Why Does C Prohibit Partial Specialization of Function Templates?

Function Templates: Unraveling the Mystery of Partial Specialization

The world of function templates in C offers a versatile mechanism for type-generic programming. However, one elusive feature is the ability to partially specialize function templates. This raises the question: why does the C language specification prohibit such partial specialization?

In exploring the rationale behind this restriction, one hypothesis is that it stemmed from an oversight. Yet, the absence of any formal documentation supporting this theory leaves us grappling for answers.

Another plausible explanation is the availability of alternative techniques to achieve partial specialization effects. By encapsulating the function within a static member of a class, programmers can effectively mimic the behavior of partial specialization.

To illustrate this approach, consider the following example:

#include <iostream>

using namespace std;

void say(char const s[]) { std::cout << s << std::endl; }

namespace detail {
    template< class T, class U >
    struct F {
        static void impl() { say( "1. primary template" ); }
    };

    template<>
    struct F<int, char> {
        static void impl() { say( "2. <int, char> explicit specialization" ); }
    };

    template< class T >
    struct F< char, T > {
        static void impl() { say( "3. <char, T> partial specialization" ); }
    };

    template< class T >
    struct F< T, int > {
        static void impl() { say( "4. <T, int> partial specialization" ); }
    };
}  // namespace detail

template< class T, class U >
void f() { detail::F<T, U>::impl(); }    

int main() {
    f<char const*, double>();       // 1
    f<int, char>();                 // 2
    f<char, double>();              // 3
    f<double, int>();               // 4
}

In this example, the function f is implemented as a static member of the F class template. The use of class specialization allows us to define explicit and partial specializations for various combinations of template parameters.

In the absence of direct support for partial specialization of function templates, this alternative approach provides a workaround to achieve similar effects. However, it may introduce additional complexities and potential code bloat.

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