Line data    Source code

       1             : #pragma once
       2             : 
       3             : // One of the clients of this library is Envoy, a service (HTTP) proxy.
       4             : //
       5             : // Envoy uses Abseil as its base C++ library, and additionally builds in C++17
       6             : // mode.  Abseil has a build option to forward its `std::string_view` and
       7             : // `std::optional` equivalents to the actual standard types when C++17 is
       8             : // available.
       9             : //
      10             : // Envoy does not use this Abseil build option, due to incomplete support for
      11             : // the C++17 standard library on iOS 11.
      12             : //
      13             : // As a result, Envoy forbids use of `std::string_view` and `std::optional`,
      14             : // instead preferring Abseil's `absl::string_view` and `absl::optional`.
      15             : //
      16             : // This presents a problem for this library, since we use `std::string_view`
      17             : // and `std::optional` in the exported interface, i.e. in header files.
      18             : //
      19             : // As a workaround, Bazel (the build tool used by Envoy) builds of this library
      20             : // will define the `DD_USE_ABSEIL_FOR_ENVOY` preprocessor macro.  When this
      21             : // macro is defined, the library-specific `StringView` and `Optional` aliases
      22             : // will refer to the Abseil types.  When the macro is not defined, the
      23             : // library-specific aliases will refer to the standard types.
      24             : //
      25             : // This file defines `datadog::tracing::Optional`, a type template that is an
      26             : // alias for either `std::optional` or `absl::optional`.
      27             : 
      28             : #ifdef DD_USE_ABSEIL_FOR_ENVOY
      29             : // Abseil examples, including usage in Envoy, include Abseil headers in quoted
      30             : // style instead of angle bracket style, per Bazel's default build behavior.
      31             : #include "absl/types/optional.h"
      32             : #else
      33             : #include <optional>
      34             : #endif  // defined DD_USE_ABSEIL_FOR_ENVOY
      35             : 
      36             : #include <utility>  // std::forward
      37             : 
      38             : namespace datadog {
      39             : namespace tracing {
      40             : 
      41             : #ifdef DD_USE_ABSEIL_FOR_ENVOY
      42             : template <typename Value>
      43             : using Optional = absl::optional<Value>;
      44             : inline constexpr auto nullopt = absl::nullopt;
      45             : #else
      46             : template <typename Value>
      47             : using Optional = std::optional<Value>;
      48             : inline constexpr auto nullopt = std::nullopt;
      49             : #endif  // defined DD_USE_ABSEIL_FOR_ENVOY
      50             : 
      51             : // Return the first non-null argument value.  The last argument must not be
      52             : // `Optional`.
      53             : template <typename Value>
      54        1429 : auto value_or(Value&& value) {
      55        1429 :   return std::forward<Value>(value);
      56             : }
      57             : 
      58             : template <typename Value, typename... Rest>
      59        4287 : auto value_or(Optional<Value> maybe, Rest&&... rest) {
      60        4287 :   return maybe.value_or(value_or(std::forward<Rest>(rest)...));
      61             : }
      62             : 
      63             : }  // namespace tracing
      64             : }  // namespace datadog