C++ :: Compare Data Members Of Objects Store In Vector

Overview of problem : I am using std::vector to hold objects of Subject. Now this vector contains lots of objects( with lots I mean 10-20 objects at max) . These objects have string member values like category and sub_category. Both category and sub_category can have string which can be same of other objects's sub_category & category.

Issue: Now I want my std::vector to have only those objects whose's sub_category are unique. If category is not unique that's not a problem .

Secondly if we found 2 objects having same sub_category then we have to delete one of them from the vector. we will delete it based on some rules example

Rules for deleting are if

i) instance of Subject ->category = " Land " OR if category = "Jungle" then delete other duplicate object ,
ii) if above condition doesn't match then delete either of them.

I am wondering , how would I compare the sub-items from the vector . For example. I have class say Subject

class Subject {
public :
// some constructors,
// functions to get ., set category and sub category
std::String get_sub_category()
std::string get_category();
private:
std::string category;
std::string sub_category;
}

I have vector which stores object of Subjects. Example : vector<Subject> copy_vector;

Now what I want is to delete the object from vector that has same sub_category I am not looking for source code buT i need a starting point,? Example:

copy_vector[0] = Animal object that has sub_category Tiger
copy_vector [1] = Animal object with Lion as sub category
copy_vector[2] = Forest object with sub_category Tiger

What I want is to based on some conditions(which I can do ) remove either Forest or Animal object containing Tiger. But for that how would I do comparison? I have written the function and have checked it.

std::vector< Subject >copy_vector;
// copy_vector contains all the objects of Subject with redundant sub_category
for( std::vector< Subject >::iterator ii = copy_vector.begin() ; ii != copy_vector.end() ; ++ii ) {
sub_category = ii->get_sub_category();

[code] ....