Problem with std::map and std::pair - c++

Question

I have a small program I want to execute to test something
#include <map>
#include <iostream>
using namespace std;
struct _pos{
float xi;
float xf;
bool operator<(_pos& other){
return this->xi < other.xi;
}
};
struct _val{
float f;
};
int main()
{
map<_pos,_val> m;
struct _pos k1 = {0,10};
struct _pos k2 = {10,15};
struct _val v1 = {5.5};
struct _val v2 = {12.3};
m.insert(std::pair<_pos,_val>(k1,v1));
m.insert(std::pair<_pos,_val>(k2,v2));
return 0;
}
The problem is that when I try to compile it, I get the following error
$ g++ m2.cpp -o mtest
In file included from /usr/include/c++/4.4/bits/stl_tree.h:64,
from /usr/include/c++/4.4/map:60,
from m2.cpp:1:
/usr/include/c++/4.4/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = _pos]’:
/usr/include/c++/4.4/bits/stl_tree.h:1170: instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = _pos, _Val = std::pair<const _pos, _val>, _KeyOfValue = std::_Select1st<std::pair<const _pos, _val> >, _Compare = std::less<_pos>, _Alloc = std::allocator<std::pair<const _pos, _val> >]’
/usr/include/c++/4.4/bits/stl_map.h:500: instantiated from ‘std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::pair<const _Key, _Tp>&) [with _Key = _pos, _Tp = _val, _Compare = std::less<_pos>, _Alloc = std::allocator<std::pair<const _pos, _val> >]’
m2.cpp:30: instantiated from here
/usr/include/c++/4.4/bits/stl_function.h:230: error: no match for ‘operator<’ in ‘__x < __y’
m2.cpp:9: note: candidates are: bool _pos::operator<(_pos&)
$
I thought that declaring the operator< on the key would solve the problem, but its still there.
What could be wrong?
Thanks in advance.

Answer 1

The problem is this:
bool operator<(_pos& other)
Should be this:
bool operator<(const _pos& other) const {
// ^^^^ ^^^^^
Without the first const, the right-hand side of the comparison (b in a < b) cannot be const, since without const the function may modify its argument.
Without the second const, the left-hand side of the comparison (a in a < b) cannot be const, since without const the function may modify this.
Internally, the key's of a map are always const.
It should be noted that you should prefer to use nonmember functions. That is, better is a free-function:
bool operator<(const _pos& lhs, const _pos& rhs)
{
return lhs.xi < rhs.xi;
}
In the same namespace as your class. (For our example, just underneath it.)
By the way, in C++ there is no need to prefix the declaration of a struct type variable with struct. This is perfect, and preferred:
_pos k1 = {0,10};
_pos k2 = {10,15};
_val v1 = {5.5};
_val v2 = {12.3};
(Though your type names are admittedly named in an unorthodox manner. :P)
Lastly, you should prefer the make_pair utility function for making pairs:
m.insert(std::make_pair(k1,v1));
m.insert(std::make_pair(k2,v2));
It saves you from having to write out the types for the pair, and is generally easier to read. (Especially when longer type names come along.)

Answer 2

Signature of the less than operator needs to be bool operator<(const _pos& other) const, otherwise map can not use this operator in const functions since this member function is declared as non-const.

Answer 3

I think that your definition of operator< is wrong - the right hand side (argument in this case) should be marked const and it should be a const member function, e.g.
bool operator<(const _pos& other) const{
return this->xi < other.xi;
}