Nick Nick - 3 months ago 17
C++ Question

How correctly to remove code repetition in template function

I have code like this ("spaceship"-like operator).

template <class T>
int comparator(const T &a, const T &b){
if (a < b){
return -1;
}else if (a > b){
return +1;
}

return 0;
}

inline int comparator(const char *a, const char *b){
return strcmp(a, b); // I never tried this, included just to get the idea
}

inline int comparator(char const a, char const b){
return a - b;
}

inline int comparator(int const a, int const b){
return a - b;
}


How can I easily remove repetition for several signed types (char, short, int, long etc). I tried with SFINAE, but result was not very encouraging.

Answer

You can mix together overloading, tag dispatching and templates, as in the following example:

#include<type_traits>
#include<utility>
#include<iostream>

template <class T>
int comparator_(char, const T &a, const T &b){
    std::cout << "catch all" << std::endl;
    return (a<b)?-1:((a>b)?1:0);
}

template<typename T>
std::enable_if_t<std::is_same<T,int>::value or std::is_same<T,char>::value, int>
comparator_(int, T const a, T const b){
    std::cout << "char or int" << std::endl;
    return a - b;
}

template<typename... A>
int comparator(A&&... args) {
    return comparator_(0, std::forward<A>(args)...);
}

int main() {
    comparator(42,0);
    comparator('c', 'g');
    comparator(42u, 0u);
}