ceph/src/boost/libs/type_traits/doc/has_post_increment.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_post_increment has_post_increment]
   9    template <class Lhs, class Ret=dont_care>
  10    struct has_post_increment : public __tof {};
  11
  12 __inherit
  13 If (i) `lhs` of type `Lhs` can be used in expression `lhs++`,
  14 and (ii) `Ret=dont_care` or the result of expression `lhs++` 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 postfix `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 f(lhs++); // is valid if has_post_increment<Lhs, 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_post_increment.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_post_increment<Lhs, Ret>::value_type` is the type `bool`.]
  35 [:`has_post_increment<Lhs, Ret>::value` is a `bool` integral constant expression.]
  36 [:`has_post_increment<int>::value` is a `bool` integral constant expression that evaluates to `true`.]
  37 [:`has_post_increment<long>` inherits from `__true_type`.]
  38
  39 [:`has_post_increment<int, int>` inherits from `__true_type`.]
  40 [:`has_post_increment<int, long>` inherits from `__true_type`.]
  41 [:`has_post_increment<double, double>` inherits from `__true_type`.]
  42 [:`has_post_increment<double, int>` inherits from `__true_type`.]
  43 [:`has_post_increment<bool>` inherits from `__true_type`.]
  44
  45 [:`has_post_increment<const int>` inherits from `__false_type`.]
  46 [:`has_post_increment<void*>` inherits from `__false_type`.]
  47 [:`has_post_increment<int, std::string>` inherits from `__false_type`.]
  48
  49
  50 [*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits]
  51
  52 [*Known issues:]
  53
  54 * This trait cannot detect whether postfix `operator++` is public or not:
  55 if `operator++` is defined as a private member of `Lhs` then
  56 instantiating `has_post_increment<Lhs>` will produce a compiler error.
  57 For this reason `has_post_increment` cannot be used to determine whether a type has a public `operator++` or not.
  58 ``
  59 struct A { private: void operator++(int); };
  60 boost::has_post_increment<A>::value; // error: A::operator++(int) is private
  61 ``
  62
  63 * There is an issue if the operator exists only for type `A` and `B` is
  64 convertible to `A`. In this case, the compiler will report an ambiguous overload.
  65 ``
  66 struct A { };
  67 void operator++(const A&, int);
  68 struct B { operator A(); };
  69 boost::has_post_increment<A>::value; // this is fine
  70 boost::has_post_increment<B>::value; // error: ambiguous overload
  71 ``
  72
  73 * There is an issue when applying this trait to template classes.
  74 If `operator++` is defined but does not bind for a given template type,
  75 it is still detected by the trait which returns `true` instead of `false`.
  76 Example:
  77 ``
  78 #include <boost/type_traits/has_post_increment.hpp>
  79 #include <iostream>
  80
  81 template <class T>
  82 struct contains { T data; };
  83
  84 template <class T>
  85 bool operator++(const contains<T> &lhs, int) {
  86         return f(lhs.data);
  87 }
  88
  89 class bad { };
  90 class good { };
  91 bool f(const good&, const good&) { }
  92
  93 int main() {
  94         std::cout<<std::boolalpha;
  95         // works fine for contains<good>
  96         std::cout<<boost::has_post_increment< contains< good > >::value<<'\n'; // true
  97         contains<good> g;
  98         g++; // ok
  99         // does not work for contains<bad>
 100         std::cout<<boost::has_post_increment< contains< bad > >::value<<'\n'; // true, should be false
 101         contains<bad> b;
 102         b++; // compile time error
 103         return 0;
 104 }
 105 ``
 106
 107 * `volatile` qualifier is not properly handled and would lead to undefined behavior
 108
 109 [endsect]
 110