be/src/gutil/template_util.h

// Copyright 2005 Google Inc.
// All rights reserved.
//
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// modification, are permitted provided that the following conditions are
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//
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// ----
//
// Template metaprogramming utility functions.
//
// This code is compiled directly on many platforms, including client
// platforms like Windows, Mac, and embedded systems.  Before making
// any changes here, make sure that you're not breaking any platforms.
//
//
// The names choosen here reflect those used in tr1 and the boost::mpl
// library, there are similar operations used in the Loki library as
// well.  I prefer the boost names for 2 reasons:
// 1.  I think that portions of the Boost libraries are more likely to
// be included in the c++ standard.
// 2.  It is not impossible that some of the boost libraries will be
// included in our own build in the future.
// Both of these outcomes means that we may be able to directly replace
// some of these with boost equivalents.
//
#ifndef BASE_TEMPLATE_UTIL_H_
#define BASE_TEMPLATE_UTIL_H_

namespace base {

// Types small_ and big_ are guaranteed such that sizeof(small_) <
// sizeof(big_)
typedef char small_;

struct big_ {
  char dummy[2];
};

// Types YesType and NoType are guaranteed such that sizeof(YesType) <
// sizeof(NoType).
typedef small_ YesType;
typedef big_ NoType;

// Identity metafunction.
template <class T>
struct identity_ {
  typedef T type;
};

// integral_constant, defined in tr1, is a wrapper for an integer
// value. We don't really need this generality; we could get away
// with hardcoding the integer type to bool. We use the fully
// general integer_constant for compatibility with tr1.

template<class T, T v>
struct integral_constant {
  static const T value = v;
  typedef T value_type;
  typedef integral_constant<T, v> type;
};

template <class T, T v> const T integral_constant<T, v>::value;


// Abbreviations: true_type and false_type are structs that represent boolean
// true and false values. Also define the boost::mpl versions of those names,
// true_ and false_.
typedef integral_constant<bool, true>  true_type;
typedef integral_constant<bool, false> false_type;
typedef true_type  true_;
typedef false_type false_;

template <class T> struct is_non_const_reference : false_type {};
template <class T> struct is_non_const_reference<T&> : true_type {};
template <class T> struct is_non_const_reference<const T&> : false_type {};

template <class T> struct is_const : false_type {};
template <class T> struct is_const<const T> : true_type {};

template <class T> struct is_void : false_type {};
template <> struct is_void<void> : true_type {};

// if_ is a templatized conditional statement.
// if_<cond, A, B> is a compile time evaluation of cond.
// if_<>::type contains A if cond is true, B otherwise.
template<bool cond, typename A, typename B>
struct if_{
  typedef A type;
};

template<typename A, typename B>
struct if_<false, A, B> {
  typedef B type;
};


// type_equals_ is a template type comparator, similar to Loki IsSameType.
// type_equals_<A, B>::value is true iff "A" is the same type as "B".
//
// New code should prefer base::is_same, defined in base/type_traits.h.
// It is functionally identical, but is_same is the standard spelling.
template<typename A, typename B>
struct type_equals_ : public false_ {
};

template<typename A>
struct type_equals_<A, A> : public true_ {
};

// and_ is a template && operator.
// and_<A, B>::value evaluates "A::value && B::value".
template<typename A, typename B>
struct and_ : public integral_constant<bool, (A::value && B::value)> {
};

// or_ is a template || operator.
// or_<A, B>::value evaluates "A::value || B::value".
template<typename A, typename B>
struct or_ : public integral_constant<bool, (A::value || B::value)> {
};

// Used to determine if a type is a struct/union/class. Inspired by Boost's
// is_class type_trait implementation.
struct IsClassHelper {
  template <typename C>
  static YesType Test(void(C::*)(void));

  template <typename C>
  static NoType Test(...);
};

template <typename T>
struct is_class
    : integral_constant<bool,
                        sizeof(IsClassHelper::Test<T>(0)) ==
                            sizeof(YesType)> {
};

}

#endif  // BASE_TEMPLATE_UTIL_H_