Pure virtual dtor

liuxinyu · #1 (**permalink**) 2008-07-18

刚才我同事问我的一个问题，分享一下：
Typically if a class has a pure virtual dtor, you can’t create instance of this class. It is designed as an abstract class.

But you must provide a default implementation for that class, or the compiler will generate error:

Example:

c++ 代码:

class Base{
public:
  virtual ~Base() = 0;
}; 
 
Base::~Base(){
}

Without default dtor implementation, following errors will be rasied:
undefined reference to `Base::~Base()`

You must over write the dtor in derived classes as the following:

c++ 代码:

#include <iostream>
 
class Base{
public:
  virtual ~Base() = 0;
};
 
Base::~Base(){
}
 
class Foo: public Base{
public:
  ~Foo(){
    std::cout<<"Foo closed\n";
  }
};
 
class Bar: public Base{
public:
  ~Bar(){
    std::cout<<"Bar closed\n";
  }
};
 
int main(int, char**){
  //Base* p=new Base; this won't pass compile
  Base* f=new Foo;
  Base* b=new Bar;
  delete f;
  delete b;
}

And the result is as below:
liuuuxin@WEIFANG ~/temp
$ g++ test.cpp -o test

liuuuxin@WEIFANG ~/temp
$ ./test
Foo closed
Bar closed

顺便引一下标准：
The standard says in section 12.4 [class.dtor] para 7 page 192:
"A destructor can be declared virtual (10.3) or pure virtual (10.4) ..."

bankrock · #2 (**permalink**) 2008-07-18

这没什么疑问吧，确实是这样的，Effectiv C++ Item14最后讲过这个问题。

cat · #3 (**permalink**) 2008-07-20

是闹明白了以后的总结吧？我也没看到问题。

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