extern access

Summary: A mechanism to make it easier conditionally to access possibly-defined names, which can be a bit tricky especially without `if constexpr`.

Reviewed By: luciang

Differential Revision: D27434753

fbshipit-source-id: 4c9aaebe89b354aa94f4f370c8fbdc8456b0e7f7

CMakeLists.txt

@@ -784,6 +784,7 @@ if (BUILD_TESTS)
       TEST lang_cast_test SOURCES CastTest.cpp
       TEST lang_checked_math_test SOURCES CheckedMathTest.cpp
       TEST lang_exception_test SOURCES ExceptionTest.cpp
+      TEST lang_extern_test SOURCES ExternTest.cpp
       TEST lang_launder_test SOURCES LaunderTest.cpp
       TEST lang_ordering_test SOURCES OrderingTest.cpp
       TEST lang_pretty_test SOURCES PrettyTest.cpp

folly/lang/Extern.h

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * 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
+ *
+ *     http://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.
+ */
+
+#pragma once
+
+#include <type_traits>
+
+//  FOLLY_CREATE_EXTERN_ACCESSOR
+//
+//  Defines a variable template which holds the address of an object or null
+//  given a condition which identifies whether that object is expected to have
+//  a definition.
+//
+//  Example:
+//
+//      extern void foobar(int); // may be defined in another library
+//      inline constexpr bool has_foobar = // ...
+//
+//      namespace myapp {
+//      FOLLY_CREATE_EXTERN_ACCESSOR(access_foobar_v, ::foobar);
+//      void try_foobar(int num) {
+//        if (auto ptr = access_foobar_v<has_foobar>) {
+//          ptr(num);
+//        }
+//      }
+//      }
+//
+//  Remarks:
+//
+//  This can be done more simply using weak symbols. But weak symbols are non-
+//  portable and the rules around the use of weak symbols are complex.
+//
+//  This can be done more simply using if-constexpr. But if-constexpr requires
+//  C++17 and there remain specific narrow cases to be supported on prior
+//  versions of C++.
+//
+//  TODO: Remove after C++17.
+#define FOLLY_CREATE_EXTERN_ACCESSOR(varname, name)                \
+  struct __folly_extern_accessor_##varname {                       \
+    template <bool E, typename N, ::std::enable_if_t<E, int> = 0>  \
+    static constexpr N* get() noexcept {                           \
+      return &name;                                                \
+    }                                                              \
+    template <bool E, typename N, ::std::enable_if_t<!E, int> = 0> \
+    static constexpr N* get() noexcept {                           \
+      return nullptr;                                              \
+    }                                                              \
+  };                                                               \
+  template <bool E, typename N = decltype(name)>                   \
+  FOLLY_INLINE_VARIABLE constexpr N* varname =                     \
+      __folly_extern_accessor_##varname::get<E, N>()

folly/lang/test/ExternTest.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * 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
+ *
+ *     http://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 <folly/lang/Extern.h>
+
+#include <folly/portability/GTest.h>
+
+namespace folly::extern_test {
+extern int foobar(int);
+}
+
+static int hasdef(int a) {
+  return -a;
+}
+
+FOLLY_CREATE_EXTERN_ACCESSOR(access_foobar_v, folly::extern_test::foobar);
+FOLLY_CREATE_EXTERN_ACCESSOR(access_hasdef_v, hasdef);
+
+class ExternTest : public testing::Test {};
+
+TEST_F(ExternTest, example) {
+  EXPECT_EQ(nullptr, access_foobar_v<false>);
+  EXPECT_EQ(nullptr, access_hasdef_v<false>);
+  EXPECT_EQ(&hasdef, access_hasdef_v<true>);
+  EXPECT_EQ(-17, access_hasdef_v<true>(17));
+}