Dune::AlwaysFalse< T > Struct Template Reference

template which always yields a false value More...

#include <dune/common/typetraits.hh>

Detailed Description

template<typename T>
struct Dune::AlwaysFalse< T >

template which always yields a false value

Template Parameters

T	Some type. It should be a type expression involving template parameters of the class or function using AlwaysFalse.

Suppose you have a template class. You want to document the required members of this class in the non-specialized template, but you know that actually instantiating the non-specialized template is an error. You can try something like this:

template<typename T>
struct Traits {
  static_assert(false,
                     "Instantiating this non-specialized template is an "
                     "error.  You should use one of the specializations "
                     "instead.");
  typedef void FrobnicateType;
};

This will trigger static_assert() as soon as the compiler reads the definition for the Traits template, since it knows that "false" can never become true, no matter what the template parameters of Traits are. As a workaround you can use AlwaysFalse: replace false by AlwaysFalse<T>::value, like this:

template<typename T>
struct Traits {
  static_assert(AlwaysFalse<T>::value,
                     "Instantiating this non-specialized template is an "
                     "error.  You should use one of the specializations "
                     "instead.");
  typedef void FrobnicateType;
};

Since there might be an specialization of AlwaysFalse for template parameter T, the compiler cannot trigger static_assert() until the type of T is known, that is, until Traits<T> is instantiated.

---

The documentation for this struct was generated from the following file:

• dune/common/typetraits.hh