struct MyStruct
{
    int key;
    std::string stringValue;

    MyStruct(int k, const std::string& s) : key(k), stringValue(s) {}
};

struct less_than_key
{
    inline bool operator() (const MyStruct& struct1, const MyStruct& struct2)
    {
        return (struct1.key < struct2.key);
    }
};

std::vector < MyStruct > vec;

vec.push_back(MyStruct(4, "test"));
vec.push_back(MyStruct(3, "a"));
vec.push_back(MyStruct(2, "is"));
vec.push_back(MyStruct(1, "this"));

std::sort(vec.begin(), vec.end(), less_than_key());

vector< cat > catSorter::SortCatsByAge(){
   vector< cat > cats_copy = cats;
   std::sort(cats_copy.begin(), cats_copy.end());
   return cats_copy;
}

class cat {
public:
    int age;
    bool operator< (const cat &other) const {
        return age < other.age;
    }
};