ceph/src/boost/libs/type_traits/doc/has_complement.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_complement has_complement]
   9    template <class Rhs, class Ret=dont_care>
  10    struct has_complement : public __tof {};
  11
  12 __inherit
  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.
  17
  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`:
  21 ``
  22 void f(Ret);
  23 Rhs rhs;
  24 f(~rhs); // is valid if has_complement<Rhs, Ret>::value==true
  25 ``
  26 If `Ret=void`, the return type is checked to be exactly `void`.
  27
  28 __header `#include <boost/type_traits/has_complement.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>`
  29
  30 [has_binary_operator_compat]
  31
  32 __examples
  33
  34 [:`has_complement<Rhs, Ret>::value_type` is the type `bool`.]
  35 [:`has_complement<Rhs, Ret>::value` is a `bool` integral constant expression.]
  36 [:`has_complement<int>::value` is a `bool` integral constant expression that evaluates to `true`.]
  37 [:`has_complement<long>` inherits from `__true_type`.]
  38
  39 [:`has_complement<int, int>` inherits from `__true_type`.]
  40 [:`has_complement<int, long>` inherits from `__true_type`.]
  41 [:`has_complement<const int>` inherits from `__true_type`.]
  42
  43 [:`has_complement<int*>` inherits from `__false_type`.]
  44 [:`has_complement<double, double>` inherits from `__false_type`.]
  45 [:`has_complement<double, int>` inherits from `__false_type`.]
  46 [:`has_complement<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 prefix `operator~` is public or not:
  54 if `operator~` is defined as a private member of `Rhs` then
  55 instantiating `has_complement<Rhs>` will produce a compiler error.
  56 For this reason `has_complement` cannot be used to determine whether a type has a public `operator~` or not.
  57 ``
  58 struct A { private: void operator~(); };
  59 boost::has_complement<A>::value; // error: A::operator~() 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&);
  67 struct B { operator A(); };
  68 boost::has_complement<A>::value; // this is fine
  69 boost::has_complement<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_complement.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> &rhs) {
  85         return f(rhs.data);
  86 }
  87
  88 class bad { };
  89 class good { };
  90 bool f(const good&) { }
  91
  92 int main() {
  93         std::cout<<std::boolalpha;
  94         // works fine for contains<good>
  95         std::cout<<boost::has_complement< contains< good > >::value<<'\n'; // true
  96         contains<good> g;
  97         ~g; // ok
  98         // does not work for contains<bad>
  99         std::cout<<boost::has_complement< contains< bad > >::value<<'\n'; // true, should be false
 100         contains<bad> b;
 101         ~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