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).
8 [section:has_post_decrement has_post_decrement]
9 template <class Lhs, class Ret=dont_care>
10 struct has_post_decrement : public __tof {};
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.
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`:
24 f(lhs--); // is valid if has_post_decrement<Lhs, Ret>::value==true
26 If `Ret=void`, the return type is checked to be exactly `void`.
28 __header `#include <boost/type_traits/has_post_decrement.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>`
30 [has_binary_operator_compat]
34 [:`has_post_decrement<Lhs, Ret>::value_type` is the type `bool`.]
35 [:`has_post_decrement<Lhs, Ret>::value` is a `bool` integral constant expression.]
36 [:`has_post_decrement<int>::value` is a `bool` integral constant expression that evaluates to `true`.]
37 [:`has_post_decrement<long>` inherits from `__true_type`.]
39 [:`has_post_decrement<int, int>` inherits from `__true_type`.]
40 [:`has_post_decrement<int, long>` inherits from `__true_type`.]
41 [:`has_post_decrement<double, double>` inherits from `__true_type`.]
42 [:`has_post_decrement<double, int>` inherits from `__true_type`.]
44 [:`has_post_decrement<bool>` inherits from `__false_type`.]
45 [:`has_post_decrement<const int>` inherits from `__false_type`.]
46 [:`has_post_decrement<void*>` inherits from `__false_type`.]
47 [:`has_post_decrement<int, std::string>` inherits from `__false_type`.]
50 [*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits]
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_decrement<Lhs>` will produce a compiler error.
57 For this reason `has_post_decrement` cannot be used to determine whether a type has a public `operator--` or not.
59 struct A { private: void operator--(int); };
60 boost::has_post_decrement<A>::value; // error: A::operator--(int) is private
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.
67 void operator--(const A&, int);
68 struct B { operator A(); };
69 boost::has_post_decrement<A>::value; // this is fine
70 boost::has_post_decrement<B>::value; // error: ambiguous overload
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`.
78 #include <boost/type_traits/has_post_decrement.hpp>
82 struct contains { T data; };
85 bool operator--(const contains<T> &lhs, int) {
91 bool f(const good&, const good&) { }
94 std::cout<<std::boolalpha;
95 // works fine for contains<good>
96 std::cout<<boost::has_post_decrement< contains< good > >::value<<'\n'; // true
99 // does not work for contains<bad>
100 std::cout<<boost::has_post_decrement< contains< bad > >::value<<'\n'; // true, should be false
102 b--; // compile time error
107 * `volatile` qualifier is not properly handled and would lead to undefined behavior