[Boost-users] Re: starting to learn Boost MPL and type_trait

From: Sharon Galtzur (sharon.galtzur_at_[hidden])
Date: 2005-02-14 08:43:48

John Maddock wrote:
>> 1. How (or rather Why) does is_class works.
> 
> 
> There are two key concepts:
> 
> 1) A class type is the only kind of type that can have a member function.
> 2) When the compiler performs template argument substition to a function 
> signature, if the signature is an invalid type then the signature is 
> ignored without a compiler error (sometimes called Substitution Failure 
> Is Not An Error or SFINAE).
> 
> So given the overloads:
> 
> template <class T>
> void f(int (T::*)(int));  / #1
> template <class T>
> void f(...); // #2
> 
> The expression:
> 
> f<T>(0);
> 
> Will call #1 if T is a class type, or #2 otherwise.
> 
>> 2. Same for is_enum
> 
> 
> An enum type is the only type that has a built-in implicit conversion to 
> int, but is not itself an integer type.  Combine that with the fact that 
> only one user-defined conversion may be performed per conversion 
> sequence, and you have the implementation.
> 
> John.

Thanks for the reply. The thing i dont understand is this :
template <class T> void f(int (T::*)(int)) will match any member 
function with specific signature (receiving int and returning int). But 
not all classes have such function. So in this case how will a class F { 
} be matched ?

The only function that always exist for all classes with the same 
signature is a destructor ( with void (void) signature ?) - is this the 
function that is used ?

Thanks

Sharon