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// Copyright (C) 2022 johannst
#include <iostream>
#include <type_traits>
// Helper meta fns.
template<typename T>
using enable_if_bool = std::enable_if_t<T::value, bool>;
template<typename T>
using disable_if_bool = std::enable_if_t<!T::value, bool>;
template<typename T>
using has_dst = std::integral_constant<bool, !std::is_same<typename T::Return, void>::value>;
// Template meta programming invoke machinery.
namespace impl {
// Invoke an OPERATION which *USES* a context.
template<typename Ctx, template<typename> class Op, typename... P,
enable_if_bool<typename Op<Ctx>::HasCtx> = true>
typename Op<Ctx>::Return Invoke(const Ctx& C, P... params) {
return Op<Ctx>()(C, params...);
}
// Invoke an OPERATION which uses *NO* context.
template<typename Ctx, template<typename> class Op, typename... P,
disable_if_bool<typename Op<Ctx>::HasCtx> = true>
typename Op<Ctx>::Return Invoke(const Ctx&, P... params) {
return Op<Ctx>()(params...);
}
} // namespace impl
// Invoke an OPERATION which *HAS* a DESTINATION with arbitrary number of arguments.
template<typename Ctx, template<typename> class Op, typename... P,
enable_if_bool<has_dst<Op<Ctx>>> = true>
void Invoke(const Ctx& C, P... params) {
std::cout << "Invoke " << Op<Ctx>::Name << '\n';
typename Op<Ctx>::Return R = impl::Invoke<Ctx, Op>(C, params...);
std::cout << "returned -> " << R << '\n';
}
// Invoke an OPERATION which has *NOT* a DESTINATION with arbitrary number of arguments.
template<typename Ctx, template<typename> class Op, typename... P,
disable_if_bool<has_dst<Op<Ctx>>> = true>
void Invoke(const Ctx& C, P... params) {
std::cout << "Invoke " << Op<Ctx>::Name << " without destination." << '\n';
impl::Invoke<Ctx, Op>(C, params...);
}
// Custom context.
struct Ctx {
void out(const char* s, unsigned v) const { printf("%s%x\n", s, v); }
};
// Operations to invoke.
template<typename Ctx>
struct OpA {
using HasCtx = std::false_type;
using Return = int;
static constexpr const char* const Name = "OpA";
constexpr Return operator()(int a, int b) const { return a + b; }
};
template<typename Ctx>
struct OpB {
using HasCtx = std::true_type;
using Return = void;
static constexpr const char* const Name = "OpB";
Return operator()(const Ctx& C, unsigned a) const { C.out("a = ", a); }
};
int main() {
Ctx C;
Invoke<Ctx, OpA>(C, 1, 2);
Invoke<Ctx, OpB>(C, 0xf00du);
return 0;
}
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