How can I use a struct as key in a std::map?

Question

I have the following code, but I get an error on on the last line:

struct coord { 
    int x, y; 

    bool operator=(const coord &o) {
        return x == o.x && y == o.y;
    }

    bool operator<(const coord &o) {
        return x < o.x || (x == o.x && y < o.y);
    }
};

map<coord, int> m;
pair<coord, int> p((coord{0,0}),123);
m.insert(p); // ERROR here

How can I use a struct as key in a map?

---

I tried to change the code to this:

struct coord { 
    int x, y; 

    bool const operator==(const coord &o) {
        return x == o.x && y == o.y;
    }

    bool const operator<(const coord &o) {
        return x < o.x || (x == o.x && y < o.y);
    }
};

But I'm still getting the following error:

C:\Users\tomc\Desktop\g>mingw32-make g++ test.cpp -std=c++0x In file included from c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/string:5 0:0, from c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/bits/loc ale_classes.h:42, from c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/bits/ios _base.h:43, from c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/ios:43, from c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/ostream: 40, from c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/iostream :40, from test.cpp:1: c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/bits/stl_function.h: In member function 'bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _ Tp = coord]': c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/bits/stl_tree.h:1184:4: inst antiated from 'std::pair, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = coord, _Val = std::pair, _KeyOfValue = std::_Select1st >, _Compare = std::less, _Alloc = std::allocator>]' c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/bits/stl_map.h:501:41: insta ntiated from 'std::pair, std::_Select1st >, _Compare, typename _Alloc::rebind ::value_type>::other>::iterator, bool> std ::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::map<_Key, _Tp, _Compare, _ Alloc>::value_type&) [with _Key = coord, _Tp = int, _Compare = std::less, _Alloc = std::allocator >, typename std::_Rb_tree<_ Key, std::pair, std::_Select1st >, _ Compare, typename _Alloc::rebind::value_ty pe>::other>::iterator = std::_Rb_tree_iterator >, st d::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair]' test.cpp:56:12: instantiated from here c:\mingw\bin../lib/gcc/mingw32/4.5.2/include/c++/bits/stl_function.h:230:22: er ror: passing 'const coord' as 'this' argument of 'const bool coord::operator<(co nst coord&)' discards qualifiers mingw32-make: *** [game] Error 1

Answer 1

Try and make operator < const:

bool operator<(const coord &o)  const {

(Your = operator should probably be == operator and const as well)

Answer 2

By far the simplest is to define a global "less than" operator for your struct in stead of as a member function.

std::map uses - by default - the 'lessthan' functor which, in turn, uses the global "operator<" defined for the key type of the map.

bool operator<(const coord& l, const coord& r) {
     return (l.x<r.x || (l.x==r.x && l.y<r.y));
}

Answer 3

Another solution, which may be used for third-party data types, is to pass a Comparison object as third template parameter. example

Answer 4

As mentioned in the answer by Andrii, you can provide a custom comparison object to the map instead of defining operator< for your struct. Since C++11, you can also use a lambda expression instead of defining a comparison object. Moreover, you don't need to define operator== for your struct to make the map work. As a result, you can keep your struct as short as this:

struct coord {
    int x, y;
};

And the rest of your code could be written as follows:

auto comp = [](const coord& c1, const coord& c2){
    return c1.x < c2.x || (c1.x == c2.x && c1.y < c2.y);
};
std::map<coord, int, decltype(comp)> m(comp);

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