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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
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‎CMake/AbseilDll.cmake

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@@ -430,6 +430,7 @@ set(ABSL_INTERNAL_DLL_FILES
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"types/span.h"
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"types/internal/span.h"
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"types/variant.h"
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"utility/internal/if_constexpr.h"
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"utility/utility.h"
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"debugging/leak_check.cc"
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)

‎absl/utility/BUILD.bazel

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@@ -52,3 +52,26 @@ cc_test(
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"@com_google_googletest//:gtest_main",
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],
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)
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cc_library(
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name = "if_constexpr",
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hdrs = [
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"internal/if_constexpr.h",
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],
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copts = ABSL_DEFAULT_COPTS,
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linkopts = ABSL_DEFAULT_LINKOPTS,
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deps = [
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"//absl/base:config",
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],
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)
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cc_test(
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name = "if_constexpr_test",
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srcs = ["internal/if_constexpr_test.cc"],
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copts = ABSL_TEST_COPTS,
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linkopts = ABSL_DEFAULT_LINKOPTS,
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deps = [
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":if_constexpr",
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"@com_google_googletest//:gtest_main",
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],
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)

‎absl/utility/CMakeLists.txt

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@@ -42,3 +42,27 @@ absl_cc_test(
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absl::strings
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GTest::gmock_main
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)
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absl_cc_library(
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NAME
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if_constexpr
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HDRS
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"internal/if_constexpr.h"
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COPTS
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${ABSL_DEFAULT_COPTS}
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DEPS
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absl::config
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PUBLIC
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)
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absl_cc_test(
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NAME
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if_constexpr_test
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SRCS
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"internal/if_constexpr_test.cc"
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COPTS
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${ABSL_TEST_COPTS}
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DEPS
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absl::if_constexpr
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GTest::gmock_main
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)

‎absl/utility/internal/if_constexpr.h

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// Copyright 2023 The Abseil Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// The IfConstexpr and IfConstexprElse utilities in this file are meant to be
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// used to emulate `if constexpr` in pre-C++17 mode in library implementation.
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// The motivation is to allow for avoiding complex SFINAE.
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//
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// The functions passed in must depend on the type(s) of the object(s) that
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// require SFINAE. For example:
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// template<typename T>
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// int MaybeFoo(T& t) {
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// if constexpr (HasFoo<T>::value) return t.foo();
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// return 0;
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// }
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//
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// can be written in pre-C++17 as:
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//
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// template<typename T>
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// int MaybeFoo(T& t) {
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// int i = 0;
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// absl::utility_internal::IfConstexpr<HasFoo<T>::value>(
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// [&](const auto& fooer) { i = fooer.foo(); }, t);
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// return i;
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// }
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#ifndef ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
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#define ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_
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#include <tuple>
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#include <utility>
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#include "absl/base/config.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace utility_internal {
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template <bool condition, typename TrueFunc, typename FalseFunc,
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typename... Args>
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auto IfConstexprElse(TrueFunc&& true_func, FalseFunc&& false_func,
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Args&&... args) {
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return std::get<condition>(std::forward_as_tuple(
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std::forward<FalseFunc>(false_func), std::forward<TrueFunc>(true_func)))(
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std::forward<Args>(args)...);
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}
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template <bool condition, typename Func, typename... Args>
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void IfConstexpr(Func&& func, Args&&... args) {
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IfConstexprElse<condition>(std::forward<Func>(func), [](auto&&...){},
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std::forward<Args>(args)...);
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}
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} // namespace utility_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_UTILITY_INTERNAL_IF_CONSTEXPR_H_

‎absl/utility/internal/if_constexpr_test.cc

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// Copyright 2023 The Abseil Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#include "absl/utility/internal/if_constexpr.h"
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#include <utility>
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#include "gtest/gtest.h"
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namespace {
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struct Empty {};
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struct HasFoo {
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int foo() const { return 1; }
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};
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TEST(IfConstexpr, Basic) {
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int i = 0;
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absl::utility_internal::IfConstexpr<false>(
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[&](const auto& t) { i = t.foo(); }, Empty{});
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EXPECT_EQ(i, 0);
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absl::utility_internal::IfConstexpr<false>(
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[&](const auto& t) { i = t.foo(); }, HasFoo{});
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EXPECT_EQ(i, 0);
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absl::utility_internal::IfConstexpr<true>(
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[&](const auto& t) { i = t.foo(); }, HasFoo{});
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EXPECT_EQ(i, 1);
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}
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TEST(IfConstexprElse, Basic) {
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EXPECT_EQ(absl::utility_internal::IfConstexprElse<false>(
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[&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; },
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Empty{}), 2);
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EXPECT_EQ(absl::utility_internal::IfConstexprElse<false>(
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[&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; },
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HasFoo{}), 2);
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EXPECT_EQ(absl::utility_internal::IfConstexprElse<true>(
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[&](const auto& t) { return t.foo(); }, [&](const auto&) { return 2; },
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HasFoo{}), 1);
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}
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struct HasFooRValue {
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int foo() && { return 1; }
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};
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struct RValueFunc {
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void operator()(HasFooRValue&& t) && { *i = std::move(t).foo(); }
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int* i = nullptr;
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};
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TEST(IfConstexpr, RValues) {
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int i = 0;
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RValueFunc func = {&i};
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absl::utility_internal::IfConstexpr<false>(
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std::move(func), HasFooRValue{});
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EXPECT_EQ(i, 0);
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func = RValueFunc{&i};
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absl::utility_internal::IfConstexpr<true>(
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std::move(func), HasFooRValue{});
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EXPECT_EQ(i, 1);
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}
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} // namespace

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