C++ Coding Tips - Chapter3. Templates

C++ Coding Tips

Betta Jin < betajin@hotmail.com>
[Draft] April {16,19,25}, 2002

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Chapter3. Templates

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Section1. Compile time routing

• Compile time if-else structure:
  

  //  The deallocator template:
  template <const bool _is_array>
  class deallocator ;
  //  Specialize for if-else switch
  template <>
  class deallocator<true>
  {
  public:
      template <class T>
      static inline void free_ptr(T * const & p)
      {
          ASSERT(p != 0) ;
          delete[] p ;
      }
  } ;
  template <>
  class deallocator<false>
  {
  public:
      template <class T>
      static inline void free_ptr(T * const & p)
      {
          ASSERT(p != 0) ;
          delete p ;
      }
  } ;
  //  Use case:
  template <class T, const bool _is_array=false>
  class CHello
  {
      ...
      T * m_p ;
      inline void free()
      {
          deallocator<_is_array>::free_ptr(m_p) ;
      }
      ...
  } ;
  

  Note: This sample demonstrates a way to wrap the delete/delete[] operation in a same function without an if-else routing. Here goes the if-else structure, and more, there will be switch-case sturcture in the same way. I think the '_is_array' parameter here can be called as 'value traits' or 'switch traits'.
• Both VC6 and VC7 cannot compile the following code. But g++3 does! So, for compatible considerations, I do not suggest such code:
  

  template <class T, const bool _is_array>
  class cls_any ;
  template <class T>
  class cls_any<T, true>
  {
      ...
  } ;
  template <class T>
  class cls_any<T, false>
  {
      ...
  } ;
  

  Note: This is something of partial specialization. The code in my way(1) for partial specializtion is nice enough for such requirements and can be accepted by both VC6/VC7/g++3. There are some tricks to implement compound class partial specialization. I'll describe it later.
• VC6, VC7 and g++3 cannot compile such code at all:

  template <class T, const bool _is_array>
  void func_any(T * const & p) ;
  template <class T>
  void func_any<T, true>(T * const & p)
  {
      ...
  }
  template <class T>
  void func_any<T, false>(T * const & p)
  {
      ...
  }
  

Section2. Any-type routing

• Description:
  There are libraries and they are likely to provide some same functions. But the providers did not make any type of conference to specify the naming of functions/APIs. This is a problem. When we write a common lib, we are forced to declare that the library is for or based on some library. It's just a burden that if some one wants the library that is based on one specified library to be used in another environment, he must write a piece of customed code to adapt the library. If the coding task is done with little ugly code, it is just so lucky. But some time, he cannot use the library in that there is no base library for him to use. So no chance a adapter, he has to write his own code instead to use the library. This is not only a compatible problem, but the library's self-adaptability.
  I'll try to give a solution for such requirement by examples.
• Any-type string:
• Use case of Any-type string: