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1 | [/ |
2 | (C) Copyright 2009-2011 Frederic Bron. | |
3 | Distributed under the Boost Software License, Version 1.0. | |
4 | (See accompanying file LICENSE_1_0.txt or copy at | |
5 | http://www.boost.org/LICENSE_1_0.txt). | |
6 | ] | |
7 | ||
8 | [section:has_greater_equal has_greater_equal] | |
9 | template <class Lhs, class Rhs=Lhs, class Ret=dont_care> | |
10 | struct has_greater_equal : public __tof {}; | |
11 | ||
12 | __inherit | |
13 | If (i) `lhs` of type `Lhs` and `rhs` of type `Rhs` can be used in expression `lhs>=rhs`, | |
14 | and (ii) `Ret=dont_care` or the result of expression `lhs>=rhs` is convertible to `Ret` | |
15 | then inherits from __true_type, | |
16 | otherwise inherits from __false_type. | |
17 | ||
18 | The default behaviour (`Ret=dont_care`) is to not check for the return value of binary `operator>=`. | |
19 | If `Ret` is different from the default `dont_care` type, the return value is checked to be convertible to `Ret`. | |
20 | Convertible to `Ret` means that the return value of the operator can be used as argument to a function expecting `Ret`: | |
21 | `` | |
22 | void f(Ret); | |
23 | Lhs lhs; | |
24 | Rhs rhs; | |
25 | f(lhs>=rhs); // is valid if has_greater_equal<Lhs, Rhs, Ret>::value==true | |
26 | `` | |
27 | If `Ret=void`, the return type is checked to be exactly `void`. | |
28 | ||
29 | __header `#include <boost/type_traits/has_greater_equal.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>` | |
30 | ||
31 | [has_binary_operator_compat] | |
32 | ||
33 | __examples | |
34 | ||
35 | [:`has_greater_equal<Lhs, Rhs, Ret>::value_type` is the type `bool`.] | |
36 | [:`has_greater_equal<Lhs, Rhs, Ret>::value` is a `bool` integral constant expression.] | |
37 | [:`has_greater_equal<int>::value` is a `bool` integral constant expression that evaluates to `true`.] | |
38 | [:`has_greater_equal<long>` inherits from `__true_type`.] | |
39 | ||
40 | [:`has_greater_equal<int, int, bool>` inherits from `__true_type`.] | |
41 | [:`has_greater_equal<int, double, bool>` inherits from `__true_type`.] | |
42 | [:`has_greater_equal<const int>` inherits from `__true_type`.] | |
43 | ||
44 | [:`has_greater_equal<int*, int>` inherits from `__false_type`.] | |
45 | [:`has_greater_equal<int*, double*>` inherits from `__false_type`.] | |
46 | [:`has_greater_equal<int, int, std::string>` inherits from `__false_type`.] | |
47 | ||
48 | ||
49 | [*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits] | |
50 | ||
51 | [*Known issues:] | |
52 | ||
53 | * This trait cannot detect whether binary `operator>=` is public or not: | |
54 | if `operator>=` is defined as a private member of `Lhs` then | |
55 | instantiating `has_greater_equal<Lhs>` will produce a compiler error. | |
56 | For this reason `has_greater_equal` cannot be used to determine whether a type has a public `operator>=` or not. | |
57 | `` | |
58 | struct A { private: void operator>=(const A&); }; | |
59 | boost::has_greater_equal<A>::value; // error: A::operator>=(const A&) is private | |
60 | `` | |
61 | ||
62 | * There is an issue if the operator exists only for type `A` and `B` is | |
63 | convertible to `A`. In this case, the compiler will report an ambiguous overload. | |
64 | `` | |
65 | struct A { }; | |
66 | void operator>=(const A&, const A&); | |
67 | struct B { operator A(); }; | |
68 | boost::has_greater_equal<A>::value; // this is fine | |
69 | boost::has_greater_equal<B>::value; // error: ambiguous overload | |
70 | `` | |
71 | ||
72 | * There is an issue when applying this trait to template classes. | |
73 | If `operator>=` is defined but does not bind for a given template type, | |
74 | it is still detected by the trait which returns `true` instead of `false`. | |
75 | Example: | |
76 | `` | |
77 | #include <boost/type_traits/has_greater_equal.hpp> | |
78 | #include <iostream> | |
79 | ||
80 | template <class T> | |
81 | struct contains { T data; }; | |
82 | ||
83 | template <class T> | |
84 | bool operator>=(const contains<T> &lhs, const contains<T> &rhs) { | |
85 | return f(lhs.data, rhs.data); | |
86 | } | |
87 | ||
88 | class bad { }; | |
89 | class good { }; | |
90 | bool f(const good&, const good&) { } | |
91 | ||
92 | int main() { | |
93 | std::cout<<std::boolalpha; | |
94 | // works fine for contains<good> | |
95 | std::cout<<boost::has_greater_equal< contains< good > >::value<<'\n'; // true | |
96 | contains<good> g; | |
97 | g>=g; // ok | |
98 | // does not work for contains<bad> | |
99 | std::cout<<boost::has_greater_equal< contains< bad > >::value<<'\n'; // true, should be false | |
100 | contains<bad> b; | |
101 | b>=b; // compile time error | |
102 | return 0; | |
103 | } | |
104 | `` | |
105 | ||
106 | * `volatile` qualifier is not properly handled and would lead to undefined behavior | |
107 | ||
108 | [endsect] | |
109 |