Question

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C++ templates that accept only certain types

For example, if we want to define a template function which we can use integers, floats, doubles but not strings. Is there an easy way to do so?

Answer 1

The way to do this to use std::enable_if in some shape or form. The selector for the supported type is then used as the return type. For example:

  template <typename T> struct is_supported { enum { value = false }; };
  template <> struct is_supported<int> { enum { value = true }; };
  template <> struct is_supported<float> { enum { value = true }; };
  template <> struct is_supported<double> { enum { value = true }; };

  template <typename T>
  typename std::enable_if<is_supported<T>::value, T>::type
  restricted_template(T const& value) {
    return value;
  }

Obviously, you want to give the traits a better name than is_supported. std::enable_if is part of C++2011 but it is easily implemented or obtained from boost in case it isn’t available with the standard library you are using.

In general, it is often unnecessary to impose explicit restrictions as the template implementation typically has implicit restrictions. However, sometimes it is helpful to disable or enable certain types.