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rocksdb/thrift/lib/cpp/util/BitwiseCast.h

64 lines
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/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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.
*/
#ifndef THRIFT_UTIL_BITWISECAST_H_
#define THRIFT_UTIL_BITWISECAST_H_
#include <boost/static_assert.hpp>
// Use this to get around strict aliasing rules.
// For example, uint64_t i = bitwise_cast<uint64_t>(returns_double());
// The most obvious implementation is to just cast a pointer,
// but that doesn't work.
// For a pretty in-depth explanation of the problem, see
// http://cellperformance.beyond3d.com/articles/2006/06/ (...)
// understanding-strict-aliasing.html
template <typename To, typename From>
static inline To bitwise_cast(From from) {
BOOST_STATIC_ASSERT(sizeof(From) == sizeof(To));
// BAD!!! These are all broken with -O2.
//return *reinterpret_cast<To*>(&from); // BAD!!!
//return *static_cast<To*>(static_cast<void*>(&from)); // BAD!!!
//return *(To*)(void*)&from; // BAD!!!
// Super clean and partially blessed by section 3.9 of the standard.
//unsigned char c[sizeof(from)];
//memcpy(c, &from, sizeof(from));
//To to;
//memcpy(&to, c, sizeof(c));
//return to;
// Slightly more questionable.
// Same code emitted by GCC.
//To to;
//memcpy(&to, &from, sizeof(from));
//return to;
// Technically undefined, but almost universally supported,
// and the most efficient implementation.
union {
From f;
To t;
} u;
u.f = from;
return u.t;
}
#endif