Add function_ref (open-telemetry#61)

* Add function_ref * Add function_ref
TuckTuckFloof · Apr 24, 2020 · 7479aa1 · 7479aa1
1 parent 9e2418b
commit 7479aa1
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Showing 4 changed files with 129 additions and 2 deletions.
diff --git a/api/include/opentelemetry/nostd/function_ref.h b/api/include/opentelemetry/nostd/function_ref.h
@@ -0,0 +1,83 @@
+#pragma once
+
+#include <memory>
+#include <type_traits>
+
+#include "opentelemetry/version.h"
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace nostd
+{
+template <class Sig>
+class function_ref;
+
+/**
+ * Non-owning function reference that can be used as a more performant
+ * replacement for std::function when ownership sematics aren't needed.
+ *
+ * See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0792r0.html
+ *
+ * Based off of https://stackoverflow.com/a/39087660/4447365
+ */
+template <class R, class... Args>
+class function_ref<R(Args...)>
+{
+  void *callable_                = nullptr;
+  R (*invoker_)(void *, Args...) = nullptr;
+
+  template <class F>
+  using FunctionPointer = decltype(std::addressof(std::declval<F &>()));
+
+  template <class F>
+  void BindTo(F &f) noexcept
+  {
+    callable_ = static_cast<void *>(std::addressof(f));
+    invoker_  = [](void *callable_, Args... args) -> R {
+      return (*static_cast<FunctionPointer<F>>(callable_))(std::forward<Args>(args)...);
+    };
+  }
+
+  template <class R_in, class... Args_in>
+  void BindTo(R_in (*f)(Args_in...)) noexcept
+  {
+    using F = decltype(f);
+    if (f == nullptr)
+    {
+      return BindTo(nullptr);
+    }
+    callable_ = reinterpret_cast<void *>(f);
+    invoker_  = [](void *callable_, Args... args) -> R {
+      return (F(callable_))(std::forward<Args>(args)...);
+    };
+  }
+
+  void BindTo(std::nullptr_t) noexcept
+  {
+    callable_ = nullptr;
+    invoker_  = nullptr;
+  }
+
+public:
+  template <
+      class F,
+      typename std::enable_if<!std::is_same<function_ref, typename std::decay<F>::type>::value,
+                              int>::type = 0,
+      typename std::enable_if<
+          std::is_convertible<typename std::result_of<F &(Args...)>::type, R>::value,
+          int>::type = 0>
+  function_ref(F &&f)
+  {
+    BindTo(f);  // not forward
+  }
+
+  function_ref(std::nullptr_t) {}
+
+  function_ref(const function_ref &) noexcept = default;
+  function_ref(function_ref &&) noexcept      = default;
+
+  R operator()(Args... args) const { return invoker_(callable_, std::forward<Args>(args)...); }
+
+  explicit operator bool() const { return invoker_; }
+};
+}  // namespace nostd
+OPENTELEMETRY_END_NAMESPACE
diff --git a/api/test/nostd/BUILD b/api/test/nostd/BUILD
@@ -1,3 +1,14 @@
+cc_test(
+    name = "function_ref_test",
+    srcs = [
+        "function_ref_test.cc",
+    ],
+    deps = [
+        "//api",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "string_view_test",
     srcs = [

diff --git a/api/test/nostd/CMakeLists.txt b/api/test/nostd/CMakeLists.txt
@@ -1,7 +1,7 @@
 include(GoogleTest)
 
-foreach(testname string_view_test unique_ptr_test utility_test span_test
-                 shared_ptr_test)
+foreach(testname function_ref_test string_view_test unique_ptr_test
+                 utility_test span_test shared_ptr_test)
   add_executable(${testname} "${testname}.cc")
   target_link_libraries(${testname} ${GTEST_BOTH_LIBRARIES}
                         ${CMAKE_THREAD_LIBS_INIT} opentelemetry_api)

diff --git a/api/test/nostd/function_ref_test.cc b/api/test/nostd/function_ref_test.cc
@@ -0,0 +1,33 @@
+#include "opentelemetry/nostd/function_ref.h"
+
+#include <gtest/gtest.h>
+using namespace opentelemetry::nostd;
+
+int Call(function_ref<int()> f)
+{
+  return f();
+}
+
+int Return3()
+{
+  return 3;
+}
+
+TEST(FunctionRefTest, Call)
+{
+  int x = 9;
+
+  auto f = [&] { return x; };
+  EXPECT_EQ(Call(f), 9);
+
+  EXPECT_EQ(Call(Return3), 3);
+}
+
+TEST(FunctionRefTest, BoolConversion)
+{
+  auto f = [] { return 0; };
+  function_ref<int()> fref1{nullptr};
+  function_ref<int()> fref2{f};
+  EXPECT_TRUE(!static_cast<bool>(fref1));
+  EXPECT_TRUE(static_cast<bool>(fref2));
+}