.. _program_listing_file_include_opentelemetry_nostd_variant.h:

Program Listing for File variant.h
==================================

|exhale_lsh| :ref:`Return to documentation for file <file_include_opentelemetry_nostd_variant.h>` (``include/opentelemetry/nostd/variant.h``)

.. |exhale_lsh| unicode:: U+021B0 .. UPWARDS ARROW WITH TIP LEFTWARDS

.. code-block:: cpp

   // Copyright 2020, OpenTelemetry 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
   //
   //     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
   
   #ifdef HAVE_CPP_STDLIB
   #  include "opentelemetry/std/variant.h"
   #elif defined(HAVE_ABSEIL_VARIANT)
   // TODO: we use a LOCAL snapshot of Abseil that is known to compile with Visual Studio 2015,
   // header-only, without compiling the actual Abseil binary. As Abseil moves on to new
   // toolchains, it may drop support for Visual Studio 2015 in future versions. Perhaps a good
   // option would be to determine if Abseil is available, then use the outside implementation,
   // but it is not guaranteed to be still compatible with 2015. Thus, the local snapshot here.
   #  include "./absl/types/variant.h"
   
   OPENTELEMETRY_BEGIN_NAMESPACE
   namespace nostd
   {
   using absl::get;
   using absl::holds_alternative;
   using absl::variant;
   using absl::visit;
   
   // nostd::bad_variant_access
   class bad_variant_access : public std::exception
   {
   public:
     virtual const char *what() const noexcept override { return "bad_variant_access"; }
   };
   
   [[noreturn]] inline void throw_bad_variant_access()
   {
     throw bad_variant_access{};
   }
   
   #  if __EXCEPTIONS
   #    define THROW_BAD_VARIANT_ACCESS opentelemetry::nostd::throw_bad_variant_access()
   #  else
   #    define THROW_BAD_VARIANT_ACCESS std::terminate()
   #  endif
   
   }  // namespace nostd
   OPENTELEMETRY_END_NAMESPACE
   
   // TODO: when we compile with Abseil variant we do not have to link the entire Abseil library.
   // The only missing funcion is the variant exception handler here, which we statically link
   // as-needed. This *should not* clash with the standard handler. However, it would make sense
   // to consider a build-time flag that allows to route the handling of ThrowBadVariantAccess to
   // the actual Abseil, in case if a product using OpenTelemetry is compiled with Abseil. That
   // way the standard Abseil exception handler can be used to catch OpenTelemetry exceptions.
   namespace absl
   {
   namespace variant_internal
   {
   static void ThrowBadVariantAccess()
   {
     THROW_BAD_VARIANT_ACCESS;
   };
   };  // namespace variant_internal
   };  // namespace absl
   
   #else
   #  include "opentelemetry/nostd/mpark/variant.h"
   #endif