| // Copyright 2016 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #ifndef BASE_BIT_CAST_H_ |
| #define BASE_BIT_CAST_H_ |
| |
| #include <string.h> |
| #include <type_traits> |
| |
| #include "base/compiler_specific.h" |
| #include "base/template_util.h" |
| #include "build/build_config.h" |
| |
| // bit_cast<Dest,Source> is a template function that implements the equivalent |
| // of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level |
| // functions like the protobuf library and fast math support. |
| // |
| // float f = 3.14159265358979; |
| // int i = bit_cast<int32_t>(f); |
| // // i = 0x40490fdb |
| // |
| // The classical address-casting method is: |
| // |
| // // WRONG |
| // float f = 3.14159265358979; // WRONG |
| // int i = * reinterpret_cast<int*>(&f); // WRONG |
| // |
| // The address-casting method actually produces undefined behavior according to |
| // the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15. |
| // (This did not substantially change in C++11.) Roughly, this section says: if |
| // an object in memory has one type, and a program accesses it with a different |
| // type, then the result is undefined behavior for most values of "different |
| // type". |
| // |
| // This is true for any cast syntax, either *(int*)&f or |
| // *reinterpret_cast<int*>(&f). And it is particularly true for conversions |
| // between integral lvalues and floating-point lvalues. |
| // |
| // The purpose of this paragraph is to allow optimizing compilers to assume that |
| // expressions with different types refer to different memory. Compilers are |
| // known to take advantage of this. So a non-conforming program quietly |
| // produces wildly incorrect output. |
| // |
| // The problem is not the use of reinterpret_cast. The problem is type punning: |
| // holding an object in memory of one type and reading its bits back using a |
| // different type. |
| // |
| // The C++ standard is more subtle and complex than this, but that is the basic |
| // idea. |
| // |
| // Anyways ... |
| // |
| // bit_cast<> calls memcpy() which is blessed by the standard, especially by the |
| // example in section 3.9 . Also, of course, bit_cast<> wraps up the nasty |
| // logic in one place. |
| // |
| // Fortunately memcpy() is very fast. In optimized mode, compilers replace |
| // calls to memcpy() with inline object code when the size argument is a |
| // compile-time constant. On a 32-bit system, memcpy(d,s,4) compiles to one |
| // load and one store, and memcpy(d,s,8) compiles to two loads and two stores. |
| |
| template <class Dest, class Source> |
| inline Dest bit_cast(const Source& source) { |
| static_assert(sizeof(Dest) == sizeof(Source), |
| "bit_cast requires source and destination to be the same size"); |
| static_assert(base::is_trivially_copyable<Dest>::value, |
| "bit_cast requires the destination type to be copyable"); |
| static_assert(base::is_trivially_copyable<Source>::value, |
| "bit_cast requires the source type to be copyable"); |
| |
| Dest dest; |
| memcpy(&dest, &source, sizeof(dest)); |
| return dest; |
| } |
| |
| #endif // BASE_BIT_CAST_H_ |