no overloaded function found-CodePudding

I have class KeyA that is from third party and will be the key in use. When I define a "<" operator, it complains:

error: no match for ‘operator<’ (operand types are ‘const KeyA’ and ‘const KeyA’)

some simplified code to show the issue:

#include <map>
using namespace std;

struct KeyA {  // defined in somewhere else
    int a;
};

namespace NS {
struct config {
    using Key = KeyA; // type alias
    using Table = map<Key, int>;
};

bool operator <(const config::Key& lhs, const config::Key& rhs) {
    return lhs.a <rhs.a ;
}
}

int main()
{
    using namespace NS;
    config::Table table;
    table[{1}]= 2;

    return 0;
}

What happens here? and how to solve this (no way to touch KeyA and most probably have to keep the overloaded function in NS)?

CodePudding user response：

One simple option is to just define your own comparator and supply that to the std::map template arguments instead:

struct config
{
    using Key = KeyA; // type alias

    struct KeyLess {
        bool operator ()(const Key& lhs, const Key& rhs) const {
            return lhs.a < rhs.a;
        }
    };

    using Table = map<Key, int, KeyLess>;
};

You can leave the other comparison function there if you want. I removed it, since it looks like you only defined it for the map anyway.

CodePudding user response：

You can use the following complete working program to solve your problem.

#include <map>
using namespace std;

struct KeyA {  // defined in somewhere else
    int a;
    //friend declaration
    friend bool operator <(const KeyA& lhs, const KeyA& rhs);
};
bool operator <(const KeyA& lhs, const KeyA& rhs) {
    return lhs.a <rhs.a ;
}
namespace NS {
struct config {
    using Key = KeyA; // type alias
    using Table = map<Key, int>;
};


}

int main()
{
    using namespace NS;
    config::Table table;
    table[{1}]= 2;

    return 0;
}

The output of the above program can be seen here.