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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_dereference has_dereference] | |
9 | template <class Rhs, class Ret=dont_care> | |
10 | struct has_dereference : public __tof {}; | |
11 | ||
12 | __inherit | |
13 | If (i) `rhs` of type `Rhs` can be used in expression `*rhs`, | |
14 | and (ii) `Ret=dont_care` or the result of expression `*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 prefix `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 | Rhs rhs; | |
24 | f(*rhs); // is valid if has_dereference<Rhs, Ret>::value==true | |
25 | `` | |
26 | If `Ret=void`, the return type is checked to be exactly `void`. | |
27 | ||
28 | __header `#include <boost/type_traits/has_dereference.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>` | |
29 | ||
30 | [has_binary_operator_compat] | |
31 | ||
32 | __examples | |
33 | ||
34 | [:`has_dereference<Rhs, Ret>::value_type` is the type `bool`.] | |
35 | [:`has_dereference<Rhs, Ret>::value` is a `bool` integral constant expression.] | |
36 | [:`has_dereference<int*>::value` is a `bool` integral constant expression that evaluates to `true`.] | |
37 | [:`has_dereference<long*>` inherits from `__true_type`.] | |
38 | ||
39 | ||
40 | [:`has_dereference<int*, int>` inherits from `__true_type`.] | |
41 | [:`has_dereference<int*, const int>` inherits from `__true_type`.] | |
42 | [:`has_dereference<int const *>` inherits from `__true_type`.] | |
43 | [:`has_dereference<int * const>` inherits from `__true_type`.] | |
44 | [:`has_dereference<int const * const>` inherits from `__true_type`.] | |
45 | ||
46 | [:`has_dereference<int>` inherits from `__false_type`.] | |
47 | [:`has_dereference<double>` inherits from `__false_type`.] | |
48 | [:`has_dereference<void*>` inherits from `__false_type`.] | |
49 | [:`has_dereference<const int*, int&>` inherits from `__false_type`.] | |
50 | ||
51 | ||
52 | [*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits] | |
53 | ||
54 | [*Known issues:] | |
55 | ||
56 | * This trait cannot detect whether prefix `operator*` is public or not: | |
57 | if `operator*` is defined as a private member of `Rhs` then | |
58 | instantiating `has_dereference<Rhs>` will produce a compiler error. | |
59 | For this reason `has_dereference` cannot be used to determine whether a type has a public `operator*` or not. | |
60 | `` | |
61 | struct A { private: void operator*(); }; | |
62 | boost::has_dereference<A>::value; // error: A::operator*() is private | |
63 | `` | |
64 | ||
65 | * There is an issue if the operator exists only for type `A` and `B` is | |
66 | convertible to `A`. In this case, the compiler will report an ambiguous overload. | |
67 | `` | |
68 | struct A { }; | |
69 | void operator*(const A&); | |
70 | struct B { operator A(); }; | |
71 | boost::has_dereference<A>::value; // this is fine | |
72 | boost::has_dereference<B>::value; // error: ambiguous overload | |
73 | `` | |
74 | ||
75 | * There is an issue when applying this trait to template classes. | |
76 | If `operator*` is defined but does not bind for a given template type, | |
77 | it is still detected by the trait which returns `true` instead of `false`. | |
78 | Example: | |
79 | `` | |
80 | #include <boost/type_traits/has_dereference.hpp> | |
81 | #include <iostream> | |
82 | ||
83 | template <class T> | |
84 | struct contains { T data; }; | |
85 | ||
86 | template <class T> | |
87 | bool operator*(const contains<T> &rhs) { | |
88 | return f(rhs.data); | |
89 | } | |
90 | ||
91 | class bad { }; | |
92 | class good { }; | |
93 | bool f(const good&) { } | |
94 | ||
95 | int main() { | |
96 | std::cout<<std::boolalpha; | |
97 | // works fine for contains<good> | |
98 | std::cout<<boost::has_dereference< contains< good > >::value<<'\n'; // true | |
99 | contains<good> g; | |
100 | *g; // ok | |
101 | // does not work for contains<bad> | |
102 | std::cout<<boost::has_dereference< contains< bad > >::value<<'\n'; // true, should be false | |
103 | contains<bad> b; | |
104 | *b; // compile time error | |
105 | return 0; | |
106 | } | |
107 | `` | |
108 | ||
109 | * `volatile` qualifier is not properly handled and would lead to undefined behavior | |
110 | ||
111 | [endsect] | |
112 |