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 #include <type_traits>
10
11 #include "base/compiler_specific.h"
12 #include "base/template_util.h"
13 #include "build/build_config.h"
14
15 // bit_cast<Dest,Source> is a template function that implements the equivalent
16 // of "*reinterpret_cast<Dest*>(&source)". We need this in very low-level
17 // functions like the protobuf library and fast math support.
18 //
19 // float f = 3.14159265358979;
20 // int i = bit_cast<int32_t>(f);
21 // // i = 0x40490fdb
22 //
23 // The classical address-casting method is:
24 //
25 // // WRONG
26 // float f = 3.14159265358979; // WRONG
27 // int i = * reinterpret_cast<int*>(&f); // WRONG
28 //
29 // The address-casting method actually produces undefined behavior according to
30 // the ISO C++98 specification, section 3.10 ("basic.lval"), paragraph 15.
31 // (This did not substantially change in C++11.) Roughly, this section says: if
32 // an object in memory has one type, and a program accesses it with a different
33 // type, then the result is undefined behavior for most values of "different
34 // type".
35 //
36 // This is true for any cast syntax, either *(int*)&f or
37 // *reinterpret_cast<int*>(&f). And it is particularly true for conversions
38 // between integral lvalues and floating-point lvalues.
39 //
40 // The purpose of this paragraph is to allow optimizing compilers to assume that
41 // expressions with different types refer to different memory. Compilers are
42 // known to take advantage of this. So a non-conforming program quietly
43 // produces wildly incorrect output.
44 //
45 // The problem is not the use of reinterpret_cast. The problem is type punning:
46 // holding an object in memory of one type and reading its bits back using a
47 // different type.
48 //
49 // The C++ standard is more subtle and complex than this, but that is the basic
50 // idea.
51 //
52 // Anyways ...
53 //
54 // bit_cast<> calls memcpy() which is blessed by the standard, especially by the
55 // example in section 3.9 . Also, of course, bit_cast<> wraps up the nasty
56 // logic in one place.
57 //
58 // Fortunately memcpy() is very fast. In optimized mode, compilers replace
59 // calls to memcpy() with inline object code when the size argument is a
60 // compile-time constant. On a 32-bit system, memcpy(d,s,4) compiles to one
61 // load and one store, and memcpy(d,s,8) compiles to two loads and two stores.
62
63 template <class Dest, class Source>
bit_cast(const Source & source)64 inline Dest bit_cast(const Source& source) {
65 static_assert(sizeof(Dest) == sizeof(Source),
66 "bit_cast requires source and destination to be the same size");
67 static_assert(base::is_trivially_copyable<Dest>::value,
68 "bit_cast requires the destination type to be copyable");
69 static_assert(base::is_trivially_copyable<Source>::value,
70 "bit_cast requires the source type to be copyable");
71
72 Dest dest;
73 memcpy(&dest, &source, sizeof(dest));
74 return dest;
75 }
76
77 #endif // BASE_BIT_CAST_H_
78