Add absl::bind_back

PiperOrigin-RevId: 905242962
Change-Id: I7ba08d1b610db5be723b2313e11f7a5d4d7ddb6e

CMake/AbseilDll.cmake

@@ -153,9 +153,11 @@ set(ABSL_INTERNAL_DLL_FILES
   "debugging/symbolize.cc"
   "debugging/symbolize.h"
   "functional/any_invocable.h"
+  "functional/bind_back.h"
   "functional/bind_front.h"
   "functional/function_ref.h"
   "functional/internal/any_invocable.h"
+  "functional/internal/back_binder.h"
   "functional/internal/front_binder.h"
   "functional/internal/function_ref.h"
   "functional/overload.h"

absl/functional/BUILD.bazel

@@ -70,6 +70,32 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "bind_back",
+    srcs = ["internal/back_binder.h"],
+    hdrs = ["bind_back.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/container:compressed_tuple",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "bind_back_test",
+    srcs = ["bind_back_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bind_back",
+        "//absl/memory",
+        "@googletest//:gtest",
+        "@googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "bind_front",
     srcs = ["internal/front_binder.h"],

absl/functional/CMakeLists.txt

@@ -50,6 +50,35 @@ absl_cc_test(
     GTest::gmock_main
 )
 
+absl_cc_library(
+  NAME
+    bind_back
+  SRCS
+    "internal/back_binder.h"
+  HDRS
+    "bind_back.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::compressed_tuple
+    absl::config
+    absl::utility
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    bind_back_test
+  SRCS
+    "bind_back_test.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::bind_back
+    absl::memory
+    GTest::gmock_main
+)
+
 absl_cc_library(
   NAME
     bind_front

absl/functional/bind_back.h

@@ -0,0 +1,79 @@
+// Copyright 2026 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.
+//
+// -----------------------------------------------------------------------------
+// File: bind_back.h
+// -----------------------------------------------------------------------------
+//
+// `absl::bind_back()` returns a functor by binding a number of arguments to
+// the back of a provided (usually more generic) functor. Unlike `std::bind`,
+// it does not require the use of argument placeholders. The simpler syntax of
+// `absl::bind_back()` allows you to avoid known misuses with `std::bind()`.
+//
+// `absl::bind_back()` is meant as a drop-in replacement for C++23's
+// `std::bind_back()`, which similarly resolves these issues with
+// `std::bind()`. Both `bind_back()` alternatives, unlike `std::bind()`, allow
+// partial function application. (See
+// https://en.wikipedia.org/wiki/Partial_application).
+
+#ifndef ABSL_FUNCTIONAL_BIND_BACK_H_
+#define ABSL_FUNCTIONAL_BIND_BACK_H_
+
+#ifdef __has_include
+#if __has_include(<version>)
+#include <version>
+#endif
+#endif
+
+#if defined(__cpp_lib_bind_back) && __cpp_lib_bind_back >= 202202L
+#include <functional>  // For std::bind_back.
+#endif
+
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/functional/internal/back_binder.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// bind_back()
+//
+// Binds the last N arguments of an invocable object and stores them by value.
+//
+// Like `std::bind()`, `absl::bind_back()` is implicitly convertible to
+// `std::function`.  In particular, it may be used as a simpler replacement for
+// `std::bind()` in most cases, as it does not require placeholders to be
+// specified. More importantly, it provides more reliable correctness guarantees
+// than `std::bind()`; while `std::bind()` will silently ignore passing more
+// parameters than expected, for example, `absl::bind_back()` will report such
+// mis-uses as errors. In C++23, `absl::bind_back` is replaced by
+// `std::bind_back`.
+//
+#if defined(__cpp_lib_bind_back) && __cpp_lib_bind_back >= 202202L
+using std::bind_back;
+#else
+template <class F, class... BoundArgs>
+constexpr functional_internal::bind_back_t<F, BoundArgs...> bind_back(
+    F&& func, BoundArgs&&... args) {
+  return functional_internal::bind_back_t<F, BoundArgs...>(
+      absl::in_place, std::forward<F>(func), std::forward<BoundArgs>(args)...);
+}
+#endif
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_BIND_BACK_H_

absl/functional/bind_back_test.cc

@@ -0,0 +1,237 @@
+// Copyright 2026 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/functional/bind_back.h"
+
+#include <stddef.h>
+
+#include <functional>
+#include <memory>
+#include <string>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/memory/memory.h"
+
+namespace {
+
+char CharAt(const char* s, size_t index) { return s[index]; }
+
+TEST(BindTest, Basics) {
+  EXPECT_EQ('C', absl::bind_back(CharAt)("ABC", 2));
+  EXPECT_EQ('C', absl::bind_back(CharAt, 2)("ABC"));
+  EXPECT_EQ('C', absl::bind_back(CharAt, "ABC", 2)());
+}
+
+TEST(BindTest, Lambda) {
+  auto lambda = [](int x, int y, int z) { return x + y + z; };
+  EXPECT_EQ(6, absl::bind_back(lambda)(1, 2, 3));
+  EXPECT_EQ(6, absl::bind_back(lambda, 3)(1, 2));
+  EXPECT_EQ(6, absl::bind_back(lambda, 2, 3)(1));
+  EXPECT_EQ(6, absl::bind_back(lambda, 1, 2, 3)());
+}
+
+struct Functor {
+  std::string operator()() & { return "&"; }
+  std::string operator()() const& { return "const&"; }
+  std::string operator()() && { return "&&"; }
+  std::string operator()() const&& { return "const&&"; }
+};
+
+TEST(BindTest, PerfectForwardingOfBoundArgs) {
+  auto f = absl::bind_back(Functor());
+  const auto& cf = f;
+  EXPECT_EQ("&", f());
+  EXPECT_EQ("const&", cf());
+  EXPECT_EQ("&&", std::move(f)());
+  EXPECT_EQ("const&&", std::move(cf)());
+}
+
+struct ArgDescribe {
+  std::string operator()(int&) const { return "&"; }             // NOLINT
+  std::string operator()(const int&) const { return "const&"; }  // NOLINT
+  std::string operator()(int&&) const { return "&&"; }
+  std::string operator()(const int&&) const { return "const&&"; }
+};
+
+TEST(BindTest, PerfectForwardingOfFreeArgs) {
+  ArgDescribe f;
+  int i;
+  EXPECT_EQ("&", absl::bind_back(f)(static_cast<int&>(i)));
+  EXPECT_EQ("const&", absl::bind_back(f)(static_cast<const int&>(i)));
+  EXPECT_EQ("&&", absl::bind_back(f)(static_cast<int&&>(i)));
+  EXPECT_EQ("const&&", absl::bind_back(f)(static_cast<const int&&>(i)));
+}
+
+struct NonCopyableFunctor {
+  NonCopyableFunctor() = default;
+  NonCopyableFunctor(const NonCopyableFunctor&) = delete;
+  NonCopyableFunctor& operator=(const NonCopyableFunctor&) = delete;
+  const NonCopyableFunctor* operator()() const { return this; }
+};
+
+TEST(BindTest, RefToFunctor) {
+  // It won't copy/move the functor and use the original object.
+  NonCopyableFunctor ncf;
+  auto bound_ncf = absl::bind_back(std::ref(ncf));
+  auto bound_ncf_copy = bound_ncf;
+  EXPECT_EQ(&ncf, bound_ncf_copy());
+}
+
+struct Struct {
+  std::string value;
+};
+
+TEST(BindTest, StoreByCopy) {
+  Struct s = {"hello"};
+  auto f = absl::bind_back(&Struct::value, s);
+  auto g = f;
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ("hello", g());
+  EXPECT_NE(&s.value, &f());
+  EXPECT_NE(&s.value, &g());
+  EXPECT_NE(&g(), &f());
+}
+
+struct NonCopyable {
+  explicit NonCopyable(const std::string& s) : value(s) {}
+  NonCopyable(const NonCopyable&) = delete;
+  NonCopyable& operator=(const NonCopyable&) = delete;
+
+  std::string value;
+};
+
+const std::string& GetNonCopyableValue(const NonCopyable& n) { return n.value; }
+
+TEST(BindTest, StoreByRef) {
+  NonCopyable s("hello");
+  auto f = absl::bind_back(&GetNonCopyableValue, std::ref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);  // NOLINT
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+TEST(BindTest, StoreByCRef) {
+  NonCopyable s("hello");
+  auto f = absl::bind_back(&GetNonCopyableValue, std::cref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);  // NOLINT
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+const std::string& GetNonCopyableValueByWrapper(
+    std::reference_wrapper<NonCopyable> n) {
+  return n.get().value;
+}
+
+TEST(BindTest, StoreByRefInvokeByWrapper) {
+  NonCopyable s("hello");
+  auto f = absl::bind_back(GetNonCopyableValueByWrapper, std::ref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+TEST(BindTest, StoreByPointer) {
+  NonCopyable s("hello");
+  auto f = absl::bind_back(&NonCopyable::value, &s);
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+}
+
+struct MyStruct {
+  int x;
+  int Add(int y) const { return x + y; }
+};
+
+TEST(BindTest, MemberFunctionFreeInstance) {
+  MyStruct s{10};
+  auto f = absl::bind_back(&MyStruct::Add, 5);
+  EXPECT_EQ(f(s), 15);
+}
+
+int Sink(std::unique_ptr<int> p) { return *p; }
+
+std::unique_ptr<int> Factory(int n) { return absl::make_unique<int>(n); }
+
+TEST(BindTest, NonCopyableArg) {
+  EXPECT_EQ(42, absl::bind_back(Sink)(absl::make_unique<int>(42)));
+  EXPECT_EQ(42, absl::bind_back(Sink, absl::make_unique<int>(42))());
+}
+
+TEST(BindTest, NonCopyableResult) {
+  EXPECT_THAT(absl::bind_back(Factory)(42), ::testing::Pointee(42));
+  EXPECT_THAT(absl::bind_back(Factory, 42)(), ::testing::Pointee(42));
+}
+
+// is_copy_constructible<FalseCopyable<unique_ptr<T>> is true but an attempt to
+// instantiate the copy constructor leads to a compile error. This is similar
+// to how standard containers behave.
+template <class T>
+struct FalseCopyable {
+  FalseCopyable() = default;
+  FalseCopyable(const FalseCopyable& other) : m(other.m) {}
+  FalseCopyable(FalseCopyable&& other) : m(std::move(other.m)) {}
+  T m;
+};
+
+int GetMember(FalseCopyable<std::unique_ptr<int>> x) { return *x.m; }
+
+TEST(BindTest, WrappedMoveOnly) {
+  FalseCopyable<std::unique_ptr<int>> x;
+  x.m = absl::make_unique<int>(42);
+  auto f = absl::bind_back(&GetMember, std::move(x));
+  EXPECT_EQ(42, std::move(f)());
+}
+
+int Plus(int a, int b) { return a + b; }
+
+TEST(BindTest, ConstExpr) {
+  constexpr auto f = absl::bind_back(CharAt);
+  EXPECT_EQ(f("ABC", 1), 'B');
+  static constexpr int five = 5;
+  constexpr auto plus5 = absl::bind_back(Plus, five);
+  EXPECT_EQ(plus5(1), 6);
+
+  static constexpr char data[] = "DEF";
+  constexpr auto g = absl::bind_back(CharAt, 1);
+  EXPECT_EQ(g(data), 'E');
+}
+
+struct ManglingCall {
+  int operator()(int, double, std::string) const { return 0; }
+};
+
+TEST(BindTest, Mangling) {
+  // We just want to generate a particular instantiation to see its mangling.
+  absl::bind_back(ManglingCall{}, 3.3, "A")(1);
+}
+
+}  // namespace

absl/functional/internal/back_binder.h

@@ -0,0 +1,95 @@
+// Copyright 2026 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.
+
+// Implementation details for `absl::bind_back()`.
+
+#ifndef ABSL_FUNCTIONAL_INTERNAL_BACK_BINDER_H_
+#define ABSL_FUNCTIONAL_INTERNAL_BACK_BINDER_H_
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/container/internal/compressed_tuple.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace functional_internal {
+
+// Invoke the method, expanding the tuple of bound arguments.
+template <class R, class Tuple, size_t... Idx, class... Args>
+constexpr R ApplyBack(Tuple&& bound, absl::index_sequence<Idx...>,
+                      Args&&... free) {
+  return std::invoke(std::forward<Tuple>(bound).template get<0>(),
+                     std::forward<Args>(free)...,
+                     std::forward<Tuple>(bound).template get<Idx + 1>()...);
+}
+
+template <class F, class... BoundArgs>
+class BackBinder {
+  using BoundArgsT = absl::container_internal::CompressedTuple<F, BoundArgs...>;
+  using Idx = absl::make_index_sequence<sizeof...(BoundArgs)>;
+
+  BoundArgsT bound_args_;
+
+ public:
+  template <class... Ts>
+  constexpr explicit BackBinder(absl::in_place_t, Ts&&... ts)
+      : bound_args_(std::forward<Ts>(ts)...) {}
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<F&, FreeArgs&&..., BoundArgs&...>>
+  constexpr R operator()(FreeArgs&&... free_args) & {
+    return functional_internal::ApplyBack<R>(
+        bound_args_, Idx(), std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<const F&, FreeArgs&&...,
+                                           const BoundArgs&...>>
+  constexpr R operator()(FreeArgs&&... free_args) const& {
+    return functional_internal::ApplyBack<R>(
+        bound_args_, Idx(), std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<F&&, FreeArgs&&..., BoundArgs&&...>>
+  constexpr R operator()(FreeArgs&&... free_args) && {
+    // This overload is called when *this is an rvalue. If some of the bound
+    // arguments are stored by value or rvalue reference, we move them.
+    return functional_internal::ApplyBack<R>(
+        std::move(bound_args_), Idx(), std::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = std::invoke_result_t<const F&&, FreeArgs&&...,
+                                           const BoundArgs&&...>>
+  constexpr R operator()(FreeArgs&&... free_args) const&& {
+    // This overload is called when *this is an rvalue. If some of the bound
+    // arguments are stored by value or rvalue reference, we move them.
+    return functional_internal::ApplyBack<R>(
+        std::move(bound_args_), Idx(), std::forward<FreeArgs>(free_args)...);
+  }
+};
+
+template <class F, class... BoundArgs>
+using bind_back_t = BackBinder<std::decay_t<F>, std::decay_t<BoundArgs>...>;
+
+}  // namespace functional_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_INTERNAL_BACK_BINDER_H_