boost::iterator_adapter not working with STL algorithms
Domanda
Consider the following "round trip" iterator, which tries to iterate over all the elements in a collection, eventually iterating over the first element again as its final step:
#include <boost/iterator/iterator_adaptor.hpp>
template<typename IteratorBase>
class roundtrip_iterator
: public boost::iterator_adaptor<
roundtrip_iterator<IteratorBase>, // the derived class overriding iterator behavior
IteratorBase, // the base class providing default behavior
boost::use_default, // iterator value type, will be IteratorBase::value_type
std::forward_iterator_tag, // iterator category
boost::use_default // iterator reference type
>
{
private:
IteratorBase m_itBegin;
IteratorBase m_itEnd;
bool m_complete;
public:
roundtrip_iterator( IteratorBase itBegin, IteratorBase itEnd )
: iterator_adaptor_(itBegin), m_itBegin(itBegin), m_itEnd(itEnd), m_complete(false)
{}
void increment()
{
if( m_complete )
{
base_reference() = m_itEnd;
return;
}
++base_reference();
if(base_reference() == m_itEnd)
{
base_reference() = m_itBegin;
m_complete = true;
}
}
};
I have only implemented increment for now.
As it stands, the iterator seems to work well in standard "for" loops, but I can't get it to work with STL algorithms. For example:
int main(int argc, char *argv[])
{
std::vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
roundtrip_iterator<std::vector<int>::iterator> roundtrip(v.begin(), v.end());
for( ; roundtrip.base() != v.end(); ++roundtrip)
std::cout << *roundtrip << std::endl;
std::cout << std::endl;
roundtrip_iterator<std::vector<int>::iterator> roundtrip2(v.begin(), v.end());
std::for_each(
roundtrip2.base(), v.end(),
print);
}
Prints:
1
2
3
1 // First element printed out using standard for loop.
1
2
3 // The for_each algo stops here for some reason.
Does anybody have any ideas as to the difference between the two?
Soluzione
By calling roundtrip2.base()
, you're effectively passing the range [v.begin(), v.end)
to std::for_each
. You need to be able to construct a one-past-the-end value e
such that you can pass [roundtrip2, e)
instead.
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