From 56c5fc620161e12844b9b92d06f8954b29f40bf4 Mon Sep 17 00:00:00 2001 From: Evan Brown Date: Fri, 28 Apr 2023 13:15:48 -0700 Subject: Introduce IfConstexpr(Else) utilities for use internally in abseil library implementation. These functions are meant to allow for avoiding complex SFINAE pre-C++17. PiperOrigin-RevId: 527968428 Change-Id: I6524206c9dc50663d7c38abf6d0e8080ca93fc30 --- CMake/AbseilDll.cmake | 1 + absl/utility/BUILD.bazel | 23 +++++++++ absl/utility/CMakeLists.txt | 24 +++++++++ absl/utility/internal/if_constexpr.h | 70 ++++++++++++++++++++++++++ absl/utility/internal/if_constexpr_test.cc | 79 ++++++++++++++++++++++++++++++ 5 files changed, 197 insertions(+) create mode 100644 absl/utility/internal/if_constexpr.h create mode 100644 absl/utility/internal/if_constexpr_test.cc diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake index 2a331435..32b51411 100644 --- a/CMake/AbseilDll.cmake +++ b/CMake/AbseilDll.cmake @@ -430,6 +430,7 @@ set(ABSL_INTERNAL_DLL_FILES "types/span.h" "types/internal/span.h" "types/variant.h" + "utility/internal/if_constexpr.h" "utility/utility.h" "debugging/leak_check.cc" ) diff --git a/absl/utility/BUILD.bazel b/absl/utility/BUILD.bazel index ca4bc0a6..061f4c5b 100644 --- a/absl/utility/BUILD.bazel +++ b/absl/utility/BUILD.bazel @@ -52,3 +52,26 @@ cc_test( "@com_google_googletest//:gtest_main", ], ) + +cc_library( + name = "if_constexpr", + hdrs = [ + "internal/if_constexpr.h", + ], + copts = ABSL_DEFAULT_COPTS, + linkopts = ABSL_DEFAULT_LINKOPTS, + deps = [ + "//absl/base:config", + ], +) + +cc_test( + name = "if_constexpr_test", + srcs = ["internal/if_constexpr_test.cc"], + copts = ABSL_TEST_COPTS, + linkopts = ABSL_DEFAULT_LINKOPTS, + deps = [ + ":if_constexpr", + "@com_google_googletest//:gtest_main", + ], +) diff --git a/absl/utility/CMakeLists.txt b/absl/utility/CMakeLists.txt index 865b758f..27ee0de5 100644 --- a/absl/utility/CMakeLists.txt +++ b/absl/utility/CMakeLists.txt @@ -42,3 +42,27 @@ absl_cc_test( absl::strings GTest::gmock_main ) + +absl_cc_library( + NAME + if_constexpr + HDRS + "internal/if_constexpr.h" + COPTS + ${ABSL_DEFAULT_COPTS} + DEPS + absl::config + PUBLIC +) + +absl_cc_test( + NAME + if_constexpr_test + SRCS + "internal/if_constexpr_test.cc" + COPTS + ${ABSL_TEST_COPTS} + DEPS + absl::if_constexpr + GTest::gmock_main +) diff --git a/absl/utility/internal/if_constexpr.h b/absl/utility/internal/if_constexpr.h new file mode 100644 index 00000000..7a26311d --- /dev/null +++ b/absl/utility/internal/if_constexpr.h @@ -0,0 +1,70 @@ +// Copyright 2023 The Abseil Authors +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +// The IfConstexpr and IfConstexprElse utilities in this file are meant to be +// used to emulate `if constexpr` in pre-C++17 mode in library implementation. +// The motivation is to allow for avoiding complex SFINAE. +// +// The functions passed in must depend on the type(s) of the object(s) that +// require SFINAE. For example: +// template +// int MaybeFoo(T& t) { +// if constexpr (HasFoo::value) return t.foo(); +// return 0; +// } +// +// can be written in pre-C++17 as: +// +// template +// int MaybeFoo(T& t) { +// int i = 0; +// absl::utility_internal::IfConstexpr::value>( +// [&](const auto& fooer) { i = fooer.foo(); }, t); +// return i; +// } + +#ifndef ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_ +#define ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_ + +#include +#include + +#include "absl/base/config.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN + +namespace utility_internal { + +template +auto IfConstexprElse(TrueFunc&& true_func, FalseFunc&& false_func, + Args&&... args) { + return std::get(std::forward_as_tuple( + std::forward(false_func), std::forward(true_func)))( + std::forward(args)...); +} + +template +void IfConstexpr(Func&& func, Args&&... args) { + IfConstexprElse(std::forward(func), [](auto&&...){}, + std::forward(args)...); +} + +} // namespace utility_internal + +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_ diff --git a/absl/utility/internal/if_constexpr_test.cc b/absl/utility/internal/if_constexpr_test.cc new file mode 100644 index 00000000..d1ee7236 --- /dev/null +++ b/absl/utility/internal/if_constexpr_test.cc @@ -0,0 +1,79 @@ +// Copyright 2023 The Abseil Authors +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +#include "absl/utility/internal/if_constexpr.h" + +#include + +#include "gtest/gtest.h" + +namespace { + +struct Empty {}; +struct HasFoo { + int foo() const { return 1; } +}; + +TEST(IfConstexpr, Basic) { + int i = 0; + absl::utility_internal::IfConstexpr( + [&](const auto& t) { i = t.foo(); }, Empty{}); + EXPECT_EQ(i, 0); + + absl::utility_internal::IfConstexpr( + [&](const auto& t) { i = t.foo(); }, HasFoo{}); + EXPECT_EQ(i, 0); + + absl::utility_internal::IfConstexpr( + [&](const auto& t) { i = t.foo(); }, HasFoo{}); + EXPECT_EQ(i, 1); +} + +TEST(IfConstexprElse, Basic) { + EXPECT_EQ(absl::utility_internal::IfConstexprElse( + [&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; }, + Empty{}), 2); + + EXPECT_EQ(absl::utility_internal::IfConstexprElse( + [&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; }, + HasFoo{}), 2); + + EXPECT_EQ(absl::utility_internal::IfConstexprElse( + [&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; }, + HasFoo{}), 1); +} + +struct HasFooRValue { + int foo() && { return 1; } +}; +struct RValueFunc { + void operator()(HasFooRValue&& t) && { *i = std::move(t).foo(); } + + int* i = nullptr; +}; + +TEST(IfConstexpr, RValues) { + int i = 0; + RValueFunc func = {&i}; + absl::utility_internal::IfConstexpr( + std::move(func), HasFooRValue{}); + EXPECT_EQ(i, 0); + + func = RValueFunc{&i}; + absl::utility_internal::IfConstexpr( + std::move(func), HasFooRValue{}); + EXPECT_EQ(i, 1); +} + +} // namespace -- cgit v1.2.3