智能指针数据成员pImpl(类Impl)和原始指针pc(CAT类)都是不完整的数据类型,在Widget.h中没有这两个类的定义
//widget.h
#ifndef W_H_
#define W_H_
#include <memory>
class Widget {
public:
Widget();
~Widget() { //
delete pc; // I know I should put ~Widget to .cpp
// I just want to show the difference in behavior
// between raw pointer and smart pointer(both has incomplete type)
// when widget object destructs
}
private:
struct Impl;
std::shared_ptr<Impl> pImpl; // use smart pointer
struct CAT;
CAT *pc; //raw pointer
};
#endif
//widget.cpp
#include "widget.h"
#include <string>
#include <vector>
#include <iostream>
using namespace std;
struct Widget::CAT
{
std::string name;
CAT(){cout<<"CAT"<<endl;}
~CAT(){cout<<"~CAT"<<endl;}
};
struct Widget::Impl {
std::string name;
Impl(){cout<<"Impl"<<endl;}
~Impl(){cout<<"~Impl"<<endl;}
};
Widget::Widget()
: pImpl(std::make_shared<Impl>()),pc(new CAT)
{}
//main.cpp
#include "widget.h"
int main()
{
Widget w;
}
//输出
Impl
CAT
~Impl
对于原始指针数据成员,当小部件对象被破坏时,它的destuctor不会被调用.
据我了解,在Widget ::〜Widget()中应该生成一些
代码自动如下：
~Widget() {
delete pc; // wrote by me
// generated by compiler
delete pImpl->get();
}
为什么在widget被破坏的时候,shared_ptr数据成员和原始数据成员有不同的行为？
我在Linux中使用g 4.8.2测试代码
================================编辑================= ==============
根据答案,原因是因为：
~Widget() {
delete pc; // wrote by me
// generated by compiler
delete pImpl->get();
}
它可能是这样的：
~Widget() {
delete pc; // wrote by me
// generated by compiler
pimpl.deleter(); //deleter will be initailized while pimpl object is initialized
}
解决方法
什么标准说
if the object being deleted has incomplete class type at the point of
deletion and the complete class has a non-trivial destructor or a
deallocation function,the behavior is undefined.
在这里可以找到详细的说明：Why,really,deleting an incomplete type is undefined behaviour?
只需将struct声明移动到类定义之前才能解决您的问题：
struct CAT
{
std::string name;
CAT(){std::cout<<"CAT"<<std::endl;}
~CAT(){std::cout<<"~CAT"<<std::endl;}
};
class Widget {
public:
Widget();
~Widget() {
delete pc; // I know we should put this code to cpp
// I am just want to show the difference behavior
// between raw pointer and smart pointer
// when widget object destruct
}
private:
struct Impl;
std::shared_ptr<Impl> pImpl; // use smart pointer
CAT *pc; //raw pointer
};
并输出
Impl CAT ~CAT ~Impl
转发声明有助于加快编译时间,但是当需要有关数据类型的更多信息时可能会导致问题.
但为什么它可以用于智能指针？
这是一个更好的解释：Deletion of pointer to incomplete type and smart pointers
基本上,在初始化或重置指针时,shared_ptr只需要声明.这意味着在声明的时刻不需要完整的类型.
This functionality isn’t free: shared_ptr has to create and store a pointer to the deleter functor; typically this is done by storing the deleter as part of the block that stores the strong and weak reference counts or by having a pointer as part of that block that points to the deleter (since you can provide your own deleter).
