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1.2.4.2 destructor
  • For destructor:
    1. If a base class has a destuctor, but sub class doesn’t have, compiler will produce an implicit default destructor, and this implicit default destructor will call base class destuctor.
    2. If you define a subclass destructor, It will call base destructor automaticlly, you don’t need to call it explicitly.
    3. The question is, How can you make sure you sub class destructor will be called if you use a base class pointer or reference, answer is below:
  • Don’t call base desctructor explicitly, It will called automatically in the reverse order of construction. And you should give a base desctructor a definition, Or linker will report error it even you don’t call it in your source code.
  • Make base class destructor public and virtual (polymorphic deletion by base class pointer or reference), or proteced and nonvirtual, base classes need not always allow polymorphic deletion. For example, consider class templates such as std::unary_function. This time, you should make destructor protected and nonvirtual.
    template <class Arg, class Result> 
    struct unary_function{ 
      typedef Arg    argument_type; 
      typedef Result result_type; 
    }; 
     
    Illegal code that you can assume will never exist. 
    void f( std::unary_function* f ){ 
      delete f; // error, illegal 
    }
  • If base class still need to build itself, You can change it back public virtual, Even without polymorphic deletion by now, you still need to declare it as virtual for the future safety(Even with a little dynamic-binding efficiency penalty. ) At same time, If a base class is not abstract class, usually, it’s BAD design.
  • Never throw exception from dtor, if exception A is thrown, then stack-unwindling, when a obj is destructed, then dtor is called, when another exception B is thrown by the dtor, application will call terminat function immediately. If you have exception, catch it inside of the dtor.

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