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| 1 // Copyright 2016 The Chromium Authors. All rights reserved. |
| 2 // Use of this source code is governed by a BSD-style license that can be |
| 3 // found in the LICENSE file. |
| 4 |
| 5 #ifndef BASE_BIT_CAST_H_ |
| 6 #define BASE_BIT_CAST_H_ |
| 7 |
| 8 #include <string.h> |
| 9 |
| 10 // bit_cast<Dest,Source> is a template function that implements the equivalent |
| 11 // of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level |
| 12 // functions like the protobuf library and fast math support. |
| 13 // |
| 14 // float f = 3.14159265358979; |
| 15 // int i = bit_cast<int32_t>(f); |
| 16 // // i = 0x40490fdb |
| 17 // |
| 18 // The classical address-casting method is: |
| 19 // |
| 20 // // WRONG |
| 21 // float f = 3.14159265358979; // WRONG |
| 22 // int i = * reinterpret_cast<int*>(&f); // WRONG |
| 23 // |
| 24 // The address-casting method actually produces undefined behavior according to |
| 25 // the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15. |
| 26 // (This did not substantially change in C++11.) Roughly, this section says: if |
| 27 // an object in memory has one type, and a program accesses it with a different |
| 28 // type, then the result is undefined behavior for most values of "different |
| 29 // type". |
| 30 // |
| 31 // This is true for any cast syntax, either *(int*)&f or |
| 32 // *reinterpret_cast<int*>(&f). And it is particularly true for conversions |
| 33 // between integral lvalues and floating-point lvalues. |
| 34 // |
| 35 // The purpose of this paragraph is to allow optimizing compilers to assume that |
| 36 // expressions with different types refer to different memory. Compilers are |
| 37 // known to take advantage of this. So a non-conforming program quietly |
| 38 // produces wildly incorrect output. |
| 39 // |
| 40 // The problem is not the use of reinterpret_cast. The problem is type punning: |
| 41 // holding an object in memory of one type and reading its bits back using a |
| 42 // different type. |
| 43 // |
| 44 // The C++ standard is more subtle and complex than this, but that is the basic |
| 45 // idea. |
| 46 // |
| 47 // Anyways ... |
| 48 // |
| 49 // bit_cast<> calls memcpy() which is blessed by the standard, especially by the |
| 50 // example in section 3.9 . Also, of course, bit_cast<> wraps up the nasty |
| 51 // logic in one place. |
| 52 // |
| 53 // Fortunately memcpy() is very fast. In optimized mode, compilers replace |
| 54 // calls to memcpy() with inline object code when the size argument is a |
| 55 // compile-time constant. On a 32-bit system, memcpy(d,s,4) compiles to one |
| 56 // load and one store, and memcpy(d,s,8) compiles to two loads and two stores. |
| 57 // |
| 58 // WARNING: if Dest or Source is a non-POD type, the result of the memcpy |
| 59 // is likely to surprise you. |
| 60 |
| 61 template <class Dest, class Source> |
| 62 inline Dest bit_cast(const Source& source) { |
| 63 static_assert(sizeof(Dest) == sizeof(Source), |
| 64 "bit_cast requires source and destination to be the same size"); |
| 65 |
| 66 Dest dest; |
| 67 memcpy(&dest, &source, sizeof(dest)); |
| 68 return dest; |
| 69 } |
| 70 |
| 71 #endif // BASE_BIT_CAST_H_ |
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