Error using custom operator< with std::less
I am trying to overload the < operator, but running into a problem.
Here is my implementation:
int Vector3D::operator < (const Vector3D &vector)
{
if(x<vector.x)
return 1;
else
return 0;
}
I am calling it using this code:
std::map<Vector3D, std::vector<const NeighborTuple *> > position;
std::set<Vector3D> pos;
for (NeighborSet::iterator it = N.begin(); it != N.end(); it++)
{
NeighborTuple const &nb_tuple = *it;
Vector exposition;
pos.insert (exposition);
position[exposition].push_back (&nb_tuple);
}
But I get this error:
/usr/include/c++/4.1.2/bits/stl_function.h: In member function 'bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = ns3::Vector3D]':
/usr/include/c++/4.1.2/bits/stl_map.h:347: instantiated from '_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = ns3::Vector3D, _Tp = std::vector<const ns3::olsr::NeighborTuple*, std::allocator<const ns3::olsr::NeighborTuple*> >, _Compare = std::less<ns3::Vector3D>, _Alloc = std::allocator<std::pair<const ns3::Vector3D, std::vector<const ns3::olsr::NeighborTuple*, std::allocator<const ns3::olsr::NeighborTuple*> > > >]'
../src/routing/olsr/olsr-routing-protocol.cc:853: instantiated from here
/usr/include/c++/4.1.2/bits/stl_function.h:227: error: passing 'const ns3::Vector3D' as 'this' argument of 'int ns3::Vector3D::operator<(const ns3::Vector3D&)' discards qualifiers
The error
passing 'const ns3::Vector3D' as 'this' argument of 'int ns3::Vector3D::operator<(const ns3::Vector3D&)' discards qualifiers
indicates that your operator< does not make the promise that the comparison won't modify the left-hand argument, whereas the map requires that the comparison operation shouldn't modify anything and is trying to use this operator for a constant instance (the map stores the key-types as const objects).
In short, such operator overloads must not mutate anything, and both operands must be declared const. As you have overloaded this as a member function, you must make the function itself const.
bool operator<(const ns3::Vector3D& rhs) const;
BTW, why don't you return a bool (the result must be true or false)?
Looks like you need to be using const_iterators:
NeighborSet::iterator
should be
NeighborSet::const_iterator
Also if your compiler supports it (C++0x) use cbegin and cend instead of begin and end.
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