CODEWITHSUNDEEP
c++
Денис Яриков
Денис Яриков

Reputation: 41

map with struct as key returns incorrect value

I have

struct data_cell{
    int owner;
    std::vector<int> shares;
};

data_cell** data; //grid, very big
std::map<data_cell, data_cell*> cells; //all uniq cells

bool operator==(const data_cell &b, const data_cell &o) {
    return (b.owner == o.owner && b.shares == o.shares);
}

bool operator<(const data_cell &b, const data_cell &o) {
    return (b.owner < o.owner && b.shares != o.shares);
}

int to_grid(float, float);

sometimes when I do:

for (int i=0;i<ids.size();i++){//example
    data_cell o=ids[i];
    //some work where fills o.shares
    data[to_grid(x,y)]=(cells[o]?:cells[o]=(new data_cell(o)));
    printf("%d |%d|%d|%d %d\n", s.id, o.owner, cells[o]->owner, data[to_grid(x,y)]->owner, to_grid(x,y));
}

I got that o.owner != cells[o]->owner, (printf shows different values), and if I print cells[o] before assign it returns nonzero pointer, but this key doesn't exist in map.

I use gcc-4.6.

what is wrong with this code?

Add1: Change to 

bool operator<(const data_cell &b, const data_cell &o) {
    return (b.owner < o.owner && b.shares < o.shares);
}

doesn't help, but 

bool operator<(const data_cell &b, const data_cell &o) {
    return (b.owner < o.owner && b.shares <= o.shares);
}

Add2: The last version that works(I hope):

bool operator<(const data_cell &b, const data_cell &o) {
    return (b.owner < o.owner && b.shares <= o.shares) ||
            (b.owner <= o.owner && b.shares < o.shares);
}

May be some additions?

Upvotes: 0

Views: 54

Answers (1)

Bathsheba
Bathsheba

Reputation: 234635

Your == and < operators have to be such that exactly one of a < b, a == b, and b < a is true, and the other two false. That is not the case here due to b.shares != o.shares in your implementation of operator<.

Upvotes: 3

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