SGI/STL __STL_CLASS_REQUIRES(_Tp, _Assignable);

[Visame]

__STL_CLASS_REQUIRES(_Tp, _Assignable);

This sentence appears many times int the SGI/STL source code.
However, this macro seems to do nothing int the cource code.
The Following is the Macro Definition in concept_checks.h:

#ifndef __STL_USE_CONCEPT_CHECKS
 
// Some compilers lack the features that are necessary for concept checks.
// On those compilers we define the concept check macros to do nothing.
#define __STL_REQUIRES(__type_var, __concept) do {} while(0)
#define __STL_CLASS_REQUIRES(__type_var, __concept) \
  static int  __##__type_var##_##__concept

and

#ifdef __STL_NO_FUNCTION_PTR_IN_CLASS_TEMPLATE
 
#define __STL_CLASS_REQUIRES(__type_var, __concept)

and

#else
 
// Use this macro inside of template classes, where you would
// like to place requirements on the template arguments to the class
// Warning: do not pass pointers and such (e.g. T*) in as the __type_var,
// since the type_var is used to construct identifiers. Instead typedef
// the pointer type, then use the typedef name for the __type_var.
#define __STL_CLASS_REQUIRES(__type_var, __concept) \
  typedef void (* __func##__type_var##__concept)( __type_var ); \
  template <__func##__type_var##__concept _Tp1> \
  struct __dummy_struct_##__type_var##__concept { }; \
  static __dummy_struct_##__type_var##__concept< \
    __concept##_concept_specification< \
      __type_var>::__concept##_requirement_violation>  \
  __dummy_ptr_##__type_var##__concept

None of the above definitions seem to be meaningful.
They only declare some variables, but does nothing?
So, Is the COMPLIER that finally defines macros like
__STL_CLASS_REQUIRES?

Thanks!C++ Geeks!

[RRick]

The compiler does supply some macros (i.e. __LINE__ and __FILE__) but these STL marcros are definitely not of that variety. There is no magic here. What is defined is what you get.

In your example from the top of the post, the macro would create the following code:

 
  typedef void (* __func_Tp_Assignable)( _Tp ); 
  template <__func_Tp_Assignable _Tp1>  struct __dummy_struct__Tp_Assignable { };
  static __dummy_struct__Tp_Assignable< _Assignable_concept_specification< _Tp>::_Assignable_requirement_violation> __dummy_ptr__Tp_Assignable
 

What's it used for is another question. Try searching in the code for __func and/or __dummy_struct. You might found more interesting bits of info there.

If you still need answers you can try a news group (i.e. c++.stl.sgi or comp.lang.c++). There are some very knowledgable people there.