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c++functiontemplatesnon-type

Non-type template argument to allow various function types?

Here is some code where a function template takes a function to call as the template non-type argument:

template <class R, R func() >
auto Call() -> R
{
   return func();
}

int f() { return 1; }

int main()
{
    Call<int, f>();      // OK

    Call<f>();           // Error
}

Is there any way to make this be callable without the need to repeat the function return type, as in Call<f>() ?

It can be done via the preprocessor, #define CALL(f) Call<decltype(f()), f>, but I would like to know if it can be done without the preprocessor.

Solution

  • This is what auto template parameters were designed to solve (C++17 and later).

    #include <cstdio>
#include <type_traits>

template <auto Fn>
auto Call() -> decltype(Fn()) {
    return Fn();
}

int f() { return 1; }

int main() {
    printf("%d\n", Call<f>());
}

    http://coliru.stacked-crooked.com/a/55808ba25f0a07cd