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_logical_and has_logical_and] 9 template <class Lhs, class Rhs=Lhs, class Ret=dont_care> 10 struct has_logical_and : public __tof {}; 11 12__inherit 13If (i) `lhs` of type `Lhs` and `rhs` of type `Rhs` can be used in expression `lhs&&rhs`, 14and (ii) `Ret=dont_care` or the result of expression `lhs&&rhs` is convertible to `Ret` 15then inherits from __true_type, 16otherwise inherits from __false_type. 17 18The default behaviour (`Ret=dont_care`) is to not check for the return value of binary `operator&&`. 19If `Ret` is different from the default `dont_care` type, the return value is checked to be convertible to `Ret`. 20Convertible to `Ret` means that the return value of the operator can be used as argument to a function expecting `Ret`: 21`` 22void f(Ret); 23Lhs lhs; 24Rhs rhs; 25f(lhs&&rhs); // is valid if has_logical_and<Lhs, Rhs, Ret>::value==true 26`` 27If `Ret=void`, the return type is checked to be exactly `void`. 28 29__header `#include <boost/type_traits/has_logical_and.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>` 30 31__examples 32 33[:`has_logical_and<Lhs, Rhs, Ret>::value_type` is the type `bool`.] 34[:`has_logical_and<Lhs, Rhs, Ret>::value` is a `bool` integral constant expression.] 35[:`has_logical_and<int>::value` is a `bool` integral constant expression that evaluates to `true`.] 36[:`has_logical_and<bool>` inherits from `__true_type`.] 37 38[:`has_logical_and<int, int, bool>` inherits from `__true_type`.] 39[:`has_logical_and<int, int, long>` inherits from `__true_type`.] 40[:`has_logical_and<int, double, bool>` inherits from `__true_type`.] 41[:`has_logical_and<const int, int>::value` inherits from `__true_type`.] 42 43[:`has_logical_and<int, int, std::string>` inherits from `__false_type`.] 44 45 46[*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits] 47 48[*Limitation:] 49 50* Requires a compiler with working SFINAE. 51 52[*Known issues:] 53 54* This trait cannot detect whether binary `operator&&` is public or not: 55if `operator&&` is defined as a private member of `Lhs` then 56instantiating `has_logical_and<Lhs>` will produce a compiler error. 57For this reason `has_logical_and` cannot be used to determine whether a type has a public `operator&&` or not. 58`` 59struct A { private: void operator&&(const A&); }; 60boost::has_logical_and<A>::value; // error: A::operator&&(const A&) is private 61`` 62 63* There is an issue if the operator exists only for type `A` and `B` is 64convertible to `A`. In this case, the compiler will report an ambiguous overload. 65`` 66struct A { }; 67void operator&&(const A&, const A&); 68struct B { operator A(); }; 69boost::has_logical_and<A>::value; // this is fine 70boost::has_logical_and<B>::value; // error: ambiguous overload 71`` 72 73* There is an issue when applying this trait to template classes. 74If `operator&&` is defined but does not bind for a given template type, 75it is still detected by the trait which returns `true` instead of `false`. 76Example: 77`` 78#include <boost/type_traits/has_logical_and.hpp> 79#include <iostream> 80 81template <class T> 82struct contains { T data; }; 83 84template <class T> 85bool operator&&(const contains<T> &lhs, const contains<T> &rhs) { 86 return f(lhs.data, rhs.data); 87} 88 89class bad { }; 90class good { }; 91bool f(const good&, const good&) { } 92 93int main() { 94 std::cout<<std::boolalpha; 95 // works fine for contains<good> 96 std::cout<<boost::has_logical_and< contains< good > >::value<<'\n'; // true 97 contains<good> g; 98 g&&g; // ok 99 // does not work for contains<bad> 100 std::cout<<boost::has_logical_and< contains< bad > >::value<<'\n'; // true, should be false 101 contains<bad> b; 102 b&&b; // compile time error 103 return 0; 104} 105`` 106 107* `volatile` qualifier is not properly handled and would lead to undefined behavior 108 109[endsect] 110 111