1 // Copyright (c) 2006-2009 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 // Template metaprogramming utility functions. 6 // 7 // This code is compiled directly on many platforms, including client 8 // platforms like Windows, Mac, and embedded systems. Before making 9 // any changes here, make sure that you're not breaking any platforms. 10 // 11 // The names choosen here reflect those used in tr1 and the boost::mpl 12 // library, there are similar operations used in the Loki library as 13 // well. I prefer the boost names for 2 reasons: 14 // 1. I think that portions of the Boost libraries are more likely to 15 // be included in the c++ standard. 16 // 2. It is not impossible that some of the boost libraries will be 17 // included in our own build in the future. 18 // Both of these outcomes means that we may be able to directly replace 19 // some of these with boost equivalents. 20 // 21 #ifndef BASE_TEMPLATE_UTIL_H_ 22 #define BASE_TEMPLATE_UTIL_H_ 23 24 namespace base { 25 26 // Types small_ and big_ are guaranteed such that sizeof(small_) < 27 // sizeof(big_) 28 typedef char small_; 29 30 struct big_ { 31 char dummy[2]; 32 }; 33 34 // integral_constant, defined in tr1, is a wrapper for an integer 35 // value. We don't really need this generality; we could get away 36 // with hardcoding the integer type to bool. We use the fully 37 // general integer_constant for compatibility with tr1. 38 39 template<class T, T v> 40 struct integral_constant { 41 static const T value = v; 42 typedef T value_type; 43 typedef integral_constant<T, v> type; 44 }; 45 46 template <class T, T v> const T integral_constant<T, v>::value; 47 48 49 // Abbreviations: true_type and false_type are structs that represent boolean 50 // true and false values. Also define the boost::mpl versions of those names, 51 // true_ and false_. 52 typedef integral_constant<bool, true> true_type; 53 typedef integral_constant<bool, false> false_type; 54 typedef true_type true_; 55 typedef false_type false_; 56 57 // if_ is a templatized conditional statement. 58 // if_<cond, A, B> is a compile time evaluation of cond. 59 // if_<>::type contains A if cond is true, B otherwise. 60 template<bool cond, typename A, typename B> 61 struct if_{ 62 typedef A type; 63 }; 64 65 template<typename A, typename B> 66 struct if_<false, A, B> { 67 typedef B type; 68 }; 69 70 71 // type_equals_ is a template type comparator, similar to Loki IsSameType. 72 // type_equals_<A, B>::value is true iff "A" is the same type as "B". 73 template<typename A, typename B> 74 struct type_equals_ : public false_ { 75 }; 76 77 template<typename A> 78 struct type_equals_<A, A> : public true_ { 79 }; 80 81 // and_ is a template && operator. 82 // and_<A, B>::value evaluates "A::value && B::value". 83 template<typename A, typename B> 84 struct and_ : public integral_constant<bool, (A::value && B::value)> { 85 }; 86 87 // or_ is a template || operator. 88 // or_<A, B>::value evaluates "A::value || B::value". 89 template<typename A, typename B> 90 struct or_ : public integral_constant<bool, (A::value || B::value)> { 91 }; 92 93 94 } // Close namespace base 95 96 #endif // BASE_TEMPLATE_UTIL_H_ 97
