type_traits.h revision a9ad04191cb56c42944b17980b8b2bb2afe11ab2 
1//===- llvm/Support/type_traits.h - Simplfied type traits -------*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file provides a template class that determines if a type is a class or
11// not. The basic mechanism, based on using the pointer to member function of
12// a zero argument to a function was "boosted" from the boost type_traits
13// library. See http://www.boost.org/ for all the gory details.
14//
15//===----------------------------------------------------------------------===//
16
17#ifndef LLVM_SUPPORT_TYPE_TRAITS_H
18#define LLVM_SUPPORT_TYPE_TRAITS_H
19
20// This is actually the conforming implementation which works with abstract
21// classes.  However, enough compilers have trouble with it that most will use
22// the one in boost/type_traits/object_traits.hpp. This implementation actually
23// works with VC7.0, but other interactions seem to fail when we use it.
24
25namespace llvm {
26
27namespace dont_use
28{
29    // These two functions should never be used. They are helpers to
30    // the is_class template below. They cannot be located inside
31    // is_class because doing so causes at least GCC to think that
32    // the value of the "value" enumerator is not constant. Placing
33    // them out here (for some strange reason) allows the sizeof
34    // operator against them to magically be constant. This is
35    // important to make the is_class<T>::value idiom zero cost. it
36    // evaluates to a constant 1 or 0 depending on whether the
37    // parameter T is a class or not (respectively).
38    template<typename T> char is_class_helper(void(T::*)());
39    template<typename T> double is_class_helper(...);
40}
41
42template <typename T>
43struct is_class
44{
45  // is_class<> metafunction due to Paul Mensonides (leavings@attbi.com). For
46  // more details:
47  // http://groups.google.com/groups?hl=en&selm=000001c1cc83%24e154d5e0%247772e50c%40c161550a&rnum=1
48 public:
49    enum { value = sizeof(char) == sizeof(dont_use::is_class_helper<T>(0)) };
50};
51
52
53// enable_if_c - Enable/disable a template based on a metafunction
54template<bool Cond, typename T = void>
55struct enable_if_c {
56  typedef T type;
57};
58
59template<typename T> struct enable_if_c<false, T> { };
60
61// enable_if - Enable/disable a template based on a metafunction
62template<typename Cond, typename T = void>
63struct enable_if : public enable_if_c<Cond::value, T> { };
64
65namespace dont_use {
66  template<typename Base> char base_of_helper(const volatile Base*);
67  template<typename Base> double base_of_helper(...);
68}
69
70/// is_base_of - Metafunction to determine whether one type is a base class of
71/// (or identical to) another type.
72template<typename Base, typename Derived>
73struct is_base_of {
74  static const bool value
75    = is_class<Base>::value && is_class<Derived>::value &&
76      sizeof(char) == sizeof(dont_use::base_of_helper<Base>((Derived*)0));
77};
78
79}
80
81#endif
82