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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 sample is licensed under the CC0 Public Domain Dedication.

Intent

Conditionally instantiate a function template depending on the template arguments.

Description

We provide two implementations of the equal function template:

  1. The template on lines 5–10 will only be instantiated when T is an integral type.
  2. 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

  • Joseph Mansfield

Last Updated

09 April 2015

Source

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