how to only enable this function for string or stringpiece

Question

I have a template function, let's say:

template <typename T>
void foo(T input) {
    // some funny processing
}

I only want to enable this function for T == string or T == stringpiece. How do I do that using std::enable_if ???

Answer 1

You can use is_same to check two types are the same and then use enable_if in the return type of the function:

#include <string>
#include <type_traits>
#include <functional>

struct stringpiece {
};

template<typename T>
typename std::enable_if<std::is_same<std::string, T>::value || std::is_same<stringpiece, T>::value>::type
foo(T input) {
  // Your stuff here
  (void)input;
}

int main() {
  foo(stringpiece());
  foo(std::string(""));
}

Answer 2

You can just use overloading for this:

template<typename T>
void foo(T);

void foo(string str) { }

void foo(stringpiece sp) { }