bslmf_nthparameter

Detailed Description

Outline

• Purpose
• Classes
• Description
  • Usage

Purpose

Metafunction to return the Nth type parameter in a parameter pack

Classes

bslmf::NthParameter<t_N, PARAM_0, t_PARAMS...>

See also

Description

This component contains a metafunction that treats a parameter pack of types as compile-time array of types, returning the Nth type (counting from zero). It is useful for implementing types like tuple that need access to a specific element of a parameter pack.

Usage

We wish to implement a tuple-like class that holds a heterogeneous collection of elements, each of which might have a different type. The metafunction, my_tuple_element<I, my_tuple<ELEMS...>>::Type would be type of the Ith element in the tuple (where I is zero-based).

First, we define our my_tuple class template. The body of the class is unimportant for this usage examples:

template <class... ELEMS>
class my_tuple {
    // ...
};

Then, we use bslmf::NthParameter to implement my_tuple_element:

#include <bslmf_nthparameter.h>
 
template <std::size_t I, class TUPLE>
struct my_tuple_element;  // Not defined
 
template <std::size_t I, class... ELEMS>
struct my_tuple_element<I, my_tuple<ELEMS...> > {
    typedef typename bslmf::NthParameter<I, ELEMS...>::Type Type;
};

Finally, we test this implementation using bsl::is_same:

#include <bslmf_issame.h>
 
int main()
{
   typedef my_tuple<int, short, char*> ttype;
 
   assert((bsl::is_same<int,    my_tuple_element<0, ttype>::Type>::value));
   assert((bsl::is_same<short,  my_tuple_element<1, ttype>::Type>::value));
   assert((bsl::is_same<char *, my_tuple_element<2, ttype>::Type>::value));
 
   assert(! (bsl::is_same<short, my_tuple_element<0, ttype>::Type>::value));
}