ceph/src/boost/libs/type_traits/doc/has_unary_plus.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_unary_plus has_unary_plus]
   9    template <class Rhs, class Ret=dont_care>
  10    struct has_unary_plus : 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_unary_plus<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_unary_plus.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_unary_plus<Rhs, Ret>::value_type` is the type `bool`.]
  35 [:`has_unary_plus<Rhs, Ret>::value` is a `bool` integral constant expression.]
  36 [:`has_unary_plus<int>::value` is a `bool` integral constant expression that evaluates to `true`.]
  37 [:`has_unary_plus<long>` inherits from `__true_type`.]
  38
  39 [:`has_unary_plus<int, int>` inherits from `__true_type`.]
  40 [:`has_unary_plus<int, long>` inherits from `__true_type`.]
  41 [:`has_unary_plus<double, double>` inherits from `__true_type`.]
  42 [:`has_unary_plus<double, int>` inherits from `__true_type`.]
  43 [:`has_unary_plus<const int>` inherits from `__true_type`.]
  44
  45 [:`has_unary_plus<int, std::string>` inherits from `__false_type`.]
  46
  47
  48 [*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits]
  49
  50 [*Known issues:]
  51
  52 * This trait cannot detect whether prefix `operator+` is public or not:
  53 if `operator+` is defined as a private member of `Rhs` then
  54 instantiating `has_unary_plus<Rhs>` will produce a compiler error.
  55 For this reason `has_unary_plus` cannot be used to determine whether a type has a public `operator+` or not.
  56 ``
  57 struct A { private: void operator+(); };
  58 boost::has_unary_plus<A>::value; // error: A::operator+() is private
  59 ``
  60
  61 * There is an issue if the operator exists only for type `A` and `B` is
  62 convertible to `A`. In this case, the compiler will report an ambiguous overload.
  63 ``
  64 struct A { };
  65 void operator+(const A&);
  66 struct B { operator A(); };
  67 boost::has_unary_plus<A>::value; // this is fine
  68 boost::has_unary_plus<B>::value; // error: ambiguous overload
  69 ``
  70
  71 * There is an issue when applying this trait to template classes.
  72 If `operator+` is defined but does not bind for a given template type,
  73 it is still detected by the trait which returns `true` instead of `false`.
  74 Example:
  75 ``
  76 #include <boost/type_traits/has_unary_plus.hpp>
  77 #include <iostream>
  78
  79 template <class T>
  80 struct contains { T data; };
  81
  82 template <class T>
  83 bool operator+(const contains<T> &rhs) {
  84         return f(rhs.data);
  85 }
  86
  87 class bad { };
  88 class good { };
  89 bool f(const good&) { }
  90
  91 int main() {
  92         std::cout<<std::boolalpha;
  93         // works fine for contains<good>
  94         std::cout<<boost::has_unary_plus< contains< good > >::value<<'\n'; // true
  95         contains<good> g;
  96         +g; // ok
  97         // does not work for contains<bad>
  98         std::cout<<boost::has_unary_plus< contains< bad > >::value<<'\n'; // true, should be false
  99         contains<bad> b;
 100         +b; // compile time error
 101         return 0;
 102 }
 103 ``
 104
 105 * `volatile` qualifier is not properly handled and would lead to undefined behavior
 106
 107 [endsect]
 108