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I am trying to create a set of a structure but my code keeps getting errors, and I can't find anything online.

struct smove {

    int src;
    int dst;
};

int main()
{

    smove moov;
    moov.dst = 1;
    moov.src = 2;
    set<struct smove> moovs = {moov};
    return 0;
}
πάντα ῥεῖ
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Space Been
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1 Answers1

4

Set's value type needs to be less-than comparable. That's how the container knows how elements relate to one another and in what order (including ensuring no duplicates).

Short story, make an operator< for smove.

The long story is, well, longer, because that operator is required to work in a certain way, but you can read up on that. For now, here's a simple example that uses std::tie to get a legal ordering quickly:

#include <set>
#include <tuple>

struct smove
{
    int src;
    int dst;
};

bool operator<(const smove& lhs, const smove& rhs)
{
    return std::tie(lhs.src, lhs.dst) < std::tie(rhs.src, rhs.dst);
}

int main()
{
    smove moov;
    moov.dst = 1;
    moov.src = 2;
    std::set<smove> moovs = {moov};
}
Lightness Races in Orbit
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    Very probable the problem identified, the question isn't yet completed though. – πάντα ῥεῖ Nov 01 '18 at 16:58
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    @πάνταῥεῖ Meh, it's obvious what the problem is. I ain't waitin'. – Lightness Races in Orbit Nov 01 '18 at 16:59
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    Further Info: Many std containers have requirements. Until you become familiar with them all, recommend googling up each and reading through the requirements. For example, you'll find that a copy constructor is required for some, others a copy constructor and less than operator. Keep in mind what is generated and what needs to be implemented for each. – Christopher Pisz Nov 01 '18 at 18:43