读陈硕书《Linux多线程服务端编程》,有一句话不能理解:析构在创建时被捕获,这意味着虚析构不再是必须的。实际代码操作了一下,发现确实通过shared_ptr去释放派生类对象,无需将析构函数置为virtual,代码如下:
#include <iostream>
#include <memory>
using namespace std;
class base
{
public:
base(){cout << "base construct" << endl;}
virtual void print(){cout << "base print" << endl;}
~base(){cout << "base destruct" << endl;}
};
class derived: public base
{
public:
derived(){cout << "derived construct " << endl;}
void print(){cout << "derived print" << endl;}
~derived(){cout << "derived destruct" << endl;}
};
int main()
{
shared_ptr<base> ptr(new derived());
return 0;
}
代码打印为:
base construct
derived construct
derived destruct
base destruct
而后阅读文章[c++ 笔记]shared_ptr 理解创建时即捕获析构动作,感觉该文章给出了一个较为合理的解释,不过模拟shared_ptr原理的代码编译不过,这边稍作修改
#include <iostream>
template <typename T>
class simple_ptr_internal_interface {
public:
virtual T* get() = 0;
virtual void destruct() = 0;
}; // class simple_ptr_internal_interface
template <typename T, typename D>
class simple_ptr_internal: public simple_ptr_internal_interface<T>
{
public:
struct DefaultDeleter {
void operator()(T* t) { delete static_cast<D*>(t); }
};
simple_ptr_internal(T* p): pointer(p) {}
virtual T* get() override { return pointer; }
virtual void destruct() override { deleter(pointer); }
private:
T* pointer;
DefaultDeleter deleter;
}; // class simple_ptr_internal
template <typename T>
class simple_ptr {
public:
template <typename Derived>
simple_ptr(Derived* d): internal(new simple_ptr_internal<T, Derived>{d}) {}
~simple_ptr() { this->destruct(); }
private:
void destruct() { internal->destruct(); }
simple_ptr_internal_interface<T>* internal;
};
class base
{
public:
base(){std::cout << "base construct" << std::endl;}
~base(){std::cout << "base destruct" << std::endl;}
};
class derived: public base
{
public:
derived(){std::cout << "derived construct " << std::endl;}
~derived(){std::cout << "derived destruct" << std::endl;}
};
int main()
{
simple_ptr<base>(new derived());
return 0;
}
代码打印为:
base construct
derived construct
derived destruct
base destruct