Class template SFINAE
123456789101112 | #include <type_traits>
template <typename T, typename Enable = void>
class foo;
template <typename T>
class foo<T, typename std::enable_if<std::is_integral<T>::value>::type>
{ };
template <typename T>
class foo<T, typename std::enable_if<std::is_floating_point<T>::value>::type>
{ }; |
This pattern is licensed under the CC0 Public Domain Dedication.
Intent
Conditionally instantiate a class template depending on the template arguments.
Description
We provide two partial specializations of the foo class template:
- The template on lines 6–8 will only be instantiated when
Tis an integral type. - The template on lines 10–12 will only be instantiated when
Tis a floating point type.
This allows us to provide different implementations of the foo
class depending on the template arguments it is instantiated with.
We have used std::enable_if on line 7 and
line 11 to force instantiation to succeed only for the appropriate
template arguments. This relies on Substitution Failure Is Not An
Error
(SFINAE), which states that
failing to instantiate a template with some particular
template arguments does not result in an error and simply discards
that instantiation.
If you want to simply prevent a template from being instantiated
for certain template arguments, consider using
static_assert instead.
