Sorting a list of a custom type

Question

I want to have a stl list of objects where each object contains two int's. Afterwards I want to sort the list with stl::sort after the value of the first int. How do I tell the sort function that it's supposed to sort after the first int?

Answer 1

You can specify a custom sort predicate. In C++11 this is best done with a lambda:

typedef std::pair<int, int> ipair;
std::list<ipair> thelist;

thelist.sort([](const ipair & a, const ipair & b) { return a.first < b.first; });

In older versions of C++ you have to write an appropriate function:

bool compFirst(const ipair & a, const ipair & b) { return a.first < b.first; }

thelist.sort(compFirst);

(Instead if ipair you can of course have your own data structure; just modify the comparison function accordingly to access the relevant data member.)

Finally, if this makes sense, you can also equip your custom class with an operator<. That allows you to use the class freely in any ordered context, but be sure to understand the consequences of that.

Answer 2

std::list<T>::sort has a one-argument form, with the first argument being the comparison function.