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