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