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_modulus has_modulus] 9 template <class Lhs, class Rhs=Lhs, class Ret=dont_care> 10 struct has_modulus : 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_modulus<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_modulus.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>` 30 31__examples 32 33[:`has_modulus<Lhs, Rhs, Ret>::value_type` is the type `bool`.] 34[:`has_modulus<Lhs, Rhs, Ret>::value` is a `bool` integral constant expression.] 35[:`has_modulus<int>::value` is a `bool` integral constant expression that evaluates to `true`.] 36[:`has_modulus<long>` inherits from `__true_type`.] 37 38[:`has_modulus<int, int, int>` inherits from `__true_type`.] 39[:`has_modulus<int, int, long>` inherits from `__true_type`.] 40[:`has_modulus<const int, int>::value` inherits from `__true_type`.] 41 42[:`has_modulus<int, double>` inherits from `__false_type`.] 43 44 45[*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits] 46 47[*Limitation:] 48 49* Requires a compiler with working SFINAE. 50 51[*Known issues:] 52 53* This trait cannot detect whether binary `operator%` is public or not: 54if `operator%` is defined as a private member of `Lhs` then 55instantiating `has_modulus<Lhs>` will produce a compiler error. 56For this reason `has_modulus` cannot be used to determine whether a type has a public `operator%` or not. 57`` 58struct A { private: void operator%(const A&); }; 59boost::has_modulus<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 63convertible to `A`. In this case, the compiler will report an ambiguous overload. 64`` 65struct A { }; 66void operator%(const A&, const A&); 67struct B { operator A(); }; 68boost::has_modulus<A>::value; // this is fine 69boost::has_modulus<B>::value; // error: ambiguous overload 70`` 71 72* There is an issue when applying this trait to template classes. 73If `operator%` is defined but does not bind for a given template type, 74it is still detected by the trait which returns `true` instead of `false`. 75Example: 76`` 77#include <boost/type_traits/has_modulus.hpp> 78#include <iostream> 79 80template <class T> 81struct contains { T data; }; 82 83template <class T> 84bool operator%(const contains<T> &lhs, const contains<T> &rhs) { 85 return f(lhs.data, rhs.data); 86} 87 88class bad { }; 89class good { }; 90bool f(const good&, const good&) { } 91 92int main() { 93 std::cout<<std::boolalpha; 94 // works fine for contains<good> 95 std::cout<<boost::has_modulus< 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_modulus< 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] 109 110