Type trait implementation

To understand type traits, we’ll look at the classic implementations of boost::is_void and boost::is_pointer.

boost::is_void

First, let’s look at one of the simplest traits classes, the is_void trait, which was created by Boost. It defines a generic template that’s used to implement the default behavior; that is, accept a void type, but not anything else. Hence, we have is_void::value = false:

//primary class template is_voidtemplate< typename T >
struct is_void{
static const bool value = false; //default value=false
};

Then, we fully specialize it for the void type, like so:

//"<>" means a full specialization of template class is_voidtemplate<>
struct is_void< void >{ //fully specialization for void
static const bool value = true; //only true for void type
};

Thus, we have a complete traits type that can be used to detect whether any given type, T, is void by checking the following expression...