pause");
return 0;
// a, b 仍然相互持有
}
Result:
The solution is as follows (using weak_ptr):
struct AClass
{
weak_ptr<BClass> pb;
~AClass() {
std::cout << "~AClass()\n"; }
};
struct CClass;
struct BClass
{
weak_ptr<AClass> pa;
~BClass() {
std::cout << "~BClass()\n"; }
};
result:
3. Use of weak_ptr
A weak_ptr can be constructed from a shared_ptr or another weak_ptr object to obtain observation rights of the resource. But weak_ptr has no shared resources, and its construction does not cause the pointer reference count to be increased. Use the member function use_count() of weak_ptr to observe the reference count of the resource. The function of another member function expired() is equivalent to use_count()==0, but faster. Indicates that the observed resource (that is, the resource managed by shared_ptr) no longer exists.
#include <iostream>
#include <memory>
using namespace std;
int main() {
{
std::shared_ptr<int> sh_ptr = std::make_shared<int>(10);
std::cout << sh_ptr.use_count() << std::endl; // 输出1
std::weak_ptr<int> wp(sh_ptr);
std::cout << wp.use_count() << std::endl; // 赋值给weak_ptr后还是输出1
if (!wp.expired()) {
// 检查sh_ptr是否还有效
std::shared_ptr<int> sh_ptr2 = wp.lock(); //将sh_ptr赋值给sh_ptr2
*sh_ptr = 100;
std::cout << wp.use_count() << std::endl; // 输出2
}
} //delete memory
std::weak_ptr<int> wp;
{
std::shared_ptr<int> sh_ptr = std::make_shared<int>(10);
wp = sh_ptr;
std::cout << std::boolalpha << wp.expired() << std::endl; // 输出false，引用对象还没删除
} //delete memory
std::cout << std::boolalpha << wp.expired() << std::endl; // 输出true，引用对象已经删除
system("pause");
return 0;
}
result:
Reference link: https://blog.csdn.net/sinat_31608641/article/details/107702175