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[/
(C) Copyright 2009-2011 Frederic Bron.
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt).
]
[section:has_pre_increment has_pre_increment]
template <class Rhs, class Ret=dont_care>
struct has_pre_increment : public __tof {};
__inherit
If (i) `rhs` of type `Rhs` can be used in expression `++rhs`,
and (ii) `Ret=dont_care` or the result of expression `++rhs` is convertible to `Ret`
then inherits from __true_type,
otherwise inherits from __false_type.
The default behaviour (`Ret=dont_care`) is to not check for the return value of prefix `operator++`.
If `Ret` is different from the default `dont_care` type, the return value is checked to be convertible to `Ret`.
Convertible to `Ret` means that the return value of the operator can be used as argument to a function expecting `Ret`:
``
void f(Ret);
Rhs rhs;
f(++rhs); // is valid if has_pre_increment<Rhs, Ret>::value==true
``
If `Ret=void`, the return type is checked to be exactly `void`.
__header `#include <boost/type_traits/has_pre_increment.hpp>` or `#include <boost/type_traits/has_operator.hpp>` or `#include <boost/type_traits.hpp>`
__examples
[:`has_pre_increment<Rhs, Ret>::value_type` is the type `bool`.]
[:`has_pre_increment<Rhs, Ret>::value` is a `bool` integral constant expression.]
[:`has_pre_increment<int>::value` is a `bool` integral constant expression that evaluates to `true`.]
[:`has_pre_increment<long>` inherits from `__true_type`.]
[:`has_pre_increment<int, int>` inherits from `__true_type`.]
[:`has_pre_increment<int, long>` inherits from `__true_type`.]
[:`has_pre_increment<double, double>` inherits from `__true_type`.]
[:`has_pre_increment<double, int>` inherits from `__true_type`.]
[:`has_pre_increment<bool>` inherits from `__true_type`.]
[:`has_pre_increment<const int>` inherits from `__false_type`.]
[:`has_pre_increment<void*>` inherits from `__false_type`.]
[:`has_pre_increment<int, std::string>` inherits from `__false_type`.]
[*See also:] [link boost_typetraits.category.value_traits.operators Operator Type Traits]
[*Limitation:]
* Requires a compiler with working SFINAE.
[*Known issues:]
* This trait cannot detect whether prefix `operator++` is public or not:
if `operator++` is defined as a private member of `Rhs` then
instantiating `has_pre_increment<Rhs>` will produce a compiler error.
For this reason `has_pre_increment` cannot be used to determine whether a type has a public `operator++` or not.
``
struct A { private: void operator++(); };
boost::has_pre_increment<A>::value; // error: A::operator++() is private
``
* There is an issue if the operator exists only for type `A` and `B` is
convertible to `A`. In this case, the compiler will report an ambiguous overload.
``
struct A { };
void operator++(const A&);
struct B { operator A(); };
boost::has_pre_increment<A>::value; // this is fine
boost::has_pre_increment<B>::value; // error: ambiguous overload
``
* There is an issue when applying this trait to template classes.
If `operator++` is defined but does not bind for a given template type,
it is still detected by the trait which returns `true` instead of `false`.
Example:
``
#include <boost/type_traits/has_pre_increment.hpp>
#include <iostream>
template <class T>
struct contains { T data; };
template <class T>
bool operator++(const contains<T> &rhs) {
return f(rhs.data);
}
class bad { };
class good { };
bool f(const good&) { }
int main() {
std::cout<<std::boolalpha;
// works fine for contains<good>
std::cout<<boost::has_pre_increment< contains< good > >::value<<'\n'; // true
contains<good> g;
++g; // ok
// does not work for contains<bad>
std::cout<<boost::has_pre_increment< contains< bad > >::value<<'\n'; // true, should be false
contains<bad> b;
++b; // compile time error
return 0;
}
``
* `volatile` qualifier is not properly handled and would lead to undefined behavior
[endsect]
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