Conditional metafunction returning the first typename if true, the second if false.
A second conditional (if_c) metafunction returns the first constant if true, the second if false;

 namespace framework {
 namespace meta
 {
    template <bool C, typename T0, typename T1>  struct if_;
    template <typename T0, typename T1> struct if_<true, T0, T1> { typedef T0 value; };
    template <typename T0, typename T1> struct if_<false, T0, T1> { typedef T1 value; };

    template <bool C, typename T, typename T C0, typename T C1> struct if_c;
    template <typename T, typename T C0, typename T C1> struct if_c<true,  T, C0, C1> { static T value; };
    template <typename T, typename T C0, typename T C1> struct if_c<false, T, C0, C1> { static T value; };
    template <typename T, typename T C0, typename T C1> T if_c<true,  T, C0, C1>::value = C0;
    template <typename T, typename T C0, typename T C1> T if_c<false, T, C0, C1>::value = C1;
 
 } // namespace meta
 } // namespace framework

For an example, see TemplateMetafunctionHasFunction.

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TemplateMetaprogrammingTechniques TemplateMetafunctionIdentity TemplateMetafunctionHasFunction CategoryCppTemplates