[boost] [enable_if] how to make sure the user supplies the correct number of template arguments

From: Stjepan Rajko (stipe_at_[hidden])
Date: 2007-11-19 20:12:45

In using enable_if, I came to use a simple technique to detect whether
the user is supplying the correct number of (non-Enable) arguments
which I think might be useful to add to the enable_if documentation.

This addresses the problem which occurs when the user erroneously
supplies an extra template argument (that ends up as the Enable
template parameter), and it either goes undetected in the compilation
or is hard to trace.  I get this especially when I decide to reduce
the number of template parameters to a class template which uses
enable_if.  E.g.

// ---- WE START WITH

template <typename T1, typename T2, typename Enable=void>
class enabled_class
{
  // base template implementation
};

// user code
enabled_class<X, Y> o;

but then we change our mind and decide that T1 should not be in the
template parameter list for enabled_class:

// ----- WE END UP WITH

template <typename T2, typename Enable=void>
class enabled_class
{
  // base template implementation
};

// we forget to change the user code
enabled_class<X, Y> o; // this should now be enabled_class<Y> o, but
we might not even get an error!

The solution I found goes like this:

// ---- A PARTIAL SOLUTION

template <typename T2, typename Enable=void>
class enabled_class
{
    BOOST_MPL_ASSERT((is_same<Enable, void>));

  // default implementation
};

// we forget to change the user code
enabled_class<X, Y> o; // ERROR (except in the unfortunate case when Y is void)

Is this a reasonable solution to this problem?  Would it be useful to
add it to the docs? I suppose this is a problem common to any time
default template arguments are used, but I ran into it a lot with
enable_if.  Having something in the docs that addresses it would have
helped me a lot.

Stjepan