1 /*
2 * Copyright 2013 Google Inc.
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8 #ifndef SkTFitsIn_DEFINED
9 #define SkTFitsIn_DEFINED
10
11 #include <limits>
12 #include <type_traits>
13
14 /**
15 * In C++ an unsigned to signed cast where the source value cannot be represented in the destination
16 * type results in an implementation defined destination value. Unlike C, C++ does not allow a trap.
17 * This makes "(S)(D)s == s" a possibly useful test. However, there are two cases where this is
18 * incorrect:
19 *
20 * when testing if a value of a smaller signed type can be represented in a larger unsigned type
21 * (int8_t)(uint16_t)-1 == -1 => (int8_t)0xFFFF == -1 => [implementation defined] == -1
22 *
23 * when testing if a value of a larger unsigned type can be represented in a smaller signed type
24 * (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true.
25 *
26 * Consider the cases:
27 * u = unsigned, less digits
28 * U = unsigned, more digits
29 * s = signed, less digits
30 * S = signed, more digits
31 * v is the value we're considering.
32 *
33 * u -> U: (u)(U)v == v, trivially true
34 * U -> u: (U)(u)v == v, both casts well defined, test works
35 * s -> S: (s)(S)v == v, trivially true
36 * S -> s: (S)(s)v == v, first cast implementation value, second cast defined, test works
37 * s -> U: (s)(U)v == v, *this is bad*, the second cast results in implementation defined value
38 * S -> u: (S)(u)v == v, the second cast is required to prevent promotion of rhs to unsigned
39 * u -> S: (u)(S)v == v, trivially true
40 * U -> s: (U)(s)v == v, *this is bad*,
41 * first cast results in implementation defined value,
42 * second cast is defined. However, this creates false positives
43 * uint16_t x = 0xFFFF
44 * (uint16_t)(int8_t)x == x
45 * => (uint16_t)-1 == x
46 * => 0xFFFF == x
47 * => true
48 *
49 * So for the eight cases three are trivially true, three more are valid casts, and two are special.
50 * The two 'full' checks which otherwise require two comparisons are valid cast checks.
51 * The two remaining checks s -> U [v >= 0] and U -> s [v <= max(s)] can be done with one op.
52 */
53
54 template <typename D, typename S>
55 static constexpr inline
56 typename std::enable_if<(std::is_integral<S>::value || std::is_enum<S>::value) &&
57 (std::is_integral<D>::value || std::is_enum<D>::value), bool>::type
SkTFitsIn(S src)58 /*bool*/ SkTFitsIn(S src) {
59 // SkTFitsIn() is used in public headers, so needs to be written targeting at most C++11.
60 return
61
62 // E.g. (int8_t)(uint8_t) int8_t(-1) == -1, but the uint8_t == 255, not -1.
63 (std::is_signed<S>::value && std::is_unsigned<D>::value && sizeof(S) <= sizeof(D)) ?
64 (S)0 <= src :
65
66 // E.g. (uint8_t)(int8_t) uint8_t(255) == 255, but the int8_t == -1.
67 (std::is_signed<D>::value && std::is_unsigned<S>::value && sizeof(D) <= sizeof(S)) ?
68 src <= (S)std::numeric_limits<D>::max() :
69
70 // else
71 (S)(D)src == src;
72 }
73
74 #endif
75