How to write `is_complete` template?

Question

After answering this question I was trying to find is_complete template in Boost library and I realized that there is no such template in Boost.TypeTraits. Why

Answer 1

Solving this requires performing the computation in the default argument of the trait template, as attempting to change the definition of a template violates the ODR rule (although a combination of __COUNTER__ and namespace {} can work around ODR).

This is written in C++11 but can be adjusted to work in C++03 mode of a moderately recent C++11-compatible compiler.

template< typename t >
typename std::enable_if< sizeof (t), std::true_type >::type
is_complete_fn( t * );

std::false_type is_complete_fn( ... );

template< typename t, bool value = decltype( is_complete_fn( (t *) nullptr ) )::value >
struct is_complete : std::integral_constant< bool, value > {};

Online demo.

The default argument is evaluated where the template is named, so it can contextually switch between different definitions. There is no need for a different specialization and definition at each use; you only need one for true and one for false.

The rule is given in §8.3.6/9, which applies equally to function default arguments and default template-arguments:

Default arguments are evaluated each time the function is called.

But beware, using this inside a template is almost sure to violate the ODR. A template instantiated on an incomplete type must not do anything differently from if it were instantiated on a complete type. I personally only want this for a static_assert.

Incidentally, this principle may also be helpful if you want to go the other way and implement the functionality of __COUNTER__ using templates and overloading.