不允许使用非类,非变量的部分专业化,但是请注意:
计算机科学中的所有问题都可以通过另一层间接解决。-大卫·惠勒
添加一个类来转发函数调用可以解决此问题,下面是一个示例:
template <class Tag, class R, class... Ts>
struct enable_fun_partial_spec;
struct fun_tag {};
template <class R, class... Ts>
constexpr R fun(Ts&&... ts) {
return enable_fun_partial_spec<fun_tag, R, Ts...>::call(
std::forward<Ts>(ts)...);
}
template <class R, class... Ts>
struct enable_fun_partial_spec<fun_tag, R, Ts...> {
constexpr static R call(Ts&&... ts) { return {0}; }
};
template <class R, class T>
struct enable_fun_partial_spec<fun_tag, R, T, T> {
constexpr static R call(T, T) { return {1}; }
};
template <class R>
struct enable_fun_partial_spec<fun_tag, R, int, int> {
constexpr static R call(int, int) { return {2}; }
};
template <class R>
struct enable_fun_partial_spec<fun_tag, R, int, char> {
constexpr static R call(int, char) { return {3}; }
};
template <class R, class T2>
struct enable_fun_partial_spec<fun_tag, R, char, T2> {
constexpr static R call(char, T2) { return {4}; }
};
static_assert(std::is_same_v<decltype(fun<int>(1, 1)), int>, "");
static_assert(fun<int>(1, 1) == 2, "");
static_assert(std::is_same_v<decltype(fun<char>(1, 1)), char>, "");
static_assert(fun<char>(1, 1) == 2, "");
static_assert(std::is_same_v<decltype(fun<long>(1L, 1L)), long>, "");
static_assert(fun<long>(1L, 1L) == 1, "");
static_assert(std::is_same_v<decltype(fun<double>(1L, 1L)), double>, "");
static_assert(fun<double>(1L, 1L) == 1, "");
static_assert(std::is_same_v<decltype(fun<int>(1u, 1)), int>, "");
static_assert(fun<int>(1u, 1) == 0, "");
static_assert(std::is_same_v<decltype(fun<char>(1, 'c')), char>, "");
static_assert(fun<char>(1, 'c') == 3, "");
static_assert(std::is_same_v<decltype(fun<unsigned>('c', 1)), unsigned>, "");
static_assert(fun<unsigned>('c', 1) == 4, "");
static_assert(std::is_same_v<decltype(fun<unsigned>(10.0, 1)), unsigned>, "");
static_assert(fun<unsigned>(10.0, 1) == 0, "");
static_assert(
std::is_same_v<decltype(fun<double>(1, 2, 3, 'a', "bbb")), double>, "");
static_assert(fun<double>(1, 2, 3, 'a', "bbb") == 0, "");
static_assert(std::is_same_v<decltype(fun<unsigned>()), unsigned>, "");
static_assert(fun<unsigned>() == 0, "");