text: // Protocol Buffers - Google's data interchange format
  // Copyright 2014 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  // copyright notice, this list of conditions and the following disclaimer
  // in the documentation and/or other materials provided with the
  // distribution.
  //     * Neither the name of Google Inc. nor the names of its
  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  #ifndef GOOGLE_PROTOBUF_CASTS_H__
  #define GOOGLE_PROTOBUF_CASTS_H__
  
  #include <type_traits>
  
  #include <google/protobuf/stubs/common.h>
  
  #include <google/protobuf/port_def.inc>
  
  namespace google {
  namespace protobuf {
  namespace internal {
  
  // Use implicit_cast as a safe version of static_cast or const_cast
  // for upcasting in the type hierarchy (i.e. casting a pointer to Foo
  // to a pointer to SuperclassOfFoo or casting a pointer to Foo to
  // a const pointer to Foo).
  // When you use implicit_cast, the compiler checks that the cast is safe.
  // Such explicit implicit_casts are necessary in surprisingly many
  // situations where C++ demands an exact type match instead of an
  // argument type convertable to a target type.
  //
  // The From type can be inferred, so the preferred syntax for using
  // implicit_cast is the same as for static_cast etc.:
  //
  //   implicit_cast<ToType>(expr)
  //
  // implicit_cast would have been part of the C++ standard library,
  // but the proposal was submitted too late.  It will probably make
  // its way into the language in the future.
  template<typename To, typename From>
  inline To implicit_cast(From const &f) {
    return f;
  }
  
  // When you upcast (that is, cast a pointer from type Foo to type
  // SuperclassOfFoo), it's fine to use implicit_cast<>, since upcasts
  // always succeed.  When you downcast (that is, cast a pointer from
  // type Foo to type SubclassOfFoo), static_cast<> isn't safe, because
  // how do you know the pointer is really of type SubclassOfFoo?  It
  // could be a bare Foo, or of type DifferentSubclassOfFoo.  Thus,
  // when you downcast, you should use this macro.  In debug mode, we
  // use dynamic_cast<> to double-check the downcast is legal (we die
  // if it's not).  In normal mode, we do the efficient static_cast<>
  // instead.  Thus, it's important to test in debug mode to make sure
  // the cast is legal!
  //    This is the only place in the code we should use dynamic_cast<>.
  // In particular, you SHOULDN'T be using dynamic_cast<> in order to
  // do RTTI (eg code like this:
  //    if (dynamic_cast<Subclass1>(foo)) HandleASubclass1Object(foo);
  //    if (dynamic_cast<Subclass2>(foo)) HandleASubclass2Object(foo);
  // You should design the code some other way not to need this.
  
  template<typename To, typename From>     // use like this: down_cast<T*>(foo);
  inline To down_cast(From* f) {                   // so we only accept pointers
    // Ensures that To is a sub-type of From *.  This test is here only
    // for compile-time type checking, and has no overhead in an
    // optimized build at run-time, as it will be optimized away
    // completely.
    if (false) {
      implicit_cast<From*, To>(0);
    }
  
  #if !defined(NDEBUG) && PROTOBUF_RTTI
    assert(f == nullptr || dynamic_cast<To>(f) != nullptr);  // RTTI: debug mode only!
  #endif
    return static_cast<To>(f);
  }
  
  template<typename To, typename From>    // use like this: down_cast<T&>(foo);
  inline To down_cast(From& f) {
    typedef typename std::remove_reference<To>::type* ToAsPointer;
    // Ensures that To is a sub-type of From *.  This test is here only
    // for compile-time type checking, and has no overhead in an
    // optimized build at run-time, as it will be optimized away
    // completely.
    if (false) {
      implicit_cast<From*, ToAsPointer>(0);
    }
  
  #if !defined(NDEBUG) && PROTOBUF_RTTI
    // RTTI: debug mode only!
    assert(dynamic_cast<ToAsPointer>(&f) != nullptr);
  #endif
    return *static_cast<ToAsPointer>(&f);
  }
  
  template<typename To, typename From>
  inline To bit_cast(const From& from) {
    GOOGLE_COMPILE_ASSERT(sizeof(From) == sizeof(To),
                          bit_cast_with_different_sizes);
    To dest;
    memcpy(&dest, &from, sizeof(dest));
    return dest;
  }
  
  }  // namespace internal
  
  // We made these internal so that they would show up as such in the docs,
  // but we don't want to stick "internal::" in front of them everywhere.
  using internal::implicit_cast;
  using internal::down_cast;
  using internal::bit_cast;
  
  }  // namespace protobuf
  }  // namespace google
  
  #include <google/protobuf/port_undef.inc>
  
  #endif  // GOOGLE_PROTOBUF_CASTS_H__
  