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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
#ifndef mozilla_FunctionTypeTraits_h
#define mozilla_FunctionTypeTraits_h
#include <cstddef> /* for size_t */
namespace mozilla {
// Main FunctionTypeTraits declaration, taking one template argument.
//
// Given a function type, FunctionTypeTraits will expose the following members:
// - ReturnType: Return type.
// - arity: Number of parameters (size_t).
// - ParameterType<N>: Type of the Nth** parameter, 0-indexed.
//
// ** `ParameterType<N>` with `N` >= `arity` is allowed and gives `void`.
// This prevents compilation errors when trying to access a type outside of the
// function's parameters, which is useful for parameters checks, e.g.:
// template<typename F>
// auto foo(F&&)
// -> enable_if(FunctionTypeTraits<F>::arity == 1 &&
// is_same<FunctionTypeTraits<F>::template ParameterType<0>,
// int>::value,
// void)
// {
// // This function will only be enabled if `F` takes one `int`.
// // Without the permissive ParameterType<any N>, it wouldn't even compile.
//
// Note: FunctionTypeTraits does not work with generic lambdas `[](auto&) {}`,
// because parameter types cannot be known until an actual invocation when types
// are inferred from the given arguments.
template <typename T>
struct FunctionTypeTraits;
// Remove reference and pointer wrappers, if any.
template <typename T>
struct FunctionTypeTraits<T&> : FunctionTypeTraits<T> {};
template <typename T>
struct FunctionTypeTraits<T&&> : FunctionTypeTraits<T> {};
template <typename T>
struct FunctionTypeTraits<T*> : FunctionTypeTraits<T> {};
// Extract `operator()` function from callables (e.g. lambdas, std::function).
template <typename T>
struct FunctionTypeTraits : FunctionTypeTraits<decltype(&T::operator())> {};
namespace detail {
template <size_t N, typename... As>
struct SafePackElement;
template <size_t N>
struct SafePackElement<N> {
using type = void;
};
template <typename A, typename... As>
struct SafePackElement<0, A, As...> {
using type = A;
};
template <size_t N, typename A, typename... As>
struct SafePackElement<N, A, As...> : SafePackElement<N - 1, As...> {};
template <size_t N, typename... As>
using SafePackElementType = typename SafePackElement<N, As...>::type;
} // namespace detail
// Specialization for free functions.
template <typename R, typename... As>
struct FunctionTypeTraits<R(As...)> {
using ReturnType = R;
static constexpr size_t arity = sizeof...(As);
template <size_t N>
using ParameterType = detail::SafePackElementType<N, As...>;
};
// Specialization for non-const member functions.
template <typename C, typename R, typename... As>
struct FunctionTypeTraits<R (C::*)(As...)> : FunctionTypeTraits<R(As...)> {};
// Specialization for const member functions.
template <typename C, typename R, typename... As>
struct FunctionTypeTraits<R (C::*)(As...) const>
: FunctionTypeTraits<R(As...)> {};
#ifdef NS_HAVE_STDCALL
// Specialization for __stdcall free functions.
template <typename R, typename... As>
struct FunctionTypeTraits<R NS_STDCALL(As...)> : FunctionTypeTraits<R(As...)> {
};
// Specialization for __stdcall non-const member functions.
template <typename C, typename R, typename... As>
struct FunctionTypeTraits<R (NS_STDCALL C::*)(As...)>
: FunctionTypeTraits<R(As...)> {};
// Specialization for __stdcall const member functions.
template <typename C, typename R, typename... As>
struct FunctionTypeTraits<R (NS_STDCALL C::*)(As...) const>
: FunctionTypeTraits<R(As...)> {};
#endif // NS_HAVE_STDCALL
} // namespace mozilla
#endif // mozilla_FunctionTypeTraits_h