c++ shared_ptr 和 左右值 关系记录

Posted 2021-06-18 村雨sup

#include <iostream>
#include <deque>
using namespace std;


template<class T>
class my_shared_ptr
{
private:
    T* m_ptr = nullptr;
    unsigned int* m_ref_count = nullptr;
public:
    my_shared_ptr():m_ptr(nullptr),m_ref_count(nullptr){}
    my_shared_ptr(T* ptr):m_ptr(ptr),m_ref_count(new unsigned int(1)){}

    //复制语义
    my_shared_ptr(const my_shared_ptr& obj) {
        m_ptr = obj.m_ptr;
        m_ref_count = obj.m_ref_count;
        if (m_ref_count != nullptr) {
            (*m_ref_count)++;
        }
    }

    my_shared_ptr& operator=(const my_shared_ptr& obj) {
        delete m_ptr;
        m_ptr = obj.m_ptr;
        delete m_ref_count;
        m_ref_count = obj.m_ref_count;

        if (m_ref_count != nullptr) {
            (*m_ref_count)++;
        }
        return *this;
    }
    //移动语义
    my_shared_ptr(my_shared_ptr&& dying_obj) :m_ptr(nullptr), m_ref_count(nullptr) {//my_shared_ptr<A> move_ctor_ptr(std::move(a_ptr));
        dying_obj.swap(*this);
    }

    my_shared_ptr& operator=(my_shared_ptr&& dying_obj) {//move_assign_ptr = std::move(b_ptr);
        my_shared_ptr(std::move(dying_obj)).swap(*this);
        return *this;
    }


    void swap(my_shared_ptr& other) {
        std::swap(m_ptr, other.m_ptr);
        std::swap(m_ref_count, other.m_ref_count);
    }

    unsigned int use_count() const {
        return  m_ref_count == nullptr? 0:*m_ref_count;
    }

    //析构函数
    ~my_shared_ptr() {
        if (m_ref_count == nullptr) return;
        (*m_ref_count)--;
        if (*m_ref_count > 0) return;
        //如果引用计数到了0，则删除指针
        if (m_ptr != nullptr) delete m_ptr;

        delete m_ref_count;
    }

};


struct A
{
    std::string m_str;
    A(std::string s) :m_str(s) {
        cout << "construct" << endl;
    }

    ~A() {
        cout << "destruct" << endl;
    }
};







int main()
{
    my_shared_ptr<A> empty_ptr;
    cout << "empty:" << empty_ptr.use_count() << endl;
    my_shared_ptr<A> a_ptr(new A("a"));
    cout << "a:" << a_ptr.use_count() << endl;
    my_shared_ptr<A> copied_ptr(a_ptr);
    cout << "copy:" << copied_ptr.use_count() << endl;
    cout << "a:" << a_ptr.use_count() << endl;

    my_shared_ptr<A> move_ptr(std::move(a_ptr));
    cout << "move:" << move_ptr.use_count() << endl;
    cout << "a:" << a_ptr.use_count() << endl;

    



    return 0;
}

————————————

左值复制

void set(const string & var1, const string & var2){
  m_var1 = var1;  //copy
  m_var2 = var2;  //copy
}
A a1;
string var1("string1");
string var2("string2");
a1.set(var1, var2); // OK to copy

————————————

右值（临时变量）移动

void set(string && var1, string && var2){
  //avoid unnecessary copy!
  m_var1 = std::move(var1);  
  m_var2 = std::move(var2);
}
A a1;
//temporary, move! no copy!
a1.set("temporary str1","temporary str2");

——————————————

std::forword 两者集大成，用模板，当左值时复制，右值时移动

template<typename T1, typename T2>
void set(T1 && var1, T2 && var2){
  m_var1 = std::forward<T1>(var1);
  m_var2 = std::forward<T2>(var2);
}

//when var1 is an rvalue, std::forward<T1> equals to static_cast<[const] T1 &&>(var1)
//when var1 is an lvalue, std::forward<T1> equals to static_cast<[const] T1 &>(var1)