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 "include/private/base/SkDebug.h" 12 13 #include <limits> 14 #include <type_traits> 15 16 /** 17 * std::underlying_type is only defined for enums. For integral types, we just want the type. 18 */ 19 template <typename T, class Enable = void> 20 struct sk_strip_enum { 21 typedef T type; 22 }; 23 24 template <typename T> 25 struct sk_strip_enum<T, typename std::enable_if<std::is_enum<T>::value>::type> { 26 typedef typename std::underlying_type<T>::type type; 27 }; 28 29 30 /** 31 * In C++ an unsigned to signed cast where the source value cannot be represented in the destination 32 * type results in an implementation defined destination value. Unlike C, C++ does not allow a trap. 33 * This makes "(S)(D)s == s" a possibly useful test. However, there are two cases where this is 34 * incorrect: 35 * 36 * when testing if a value of a smaller signed type can be represented in a larger unsigned type 37 * (int8_t)(uint16_t)-1 == -1 => (int8_t)0xFFFF == -1 => [implementation defined] == -1 38 * 39 * when testing if a value of a larger unsigned type can be represented in a smaller signed type 40 * (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true. 41 * 42 * Consider the cases: 43 * u = unsigned, less digits 44 * U = unsigned, more digits 45 * s = signed, less digits 46 * S = signed, more digits 47 * v is the value we're considering. 48 * 49 * u -> U: (u)(U)v == v, trivially true 50 * U -> u: (U)(u)v == v, both casts well defined, test works 51 * s -> S: (s)(S)v == v, trivially true 52 * S -> s: (S)(s)v == v, first cast implementation value, second cast defined, test works 53 * s -> U: (s)(U)v == v, *this is bad*, the second cast results in implementation defined value 54 * S -> u: (S)(u)v == v, the second cast is required to prevent promotion of rhs to unsigned 55 * u -> S: (u)(S)v == v, trivially true 56 * U -> s: (U)(s)v == v, *this is bad*, 57 * first cast results in implementation defined value, 58 * second cast is defined. However, this creates false positives 59 * uint16_t x = 0xFFFF 60 * (uint16_t)(int8_t)x == x 61 * => (uint16_t)-1 == x 62 * => 0xFFFF == x 63 * => true 64 * 65 * So for the eight cases three are trivially true, three more are valid casts, and two are special. 66 * The two 'full' checks which otherwise require two comparisons are valid cast checks. 67 * The two remaining checks s -> U [v >= 0] and U -> s [v <= max(s)] can be done with one op. 68 */ 69 70 template <typename D, typename S> 71 static constexpr inline 72 typename std::enable_if<(std::is_integral<S>::value || std::is_enum<S>::value) && 73 (std::is_integral<D>::value || std::is_enum<D>::value), bool>::type 74 /*bool*/ SkTFitsIn(S src) { 75 // Ensure that is_signed and is_unsigned are passed the arithmetic underlyng types of enums. 76 using Sa = typename sk_strip_enum<S>::type; 77 using Da = typename sk_strip_enum<D>::type; 78 79 // SkTFitsIn() is used in public headers, so needs to be written targeting at most C++11. 80 return 81 82 // E.g. (int8_t)(uint8_t) int8_t(-1) == -1, but the uint8_t == 255, not -1. 83 (std::is_signed<Sa>::value && std::is_unsigned<Da>::value && sizeof(Sa) <= sizeof(Da)) ? 84 (S)0 <= src : 85 86 // E.g. (uint8_t)(int8_t) uint8_t(255) == 255, but the int8_t == -1. 87 (std::is_signed<Da>::value && std::is_unsigned<Sa>::value && sizeof(Da) <= sizeof(Sa)) ? 88 src <= (S)std::numeric_limits<Da>::max() : 89 90 // This trips up MSVC's /RTCc run-time checking, which we don't support. 91 (S)(D)src == src; 92 } 93 94 #endif 95