Standard library sort and user defined types

It is possible to use standard function if your type implements "bool operator < (...) const" and a copy constructor (compiler-generated or custom).

struct MyType {
    int a;
    int b;
    bool operator < (const MyType& other) const {
        ... // a meaningful implementation for your type
    }
    // Copy constructor (unless it's a POD type).
    MyType(const MyType &other)
        : a(other.a), b(other.b) { }
    // Some other form of construction apart from copy constructor.
    MyType()
        : a(0), b(0) { }
};

Alternatively, you can pass an ordering function (or a functor) as a third argument to sort() instead of implementing operator "<".

bool type_is_less(const MyType& t1, const MyType& t2) { ... }
...
std::sort(c.begin(), c.end(), type_is_less);

This is useful in the following situations:

1. you don’t want to implement operator "<" for whatever reason,
2. you need to sort a container of built-in or pointer types for which you can’t overload operators.
3. you wish to sort the sequence using different orderings. ex: sometimes you want a struct with first/last name members sorted by first name, other times by last name. two different functions (or functors) make such options trivial.