Brian Silverman | 4a2409e | 2018-08-04 23:24:02 -0700 | [diff] [blame^] | 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 | |