1 // Copyright 2014 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_NUMERICS_SAFE_CONVERSIONS_H_
6 #define BASE_NUMERICS_SAFE_CONVERSIONS_H_
7
8 #include <stddef.h>
9
10 #include <limits>
11 #include <type_traits>
12
13 #include "base/logging.h"
14 #include "base/numerics/safe_conversions_impl.h"
15
16 namespace base {
17
18 // Convenience function that returns true if the supplied value is in range
19 // for the destination type.
20 template <typename Dst, typename Src>
IsValueInRangeForNumericType(Src value)21 inline bool IsValueInRangeForNumericType(Src value) {
22 return internal::DstRangeRelationToSrcRange<Dst>(value) ==
23 internal::RANGE_VALID;
24 }
25
26 // Convenience function for determining if a numeric value is negative without
27 // throwing compiler warnings on: unsigned(value) < 0.
28 template <typename T>
29 typename std::enable_if<std::numeric_limits<T>::is_signed, bool>::type
IsValueNegative(T value)30 IsValueNegative(T value) {
31 static_assert(std::numeric_limits<T>::is_specialized,
32 "Argument must be numeric.");
33 return value < 0;
34 }
35
36 template <typename T>
37 typename std::enable_if<!std::numeric_limits<T>::is_signed, bool>::type
IsValueNegative(T)38 IsValueNegative(T) {
39 static_assert(std::numeric_limits<T>::is_specialized,
40 "Argument must be numeric.");
41 return false;
42 }
43
44 // checked_cast<> is analogous to static_cast<> for numeric types,
45 // except that it CHECKs that the specified numeric conversion will not
46 // overflow or underflow. NaN source will always trigger a CHECK.
47 template <typename Dst, typename Src>
checked_cast(Src value)48 inline Dst checked_cast(Src value) {
49 CHECK(IsValueInRangeForNumericType<Dst>(value));
50 return static_cast<Dst>(value);
51 }
52
53 // HandleNaN will cause this class to CHECK(false).
54 struct SaturatedCastNaNBehaviorCheck {
55 template <typename T>
HandleNaNSaturatedCastNaNBehaviorCheck56 static T HandleNaN() {
57 CHECK(false);
58 return T();
59 }
60 };
61
62 // HandleNaN will return 0 in this case.
63 struct SaturatedCastNaNBehaviorReturnZero {
64 template <typename T>
HandleNaNSaturatedCastNaNBehaviorReturnZero65 static T HandleNaN() {
66 return T();
67 }
68 };
69
70 // saturated_cast<> is analogous to static_cast<> for numeric types, except
71 // that the specified numeric conversion will saturate rather than overflow or
72 // underflow. NaN assignment to an integral will defer the behavior to a
73 // specified class. By default, it will return 0.
74 template <typename Dst,
75 class NaNHandler = SaturatedCastNaNBehaviorReturnZero,
76 typename Src>
saturated_cast(Src value)77 inline Dst saturated_cast(Src value) {
78 // Optimization for floating point values, which already saturate.
79 if (std::numeric_limits<Dst>::is_iec559)
80 return static_cast<Dst>(value);
81
82 switch (internal::DstRangeRelationToSrcRange<Dst>(value)) {
83 case internal::RANGE_VALID:
84 return static_cast<Dst>(value);
85
86 case internal::RANGE_UNDERFLOW:
87 return std::numeric_limits<Dst>::min();
88
89 case internal::RANGE_OVERFLOW:
90 return std::numeric_limits<Dst>::max();
91
92 // Should fail only on attempting to assign NaN to a saturated integer.
93 case internal::RANGE_INVALID:
94 return NaNHandler::template HandleNaN<Dst>();
95 }
96
97 NOTREACHED();
98 return static_cast<Dst>(value);
99 }
100
101 // strict_cast<> is analogous to static_cast<> for numeric types, except that
102 // it will cause a compile failure if the destination type is not large enough
103 // to contain any value in the source type. It performs no runtime checking.
104 template <typename Dst, typename Src>
strict_cast(Src value)105 inline Dst strict_cast(Src value) {
106 static_assert(std::numeric_limits<Src>::is_specialized,
107 "Argument must be numeric.");
108 static_assert(std::numeric_limits<Dst>::is_specialized,
109 "Result must be numeric.");
110 static_assert((internal::StaticDstRangeRelationToSrcRange<Dst, Src>::value ==
111 internal::NUMERIC_RANGE_CONTAINED),
112 "The numeric conversion is out of range for this type. You "
113 "should probably use one of the following conversion "
114 "mechanisms on the value you want to pass:\n"
115 "- base::checked_cast\n"
116 "- base::saturated_cast\n"
117 "- base::CheckedNumeric");
118
119 return static_cast<Dst>(value);
120 }
121
122 // StrictNumeric implements compile time range checking between numeric types by
123 // wrapping assignment operations in a strict_cast. This class is intended to be
124 // used for function arguments and return types, to ensure the destination type
125 // can always contain the source type. This is essentially the same as enforcing
126 // -Wconversion in gcc and C4302 warnings on MSVC, but it can be applied
127 // incrementally at API boundaries, making it easier to convert code so that it
128 // compiles cleanly with truncation warnings enabled.
129 // This template should introduce no runtime overhead, but it also provides no
130 // runtime checking of any of the associated mathematical operations. Use
131 // CheckedNumeric for runtime range checks of tha actual value being assigned.
132 template <typename T>
133 class StrictNumeric {
134 public:
135 typedef T type;
136
StrictNumeric()137 StrictNumeric() : value_(0) {}
138
139 // Copy constructor.
140 template <typename Src>
StrictNumeric(const StrictNumeric<Src> & rhs)141 StrictNumeric(const StrictNumeric<Src>& rhs)
142 : value_(strict_cast<T>(rhs.value_)) {}
143
144 // This is not an explicit constructor because we implicitly upgrade regular
145 // numerics to StrictNumerics to make them easier to use.
146 template <typename Src>
StrictNumeric(Src value)147 StrictNumeric(Src value)
148 : value_(strict_cast<T>(value)) {}
149
150 // The numeric cast operator basically handles all the magic.
151 template <typename Dst>
Dst()152 operator Dst() const {
153 return strict_cast<Dst>(value_);
154 }
155
156 private:
157 T value_;
158 };
159
160 // Explicitly make a shorter size_t typedef for convenience.
161 typedef StrictNumeric<size_t> SizeT;
162
163 } // namespace base
164
165 #endif // BASE_NUMERICS_SAFE_CONVERSIONS_H_
166