FOLLY_INVOKE_MEMBER

Summary: A lambda-expression parallel to `FOLLY_CREATE_MEMBER_INVOKER_SUITE`.

Reviewed By: Gownta

Differential Revision: D32601861

fbshipit-source-id: 8bb5876d800215346b19c42090a1cc13b6aa9042

folly/Portability.h

@@ -541,6 +541,19 @@ constexpr auto kCpplibVer = 0;
 #define FOLLY_STORAGE_CONSTEXPR constexpr
 #endif
 
+//  FOLLY_LAMBDA_CONSTEXPR
+//
+//  C++17 permits a lambda to be marked constexpr.
+//
+//  Example:
+//
+//    []() FOLLY_LAMBDA_CONSTEXPR { /* ... */ }
+#if FOLLY_CPLUSPLUS >= 201703L
+#define FOLLY_LAMBDA_CONSTEXPR constexpr
+#else
+#define FOLLY_LAMBDA_CONSTEXPR
+#endif
+
 #if __cplusplus >= 201703L
 // folly::coro requires C++17 support
 #if defined(_WIN32) && defined(__clang__) && !defined(LLVM_COROUTINES)

folly/functional/Invoke.h

@@ -32,6 +32,10 @@
 #include <folly/Utility.h>
 #include <folly/lang/CustomizationPoint.h>
 
+#define FOLLY_DETAIL_FORWARD_REF(a) static_cast<decltype(a)&&>(a)
+#define FOLLY_DETAIL_FORWARD_BODY(e) \
+  noexcept(noexcept(e))->decltype(e) { return e; }
+
 /**
  *  include or backport:
  *  * std::invoke
@@ -461,6 +465,36 @@ struct invoke_traits : detail::invoke_traits_base<I> {
   FOLLY_MAYBE_UNUSED FOLLY_INLINE_VARIABLE constexpr membername##_fn \
       membername {}
 
+/***
+ *  FOLLY_INVOKE_MEMBER
+ *
+ *  An invoker expression resulting in an invocable which, when invoked, invokes
+ *  the member on the object with the given arguments.
+ *
+ *  Example:
+ *
+ *    FOLLY_INVOKE_MEMBER(find)(map, key)
+ *
+ *  Equivalent to:
+ *
+ *    map.find(key)
+ *
+ *  But also equivalent to:
+ *
+ *    std::invoke(FOLLY_INVOKE_MEMBER(find), map, key)
+ *
+ *  As an implementation detail, the resulting callable is a lambda. This has
+ *  two observable consequences.
+ *  * Since C++17 only, lambda invocations may be marked constexpr.
+ *  * Since C++20 only, lambda definitions may appear in an unevaluated context,
+ *    namely, in an operand to decltype, noexcept, sizeof, or typeid.
+ */
+#define FOLLY_INVOKE_MEMBER(membername)                 \
+  [](auto&& __folly_param_o, auto&&... __folly_param_a) \
+      FOLLY_LAMBDA_CONSTEXPR FOLLY_DETAIL_FORWARD_BODY( \
+          FOLLY_DETAIL_FORWARD_REF(__folly_param_o)     \
+              .membername(FOLLY_DETAIL_FORWARD_REF(__folly_param_a)...))
+
 /***
  *  FOLLY_CREATE_STATIC_MEMBER_INVOKER
  *

folly/functional/test/InvokeTest.cpp

@@ -275,6 +275,18 @@ TEST_F(InvokeTest, member_is_nothrow_invocable_r) {
   EXPECT_FALSE((traits::is_nothrow_invocable_r_v<int, Obj, int>));
 }
 
+TEST_F(InvokeTest, invoke_member) {
+  auto test = FOLLY_INVOKE_MEMBER(test);
+
+  Obj fn;
+
+  EXPECT_TRUE(noexcept(test(fn, 1, 2)));
+  EXPECT_FALSE(noexcept(test(fn, 1, "2")));
+
+  EXPECT_EQ('a', test(fn, 1, 2));
+  EXPECT_EQ(17, test(fn, 1, "2"));
+}
+
 namespace {
 
 namespace invoker {