7.shared_ptr自定义Deleter

shared_ptr允许自定义一个Deleter

#include <iostream>
#include <memory>
using namespace std;
class T {
private:
    static int id;
    int tid;
public:
    T() {
        tid = id++;
        cout << "id = " << tid << " constructor called" << endl;
    }
    ~T() {
        cout << "id = " << tid << " destructor called" << endl;
    }
};
int T::id = 1;
void delete_T(T *){
    cout << "delete T" <<endl;
}
int main() {
    cout << "--------begin-----------" << endl;
    {
        shared_ptr<T> t1(new T,delete_T);
    }
    cout << "--------end-----------" << endl;
    return 0;
}

在 delete_T 中没有使用delete，t1的析构函数没有调用。

正确做法

#include <iostream>
#include <memory>
using namespace std;
class T {
private:
    static int id;
    int tid;
public:
    T() {
        tid = id++;
        cout << "id = " << tid << " constructor called" << endl;
    }
    ~T() {
        cout << "id = " << tid << " destructor called" << endl;
    }
};
int T::id = 1;
void delete_T(T *t){
    cout << "delete T" <<endl;
    delete t;    // 在deleter中要手动delete
}
int main() {
    cout << "--------begin-----------" << endl;
    {
        shared_ptr<T> t1(new T,delete_T);
    }
    cout << "--------end-----------" << endl;
    return 0;
}

写成模板

#include <iostream>
#include <memory>
using namespace std;
class T {
private:
    static int id;
    int tid;
public:
    T() {
        tid = id++;
        cout << "id = " << tid << " constructor called" << endl;
    }
    ~T() {
        cout << "id = " << tid << " destructor called" << endl;
    }
};
int T::id = 1;
template<typename _T>
void delete_T(_T *t){
    cout << "delete T" <<endl;
    delete t;
}
int main() {
    cout << "--------begin-----------" << endl;
    {
        shared_ptr<T> t1(new T,delete_T<T>);
    }
    cout << "--------end-----------" << endl;
    return 0;
}

#include <iostream>
#include <memory>
using namespace std;
class T {
private:
    static int id;
    int tid;
public:
    T() {
        tid = id++;
        cout << "id = " << tid << " constructor called" << endl;
    }
    ~T() {
        cout << "id = " << tid << " destructor called" << endl;
    }
};
int T::id = 1;
void deleter1(T *t) {
    cout << "deleter 1" << endl;
    delete t;
}
void deleter2(T *t) {
    cout << "deleter 2" << endl;
    delete t;
}
int main() {
    cout << "--------begin-----------" << endl;
    {
        cout << 1 << endl;
        shared_ptr<T> t1(new T, deleter1);
        cout << 2 << endl;
        shared_ptr<T> t2(new T, deleter2);
        cout << 3 << endl;
        shared_ptr<T> t3 = t2;
        cout << 4 << endl;
        t2 = t1;
        cout << 5 << endl;
    }
    cout << "--------end-----------" << endl;
    return 0;
}

结果：

#include <iostream>
#include <memory>
using namespace std;
class T {
private:
    static int id;
    int tid;
public:
    T() {
        tid = id++;
        cout << "id = " << tid << " constructor called" << endl;
    }
    ~T() {
        cout << "id = " << tid << " destructor called" << endl;
    }
};
int T::id = 1;
void deleter1(T *t) {
    cout << "deleter 1" << endl;
    delete t;
}
void deleter2(T *t) {
    cout << "deleter 2" << endl;
    delete t;
}
int main() {
    cout << "--------begin-----------" << endl;
    {
        cout << 1 << endl;
        T * trow = new T;
        shared_ptr<T> t1(nullptr, deleter1);
        cout << 2 << endl;
        shared_ptr<T> t2(nullptr, deleter2);
        cout << 3 << endl;
        t1.reset(trow);
        cout << 4 << endl;
        t2=t1;
        cout << 5 << endl;
    }
    cout << "--------end-----------" << endl;
    return 0;
}

似乎是只会绑定对象最初被创建时的那个deleter，同时 reset() 方法有一个可以指定deleter的重载