casts.h

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#ifndef GOOGLE_PROTOBUF_CASTS_H__
#define GOOGLE_PROTOBUF_CASTS_H__

#include <google/protobuf/stubs/common.h>

#include <google/protobuf/port_def.inc>
#include <type_traits>

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 convertible 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) {
  static_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__