Re: Template default constructor possible?

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Sun, 21 Oct 2007 23:31:29 +0200

Message-ID:

<13hnh5oaha4hnea@corp.supernews.com>

* Ian Collins:

I've never had cause to try this before, but is it possible to have a
template default constructor?

I wish to pass the type of a singleton to a class and use the static
methods of that singleton object in the class, for example:

template <typename Item>
bool getItemValue() {
  return Item::getValue();
}

struct Item { static bool getValue(); };

struct X
{
  typedef bool (*GetFn)();

  const GetFn getFn;

  template <typename T> X( const T& )
    : getFn( getItemValue<T> ) {}
};

works, but requires a public default constructor for T.

There's no way to call a templated constructor with no arguments. So
you can have that templated constructor, but not use it. However, it's
no big deal create an argument type that can carry a type for you, e.g.

   template< typename T > struct UseMethodsOf {};
   template< typename T >
   UseMethodsOf<T> useMethodsOf(){ return UseMethodsOf<T>(); }

   struct X
   {
       ...
       template< typename T >
       X( UseMethodsOf<T> ): getFn( &getItemValue<T> ) {}
   };

   ...
   X foo( useMethodsOf<Whatever>() );

Cheers, & hth.,

- Alf

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