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

From: John Maddock (john_at_[hidden])
Date: 2005-02-14 06:30:18

> 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.