如何使用const getters对std :: set进行排序 [英] How to sort a std::set with const getters
问题描述
我有一个std :: set容器,其元素是以下类的对象:
I have a std::set container whose elements are objects of the following class:
class LaneConnector {
public:
const Lane* getLaneFrom() const {
return From;
}
const Lane* getLaneTo() const {
return To;
}
private:
Lane* From;
Lane* To;
}
我的比较器功能如下:
struct MyLaneConectorSorter {
bool operator() (LaneConnector * c, LaneConnector * d)
{
Lane* a = const_cast<Lane*>(c->getLaneFrom());
Lane* b = const_cast<Lane*>(d->getLaneFrom());
return (a->getLaneID() < b->getLaneID());
}
} myLaneConnectorSorter;
现在,当我尝试使用以下方法对集合中的元素进行排序时:
Now when I try to sort the elements in the set with:
//dont panic, the container just came through a const_iterator of a std::map :)
const std::set<LaneConnector*> & tempLC = (*it_cnn).second;
std::sort(tempLC.begin(), tempLC.end(), myLaneConnectorSorter);
从以下几行开始,我疯狂地犯了错误,如果您能帮助我解决此问题,请感谢. 谢谢:
I get a frenzy of errors starting with the following lines, Appreciate if you help me solve this problem. Thanks:
/usr/include/c++/4.6/bits/stl_algo.h: In function ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = std::_Rb_tree_const_iterator<LaneConnector*>, _Compare = {anonymous}::MyLaneConectorSorter]’:
/home/.../dev/Basic/shared/conf/simpleconf.cpp:1104:65: instantiated from here
/usr/include/c++/4.6/bits/stl_algo.h:5368:4: error: no match for ‘operator-’ in ‘__last - __first’
/usr/include/c++/4.6/bits/stl_algo.h:5368:4: note: candidates are:
/usr/include/c++/4.6/bits/stl_iterator.h:321:5: note: template<class _Iterator> typename std::reverse_iterator::difference_type std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
/usr/include/c++/4.6/bits/stl_iterator.h:378:5: note: template<class _IteratorL, class _IteratorR> typename std::reverse_iterator<_IteratorL>::difference_type std::operator-(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&)
/usr/include/c++/4.6/bits/stl_bvector.h:181:3: note: std::ptrdiff_t std::operator-(const std::_Bit_iterator_base&, const std::_Bit_iterator_base&)
/usr/include/c++/4.6/bits/stl_bvector.h:181:3: note: no known conversion for argument 1 from ‘std::_Rb_tree_const_iterator<LaneConnector*>’ to ‘const std::_Bit_iterator_base&’
推荐答案
First, you cannot sort an std::set. It is a sorted structure, sorting happens upon construction or insertion.
第二,您可以使用自己的排序函子构造一个std::set
,并且可以通过将其取为const pointers
来避免不必要的const_casts
:
Second, you can construct an std::set
with your own sorting functor, and you can avoid unnecessary const_casts
by making it take const pointers
:
struct MyLaneConectorSorter {
bool operator() (const LaneConnector* lhs, const LaneConnector* rhs) const
{
// you may want to put some null pointer checks in here
const Lane* a = lhs->getLaneFrom();
const Lane* b = rhs->getLaneFrom();
return a->getLaneID() < b->getLaneID();
}
};
并实例化集合:
std::set<LaneConnector*, MyLaneConectorSorter> s(MyLaneConectorSorter());
或者,如果您想以不同的顺序从另一个集合构造它,
or, if you want to construct it from a different set, with a different ordering,
std::set<LaneConnector*> orig = ..... ;
....
std::set<LaneConnector*, MyLaneConectorSorter> s(orig.begin(), orig.end(), MyLaneConectorSorter());
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