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FunctionTypeTraits< function type > makes it easier to inspect a function's return type, arity, and parameter types. It works with free functions, struct/class methods, function objects like non-generic lambdas and std::function. Differential Revision: https://phabricator.services.mozilla.com/D2998 --HG-- extra : moz-landing-system : lando
137 lines
4.1 KiB
C++
137 lines
4.1 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* Helpers to manipulate function types that don't fit in TypeTraits.h */
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#ifndef mozilla_FunctionTypeTraits_h
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#define mozilla_FunctionTypeTraits_h
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#include <tuple>
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namespace mozilla {
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// Main FunctionTypeTraits declaration, taking one template argument.
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//
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// Given a function type, FunctionTypeTraits will expose the following members:
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// - ReturnType: Return type.
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// - arity: Number of parameters (size_t).
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// - ParameterType<N>: Type of the Nth** parameter, 0-indexed.
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//
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// ** `ParameterType<N>` with `N` >= `arity` is allowed and gives `void`.
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// This prevents compilation errors when trying to access a type outside of the
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// function's parameters, which is useful for parameters checks, e.g.:
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// template<typename F>
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// auto foo(F&&)
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// -> enable_if(FunctionTypeTraits<F>::arity == 1 &&
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// is_same<FunctionTypeTraits<F>::template ParameterType<0>,
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// int>::value,
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// void)
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// {
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// // This function will only be enabled if `F` takes one `int`.
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// // Without the permissive ParameterType<any N>, it wouldn't even compile.
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//
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// Note: FunctionTypeTraits does not work with generic lambdas `[](auto&) {}`,
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// because parameter types cannot be known until an actual invocation when types
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// are inferred from the given arguments.
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template<typename T>
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struct FunctionTypeTraits;
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// Remove reference and pointer wrappers, if any.
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template<typename T>
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struct FunctionTypeTraits<T&> : public FunctionTypeTraits<T>
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{
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};
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template<typename T>
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struct FunctionTypeTraits<T&&> : public FunctionTypeTraits<T>
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{
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};
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template<typename T>
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struct FunctionTypeTraits<T*> : public FunctionTypeTraits<T>
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{
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};
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// Extract `operator()` function from callables (e.g. lambdas, std::function).
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template<typename T>
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struct FunctionTypeTraits : public FunctionTypeTraits<decltype(&T::operator())>
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{
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};
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namespace detail {
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// If `safe`, retrieve the `N`th type from `As`, otherwise `void`.
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// See top description for reason.
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template<bool safe, size_t N, typename... As>
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struct TupleElementSafe;
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template<size_t N, typename... As>
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struct TupleElementSafe<true, N, As...>
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{
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using Type = typename std::tuple_element<N, std::tuple<As...>>::type;
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};
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template<size_t N, typename... As>
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struct TupleElementSafe<false, N, As...>
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{
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using Type = void;
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};
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template<typename R, typename... As>
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struct FunctionTypeTraitsHelper
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{
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using ReturnType = R;
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static constexpr size_t arity = sizeof...(As);
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template<size_t N>
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using ParameterType =
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typename TupleElementSafe<(N < sizeof...(As)), N, As...>::Type;
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};
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} // namespace detail
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// Specialization for free functions.
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template<typename R, typename... As>
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struct FunctionTypeTraits<R(As...)> : detail::FunctionTypeTraitsHelper<R, As...>
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{
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};
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// Specialization for non-const member functions.
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template<typename C, typename R, typename... As>
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struct FunctionTypeTraits<R (C::*)(As...)>
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: detail::FunctionTypeTraitsHelper<R, As...>
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{
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};
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// Specialization for const member functions.
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template<typename C, typename R, typename... As>
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struct FunctionTypeTraits<R (C::*)(As...) const>
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: detail::FunctionTypeTraitsHelper<R, As...>
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{
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};
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#ifdef NS_HAVE_STDCALL
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// Specialization for __stdcall free functions.
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template<typename R, typename... As>
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struct FunctionTypeTraits<R NS_STDCALL(As...)>
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: detail::FunctionTypeTraitsHelper<R, As...>
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{
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};
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// Specialization for __stdcall non-const member functions.
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template<typename C, typename R, typename... As>
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struct FunctionTypeTraits<R (NS_STDCALL C::*)(As...)>
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: detail::FunctionTypeTraitsHelper<R, As...>
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{
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};
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// Specialization for __stdcall const member functions.
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template<typename C, typename R, typename... As>
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struct FunctionTypeTraits<R (NS_STDCALL C::*)(As...) const>
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: detail::FunctionTypeTraitsHelper<R, As...>
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{
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};
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#endif // NS_HAVE_STDCALL
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} // namespace mozilla
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#endif // mozilla_FunctionTypeTraits_h
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