third_party/cld/base/template_util.h

   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_