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).
8 [section:has_dereference has_dereference]
9 template <class Rhs, class Ret=dont_care>
10 struct has_dereference : public __tof {};
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.
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`:
24 f(*rhs); // is valid if has_dereference<Rhs, Ret>::value==true
26 If `Ret=void`, the return type is checked to be exactly `void`.
28 __header `#include <boost/type_traits/has_dereference.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>`
30 [has_binary_operator_compat]
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`.]
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`.]
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`.]
52 [*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits]
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.
61 struct A { private: void operator*(); };
62 boost::has_dereference<A>::value; // error: A::operator*() is private
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.
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
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`.
80 #include <boost/type_traits/has_dereference.hpp>
84 struct contains { T data; };
87 bool operator*(const contains<T> &rhs) {
93 bool f(const good&) { }
96 std::cout<<std::boolalpha;
97 // works fine for contains<good>
98 std::cout<<boost::has_dereference< contains< good > >::value<<'\n'; // true
101 // does not work for contains<bad>
102 std::cout<<boost::has_dereference< contains< bad > >::value<<'\n'; // true, should be false
104 *b; // compile time error
109 * `volatile` qualifier is not properly handled and would lead to undefined behavior