Problem with std::map and std::pair

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.)