目录
1. 几种智能指针
1. auto_ptr: c++11中推荐不使用他
2. shared_ptr: 每添加一次引用 就+1,减少一次引用,就-1;做到指针进行共享
3. unique_ptr: 一个指针同时只能有一个使用者使用
4. weaked_ptr: 与shared_ptr搭配使用
1.1 shared_ptr
参考:https://zh.cppreference.com/w/cpp/memory/shared_ptr
std::shared_ptr
是通过指针保持对象共享所有权的智能指针。多个 shared_ptr
对象可占有同一对象。下列情况之一出现时销毁对象并解分配其内存:
#include <iostream>
#include <memory>
#include <thread>
#include <chrono>
#include <mutex>
class Object
{
public:
Object(int id) : m_id(id) {
std::cout << "init obj " << m_id << std::endl;
}
~Object() {
std::cout << "bye bye " << m_id << std::endl;
}
int id() const {
return m_id;
}
private:
int m_id;
};
typedef std::shared_ptr<Object> ObjectPtr;
void print(ObjectPtr obj);
void printRef(const ObjectPtr& obj);
void interfaceOfSharedPtr()
{
ObjectPtr null;
std::cout << "ref count is " << null.use_count() << std::endl; // 0
ObjectPtr obj(new Object(1));
std::cout << "ref count is " << obj.use_count() << std::endl; // 1
ObjectPtr obj2(obj);
std::cout << "ref count is " << obj.use_count() << std::endl; // 2
ObjectPtr obj3 = obj;
std::cout << "ref count is " << obj.use_count() << std::endl; // 3
obj2.reset(); // 不需要了
//obj2 = nullptr; // c++11写法
std::cout << "ref count is " << obj.use_count() << std::endl; // 2
ObjectPtr obj4;
obj3.swap(obj4);
//std::swap(obj3, obj4); // 本身管理资源,引用数没有变化
std::cout << "ref count is " << obj.use_count() << std::endl; // 2
auto p = obj.get();
if (p)
{
std::cout << "id is " << p->id() << std::endl;
}
if (obj)
{
std::cout << "p id is " << obj->id() << std::endl; // operator ->
std::cout << "ref id is " << (*obj).id() << std::endl; // operator *
}
obj4 = nullptr;
// if (obj.unique()) //效率更高
// if (obj.use_count() == 1) // 会有效率损失
std::cout << "only one hold ptr " << obj.unique() << std::endl;
// 值传入
print(obj);
std::cout << "ref count is " << obj.use_count() << std::endl;
// 引用传入, 推荐
printRef(obj);
}
void print(ObjectPtr obj) // 值传入,会临时改变引用数量+1
{
std::cout << "count " << obj.use_count() << " id " << obj->id()
<< std::endl; // 3
}
void printRef(const ObjectPtr& obj) // 引用传递不会改变引用数量
{
std::cout << "ref count " << obj.use_count() << " id " << obj->id()
<< std::endl; // 2
}
void deleteOfObject(Object* obj)
{
if (obj) {
std::cout << "delete obj " << obj->id() << std::endl;
delete obj;
}
}
void useDeleter()
{
// 管理 裸指针 和 处理这个指针的函数 -- 可以管理应用delete资源,也可以管理其他类似资源
ObjectPtr obj(new Object(2), deleteOfObject);
ObjectPtr obj2 = obj;
}
int main()
{
interfaceOfSharedPtr();
return 0;
}
ref count is 0
init obj 1
ref count is 1
ref count is 2
ref count is 3
ref count is 2
ref count is 2
id is 1
p id is 1
ref id is 1
only one hold ptr 1
count 2 id 1
ref count is 1
ref count 1 id 1
bye bye 1