Re: Constructor syntax woes.

Tue, 15 Jul 2008 19:23:01 -0700 (PDT)
On Jul 15, 8:24 pm, "Alf P. Steinbach" <> wrote:


Below is a condensed version of some code I'm having difficulty with:



#include <cstddef> // size_t

Well, with the current standard formally this only gives you std::size_t,=


all or nearly all compilers also place it in the global namespace. With C=


it's allowed to also give you size_t in global namespace. Because of that=


now, IMHO, misleading to use the <c...> headers; just use <stddef.h>.

#include <functional> // binary_function

// the input parameter type to a class constructor.
template< typename T >
struct ABinaryFunction : public std::binary_function< T, size_t,
double >
  typename ABinaryFunction::result_type
  operator()( typename ABinaryFunction< T >::first_argument_type lhs,
              typename ABinaryFunction< T >::second_argum=


rhs )
    return ABinaryFunction< T >::result_type();

OK, except you'll probably want to add a 'const' for the operator() (pres=


it's not changing the functor object, but just computing something).

template< typename T >
class AClass

  // constructor takes one specialized binary_function type parameter=


  AClass( std::binary_function< T, size_t, double > function )

If you want to somehow retain the functor object then you need to use som=

e other

kind of argument, possibly a templated constructor. As it is your actual
argument will be sliced to std::binary_function. Not much you can do with=

 a pure


  // arbitrary method to test instantiation.
  bool True()
    return true;

int main()
  // this won't create a class of type "AClass".
  AClass< int > instance_a( ABinaryFunction< int >() );

This is an example of what's been called "the most vexing parse" in C++. =


rule is that if the compiler can treat a declaration as a function declar=


it will. And here it can, so it does.

You can fiddle with extra parentheses and the like, to convince the compi=


that that argument can't possibly be a type, but that yields unclear code=


So instead just do

   ABinaryFunction<int> foo;
   AClass<int> instance_a( foo );

Or you can introduce a factory function for your binary functors,

   template< typename T >
   ABinaryFunction<T> aBinaryFunction() { return ABinaryFunction<T>()=

; }

and then in main you can declare

   AClass<int> instance_a( aBinaryFunction<int>() );

Anyway, as noted earlier, this actual argument will be sliced.

  // this will not compile.
  bool bool_a = instance_a.True();

  // this will create a class of type "AClass", along with an unwante=


  ABinaryFunction< int > binary_function_b;
  AClass< int > instance_b( binary_function_b );
  // this will compile.
  bool bool_b = instance_b.True();

Yes, that's OK.

  return 0;

Cheers, & hth.,

- Alf

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Thanks for the pointers and suggestions. I know the code I provided
wasn't terribly functional; I just stripped out everything I could
think of and still demonstrate the problem. You're right on the
ABinaryFunction inheritance bit-slicing; however most derived versions
of std::binary_function() are to re-define 'operator()', so it's
okay. If I had state, I'd virtualize the destructor.

As you mentioned, and Sam showed, the double-parens trick also works.
Kinda subtle. Not sure if I like that over my explicit declaration
and "construction by assignment" version more. But the point is, now
I have some options; I'll have to comment the code accordingly for
either fix.

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