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Reference documentation for deal.II version 9.1.1
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#include <deal.II/base/template_constraints.h>
This specialization of the general template for the case of a true
first template argument declares a local alias type
to the second template argument. It is used in order to construct constraints on template arguments in template (and member template) functions. The negative specialization is missing.
Here's how the trick works, called SFINAE (substitution failure is not an error): The C++ standard prescribes that a template function is only considered in a call, if all parts of its signature can be instantiated with the template parameter replaced by the respective types/values in this particular call. Example:
The compiler should detect that in this call, the template parameter T must be identified with the type "int". However, the return type T::type does not exist. The trick now is that this is not considered an error: this template is simply not considered, the compiler keeps on looking for another possible function foo.
The idea is then to make the return type un-instantiatable if certain constraints on the template types are not satisfied:
constraint_and_return_value<false,T> is not defined. Given something like
we can write a template
which can only be instantiated if T=int or T=double. A call to f('c') will just fail with a compiler error: "no instance of f(char) found". On the other hand, if the predicate in the first argument to the constraint_and_return_value template is true, then the return type is just the second type in the template.
Definition at line 201 of file template_constraints.h.