STL set of structs

[creativeinspira]

Hi Everybody. If any of you know how to make an STL set of classes or structs, that would be cool. Which operators do you need to implements so if I have myclass, I can have an stl set of type myclass. Examples are welcome too.

[Daved]

You need a way to compare instances of your class. One way is an operator<, which makes sense if there is one obvious way to compare two class objects. Another option is to write a comparison class whose operator() compares two class instances like a global operator< would. If you use that then you have to specify it when you declare your set. If you just use operator< then the set will automatically use that for its sorting.

[hk_mp5kpdw]

First method as described by Daved...

#include <set>
...

struct foo
{
    int bar;
};

bool operator<(const foo& left, const foo& right)
{
    return left.bar < right.bar;
}

int main()
{
    std::set<foo> fooset;  // Uses operator< automatically for sorting items in the container

    ...

    return 0;
}

Or, second method as described above...

#include <set>
...

struct foo
{
    int bar;
};

struct comp
{
    inline bool operator()(const foo& left,const foo& right)
    {
        return left.bar < right.bar;
    }
};

int main()
{
    std::set<foo,comp> fooset;  // Uses comparison struct/class object comp to sort the container

    ...

    return 0;
}

If the data member you are comparing happens to be private then you'll need to make adjustments, either declaring operator< as a friend to the class/struct if you use that method, or creating accessor functions for returning the value of those private data members so they can be compared.

[edit]Yay, my 2000th post, I thought for sure I was still around 1997 or so...[/edit]

[laserlight]

To be pedantic: a predicate like comp should not change its state after a call or a copy. After all, if a and b do not change, then (a < b) should likewise not change. As such, it would be better to write:

struct comp
{
    inline bool operator()(const foo& left,const foo& right) const
    {
        return left.bar < right.bar;
    }
};