GeezerVison

The World Wide Web, restored to 800 pixels of glory.

GeezerVison is showing cpppatterns.com — Function template SFINAE - C++ Patterns

← Patterns

Function template SFINAE

1234567891011121314151617 #include <type_traits> #include <limits> #include <cmath> template <typename T> typename std::enable_if<std::is_integral<T>::value, bool>::type equal(T lhs, T rhs) { return lhs == rhs; } template <typename T> typename std::enable_if<std::is_floating_point<T>::value, bool>::type equal(T lhs, T rhs) { return std::abs(lhs - rhs) < 0.0001; }

This pattern is licensed under the CC0 Public Domain Dedication.

Requires c++11 or newer.

Intent

Conditionally instantiate a function template depending on the template arguments.

Description

We provide two implementations of the equal function template:

The template on lines 5–10 will only be instantiated when T is an integral type.
The template on lines 12–17 will only be instantiated when T is a floating point type.

We have used std::enable_if on line 6 and line 13 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.

The second template argument of std::enable_if — in this case, bool — is what the full std::enable_if<...>::type evaluates to when the first template argument is true. This means that the return type of equal will be bool.

If you want to simply prevent a template from being instantiated for certain template arguments, consider using static_assert instead.

Contributors

Last Updated

09 December 2017

Source

Fork this pattern on GitHub

You are visitor 01402503 — GeezerVison does not store the pages you look up.
Best viewed with Netscape Navigator 4.7 or Internet Explorer 6 at 800×600. No cookies required.