[Boost-users] Constraining unordered template constructors

Subject: [Boost-users] Constraining unordered template constructors
From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2013-05-12 00:37:10

I have a class that I want to have accept 0-5 arguments in any order for 
it's constructor. Each argument is a const references to one of 5 
possible class types. All arguments must be distinct in type, passing 
the same type twice in an error. Passing an argument that is not one of 
the 5 supported types is also an error.

How do you go about implementing something like this?

I figure first you have to create a constructor for each arity but then 
how do you constrain the constructors as described above? I imagine it 
would involve enable_if.

class widget
{
   widget();

   template< class A >
   widget( const A& a );

   template< class A, class B >
   widget( const A& a, const B& b );

   template< class A, class B, class C >
   widget( const A& a, const B& b, const C& c );

   template< class A, class B, class C, class D >
   widget( const A& a, const B& b, const C& c, const D& d );

   template< class A, class B, class C, class D, class E >
   widget( const A& a, const B& b, const C& c, const D& d, const E& e );
};