Why can't I map a structure in C++?

Question

I've declared a structure like this ->

struct data{
    int x,y;
    bool operator < (const data& other) {
        return x<other.x or y<other.y;
    }
};

Now I want to map it as a key and with a bool value.

int main()
{
    data a;
    map<data,bool>mp;
    a.x=12, a.y=24;
    mp[a]=true;
}

The last line gives me this error ->

error: passing 'const' as 'this' argument of 'bool data::operator<(const data&)' discards qualifiers

How can I fix this ??

@Ajay [See this](http://en.cppreference.com/w/cpp/language/operator_alternative) — Rakete1111, Aug 11 '16 at 05:10
Yeah, but from where it is coming? The code you post here should compile everywhere! :) — Ajay, Aug 11 '16 at 05:10
@Oktalist Thanks a lot. Yeah, I know that it was wrong. I wrote it just as a demo. But I also don't know the logic behind mapping a structure! Would you please tell me how to design the logic to map a structure ?? — jbsu32, Aug 13 '16 at 07:14

score 11 · Accepted Answer · answered Aug 11 '16 at 04:56

11

std::map<Key, Value> internally stores them as std::map<const Key, Value>. The important thing here is that the Key is const.

So, in your example, data is const, but operator< is not! You cannot call a non-const method from a const object, so the compiler complains.

You'll have to specify operator< as const:

bool operator<(const data& other) const { /*...*/ }
                                  ^^^^^

answered Aug 11 '16 at 04:56

Rakete1111

Thank! It works :) Just tell me one more thing! Does it work fine when I compare two `data` type structures outside like `a – jbsu32 Aug 11 '16 at 05:11
1

@JishnuBanerjee why not? :) – Rakete1111 Aug 11 '16 at 05:12
2

@JishnuBanerjee a `const` method can be called on a non-const variable, but the reverse is not allowed. – Remy Lebeau Aug 11 '16 at 07:03

1 Answers1