Can we define a template function for some data types only?

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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?

As far as I know, not in C++ even C++11, but the old removed features concerning concepts do what you want if I remember well. — Jaffa, Dec 29 '11 at 18:10
@Geoffroy: there is no core language level support for this but using enable_if it can be done (see my answer). It may be a bit cumbersome but I can imagine some helper types which could give this nicer syntax in C++2011 using variadic templates (I haven't experimented with this, yet, however). — Dietmar Kühl, Dec 29 '11 at 18:16
Do you want the same definition to apply for the supported types, or do you need to define it differently for each one? i.e. does `f` have exactly the same body as `f`? — Alan Stokes, Dec 29 '11 at 18:18
@AlanStokes, f has the same body as f, but has a different body as f and that's why I'm trying to separate them out... — derekhh, Dec 29 '11 at 18:22
Exact Match :http://stackoverflow.com/questions/874298/c-templates-that-accept-only-certain-types — Abhishek Chandel, Dec 29 '11 at 18:15

Dietmar Kühl · Accepted Answer · 2013-12-17T00:44:37.640

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.

score 0 · Answer 2 · answered Dec 29 '11 at 18:12

You can check the types of the values. If they are one of your designated types you can go on, if not you can return the function. See here for more information: http://publib.boulder.ibm.com/infocenter/lnxpcomp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8l.doc%2Flanguage%2Fref%2Fthe_typeid_operator.htm

With the use of typeid you should also be able to throw a compileerror.

score 0 · Answer 3 · answered Dec 29 '11 at 18:33

Usually whitelisting certain types restricts the usage of templates very much.

Boost has so called concepts which are basically interfaces for templates. instead of whitelisting certain types you can create compile time errors if certain conditions (functions missing, or with wrong arguments etc.) are not met. of course you can also use this to restrict your template arguments to certain types.

Can we define a template function for some data types only?

3 Answers3