Why are Partial Specializations of Function Templates Disallowed in C ?

Non-Existence of Partial Function Template Specialization: Rationale and Workarounds

Why are partial specializations of function templates prohibited in the C language? While the primary template and specific specializations are allowed, partial specializations that partially constrain types within the template parameters are not.

The C language specification explicitly disallows partial specializations because it was deemed unnecessary. The functionality achieved by partial specialization can be replicated using alternative methods, such as enclosing individual function instances within classes or namespaces or using conditional code blocks to selectively instantiate the template.

Consider the following example, where partial specialization is not allowed:

template<typename T, typename U>
void f() {}   //allowed!
template<>
void f<int, char>() {}   //allowed!
template<typename T>
void f<char, T>() {}   //not allowed!
template<typename T>
void f<T, int>() {}     //not allowed!

This behavior may seem counterintuitive, but the rationale is clear. Partial specializations can create ambiguities during template instantiation and can lead to unintended behavior. By enforcing a strict separation between primary templates and specific specializations, the language ensures predictable and well-defined template instantiation.

However, this does not mean that partial specialization is completely useless. By incorporating the function within a class or namespace, you can effectively achieve a similar effect:

#include <iostream>
using namespace std;

void say(const char 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
}

This code demonstrates how to use class members to emulate partial specialization. Each specific instantiation of the function is enclosed within its own static member function, providing a similar level of specialization without violating the language restrictions. By adopting these workarounds, you can effectively achieve the desired functionality of partial specialization while adhering to the established language conventions.

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