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 <stdint.h> 13 #include <type_traits> 14 15 /** 16 * std::underlying_type is only defined for enums. For integral types, we just want the type. 17 */ 18 template <typename T, class Enable = void> 19 struct sk_strip_enum { 20 typedef T type; 21 }; 22 23 template <typename T> 24 struct sk_strip_enum<T, typename std::enable_if<std::is_enum<T>::value>::type> { 25 typedef typename std::underlying_type<T>::type type; 26 }; 27 28 29 /** 30 * In C++ an unsigned to signed cast where the source value cannot be represented in the destination 31 * type results in an implementation defined destination value. Unlike C, C++ does not allow a trap. 32 * This makes "(S)(D)s == s" a possibly useful test. However, there are two cases where this is 33 * incorrect: 34 * 35 * when testing if a value of a smaller signed type can be represented in a larger unsigned type 36 * (int8_t)(uint16_t)-1 == -1 => (int8_t)0xFFFF == -1 => [implementation defined] == -1 37 * 38 * when testing if a value of a larger unsigned type can be represented in a smaller signed type 39 * (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true. 40 * 41 * Consider the cases: 42 * u = unsigned, less digits 43 * U = unsigned, more digits 44 * s = signed, less digits 45 * S = signed, more digits 46 * v is the value we're considering. 47 * 48 * u -> U: (u)(U)v == v, trivially true 49 * U -> u: (U)(u)v == v, both casts well defined, test works 50 * s -> S: (s)(S)v == v, trivially true 51 * S -> s: (S)(s)v == v, first cast implementation value, second cast defined, test works 52 * s -> U: (s)(U)v == v, *this is bad*, the second cast results in implementation defined value 53 * S -> u: (S)(u)v == v, the second cast is required to prevent promotion of rhs to unsigned 54 * u -> S: (u)(S)v == v, trivially true 55 * U -> s: (U)(s)v == v, *this is bad*, 56 * first cast results in implementation defined value, 57 * second cast is defined. However, this creates false positives 58 * uint16_t x = 0xFFFF 59 * (uint16_t)(int8_t)x == x 60 * => (uint16_t)-1 == x 61 * => 0xFFFF == x 62 * => true 63 * 64 * So for the eight cases three are trivially true, three more are valid casts, and two are special. 65 * The two 'full' checks which otherwise require two comparisons are valid cast checks. 66 * The two remaining checks s -> U [v >= 0] and U -> s [v <= max(s)] can be done with one op. 67 */ 68 69 template <typename D, typename S> 70 static constexpr inline 71 typename std::enable_if<(std::is_integral<S>::value || std::is_enum<S>::value) && 72 (std::is_integral<D>::value || std::is_enum<D>::value), bool>::type 73 /*bool*/ SkTFitsIn(S src) { 74 // SkTFitsIn() is used in public headers, so needs to be written targeting at most C++11. 75 return 76 77 // E.g. (int8_t)(uint8_t) int8_t(-1) == -1, but the uint8_t == 255, not -1. 78 (std::is_signed<S>::value && std::is_unsigned<D>::value && sizeof(S) <= sizeof(D)) ? 79 (S)0 <= src : 80 81 // E.g. (uint8_t)(int8_t) uint8_t(255) == 255, but the int8_t == -1. 82 (std::is_signed<D>::value && std::is_unsigned<S>::value && sizeof(D) <= sizeof(S)) ? 83 src <= (S)std::numeric_limits<typename sk_strip_enum<D>::type>::max() : 84 85 #if !defined(SK_DEBUG) && !defined(__MSVC_RUNTIME_CHECKS ) 86 // Correct (simple) version. This trips up MSVC's /RTCc run-time checking. 87 (S)(D)src == src; 88 #else 89 // More complex version that's safe with /RTCc. Used in all debug builds, for coverage. 90 (std::is_signed<S>::value) ? 91 (intmax_t)src >= (intmax_t)std::numeric_limits<typename sk_strip_enum<D>::type>::min() && 92 (intmax_t)src <= (intmax_t)std::numeric_limits<typename sk_strip_enum<D>::type>::max() : 93 94 // std::is_unsigned<S> ? 95 (uintmax_t)src <= (uintmax_t)std::numeric_limits<typename sk_strip_enum<D>::type>::max(); 96 #endif 97 } 98 99 #endif 100