Template Metaprogramming

[Imposter]

Hi, I was going through the TMP . I got stuck with the following code . I coundn't understand the declaration of

Class if_

in the following code ,can anyone help me in figuring out the FLOW OF THE PROGRAM

 
template <bool Condition, typename TrueResult, typename FalseResult>
class if_;
 
template <typename TrueResult, typename FalseResult>
struct if_<true, TrueResult, FalseResult>
{
  typedef TrueResult result;
};
 
template <typename TrueResult, typename FalseResult>
struct if_<false, TrueResult, FalseResult>
{
  typedef FalseResult result;
};
 
int main()
{
  typename if_<true, int, void*>::result number(3);
  typename if_<false, int, void*>::result pointer(&number);
 
   typedef typename if_<(sizeof(void *) > sizeof(uint32_t)), uint64_t, uint32_t>::result
      integral_ptr_t;
 
   integral_ptr_t converted_pointer = reinterpret_cast<integral_ptr_t>(pointer);
}
 
 

[weaknessforcats]

This code:

class if_;

is called a forward reference.

The rule is you cannot use a class name unless the class has already been defined. Th forwrd reference says the class has been defined bu just not in this file o in ts includes.

This assurance by you that class if_ exists somewhere is enough for the compiler to allow and if_* or an if_&. However, if you start executing member functons of if_ you will need the full class definiion.

Usually you see this where the details of if_ need to remain hidden because if_ is a commercial purchase and the seller does not want you to know how if_ is designed. So you are given a header with a forward reference and a library. The library was built using the complete definition soit's OK for you to call a library function using an if_* or an if_& because it is the library code (not you) that calls the member function.

[Imposter]

hi ..
when you say IF_* , and IF_& , WHAT DOES IT ESSENTIALLY mean... does it mean if_pointer and if_address(reference)

[weaknessforcats]

if_ is the name of your class:

class if_;
 

so if_ is a type like char is a type. Therefore, if_* is a pointer to if_ just like char* is a pointer to char. The & is a reference: if_& means reference to if_ just like char& is a reference to char.

BTW: Sorry for the delay but there have been site problems norifying me of posts needing replies.