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#if 0
set -xe
g++ -std=c++11 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
clang++ -std=c++11 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
g++ -std=c++20 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
clang++ -std=c++20 -fsyntax-only fn_signature_checker.cc || echo "EXPECTED FAILURE"
exit 0
#endif
#include <type_traits>
// Function signature checker for generic functions.
//
// This demonstrates a way to do some simple function signature checking on
// template arguments w/o implementing the whole concept emulation with
// std::void_t prior to cpp20. This may not be the best solution, but it gives a
// small tool at hand which can easily be employed into older cpp code bases.
//
// Starting with cpp20, one can fully switch to concepts for this task.
// -- PRE CPP20 ----------------------------------------------------------------
// (1) Primary template to extract the function signature type.
template <typename Fn>
struct fn;
// (2) Partial specialization for function types.
template <typename Ret, typename... Args>
struct fn<Ret(Args...)> {
using type = Ret(Args...);
};
// (3) Partial specialization for function pointer types.
//
// This specialization is disabled on purpose as it allows for writing ambiguous
// checks for either non-static or static member functions.
//
// template <typename Ret, typename... Args>
// struct fn<Ret (*)(Args...)> {
// using type = Ret(Args...);
// };
// (4) Partial specialization for member function pointer types.
template <typename T, typename Ret, typename... Args>
struct fn<Ret (T::*)(Args...)> {
using type = Ret(Args...);
};
template <typename Fn>
using fn_t = typename fn<Fn>::type;
template <typename T>
void do_work() {
// Check for non-static member function.
//
// If the partial specialization (3) is enabled, this could also match a
// static member function T::get.
static_assert(std::is_same<fn_t<int(int)>, fn_t<decltype(&T::get)>>::value,
"T has no member function 'int get(int)'");
// Check for non-static member function.
static_assert(std::is_same<fn_t<void(int)>, fn_t<decltype(&T::set)>>::value,
"T has no member function 'void set(int)'");
// Check for static member function.
static_assert(std::is_same<fn_t<int()>, fn_t<decltype(T::cnt)>>::value,
"T has no static function 'static int cnt()'");
}
struct good {
int get(int);
void set(int);
static int cnt();
};
struct bad {
int get(int);
int set();
static int cnt();
};
void check() {
do_work<good>();
// TODO: Enable to see a failing check due to wrong function signature.
// do_work<bad>();
}
// -- SINCE CPP20 --------------------------------------------------------------
#if __cplusplus >= 202002L
#include <concepts>
template <typename T>
concept MyType = requires(T t) {
{ t.get(int{}) } -> std::same_as<int>;
{ t.set(int{}) } -> std::same_as<void>;
{ T::cnt() } -> std::same_as<int>;
};
template <MyType T>
void do_work2() {
}
void check2() {
do_work2<good>();
// TODO: Enable to see failing to satisfy the concept MyType.
// do_work2<bad>();
}
#endif
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