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_logical_not has_logical_not]
9 template <class Rhs, class Ret=dont_care>
10 struct has_logical_not : 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_logical_not<Rhs, Ret>::value==true
26 If `Ret=void`, the return type is checked to be exactly `void`.
28 __header `#include <boost/type_traits/has_logical_not.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>`
30 [has_binary_operator_compat]
34 [:`has_logical_not<Rhs, Ret>::value_type` is the type `bool`.]
35 [:`has_logical_not<Rhs, Ret>::value` is a `bool` integral constant expression.]
36 [:`has_logical_not<int>::value` is a `bool` integral constant expression that evaluates to `true`.]
37 [:`has_logical_not<bool>` inherits from `__true_type`.]
39 [:`has_logical_not<int, bool>` inherits from `__true_type`.]
40 [:`has_logical_not<int, long>` inherits from `__true_type`.]
41 [:`has_logical_not<double, double>` inherits from `__true_type`.]
42 [:`has_logical_not<double, bool>` inherits from `__true_type`.]
43 [:`has_logical_not<const bool>` inherits from `__true_type`.]
45 [:`has_logical_not<int, std::string>` inherits from `__false_type`.]
48 [*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits]
52 * This trait cannot detect whether prefix `operator!` is public or not:
53 if `operator!` is defined as a private member of `Rhs` then
54 instantiating `has_logical_not<Rhs>` will produce a compiler error.
55 For this reason `has_logical_not` cannot be used to determine whether a type has a public `operator!` or not.
57 struct A { private: void operator!(); };
58 boost::has_logical_not<A>::value; // error: A::operator!() is private
61 * There is an issue if the operator exists only for type `A` and `B` is
62 convertible to `A`. In this case, the compiler will report an ambiguous overload.
65 void operator!(const A&);
66 struct B { operator A(); };
67 boost::has_logical_not<A>::value; // this is fine
68 boost::has_logical_not<B>::value; // error: ambiguous overload
71 * There is an issue when applying this trait to template classes.
72 If `operator!` is defined but does not bind for a given template type,
73 it is still detected by the trait which returns `true` instead of `false`.
76 #include <boost/type_traits/has_logical_not.hpp>
80 struct contains { T data; };
83 bool operator!(const contains<T> &rhs) {
89 bool f(const good&) { }
92 std::cout<<std::boolalpha;
93 // works fine for contains<good>
94 std::cout<<boost::has_logical_not< contains< good > >::value<<'\n'; // true
97 // does not work for contains<bad>
98 std::cout<<boost::has_logical_not< contains< bad > >::value<<'\n'; // true, should be false
100 !b; // compile time error
105 * `volatile` qualifier is not properly handled and would lead to undefined behavior