ceph/src/boost/libs/type_traits/doc/has_modulus_assign.qbk

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