Can I pass a parameter to an std::vector sort function?

Question

Consider the class:

MyClass {
    int varA;
    int varB;
};

I have a vector of pointers to MyClass objects:

std::vector<MyClass*> Vec;

I want to sort the vector according to varA or varB using the same sort function, i.e. :

bool SortFunction(const MyClass* obj1, const MyClass* obj2, const short type) {
     if( type == VARA_ID )
         return obj1->varA < obj2->varA;
     else if( type == VARB_ID )
         return obj1->varB < obj2->varB;
}

AFAICT this is not possible. What would be the most elegant way to this without using external libraries?

Answer 1

class sorter {
      short type_;
public:
      sorter(short type) : type_(type) {}
      bool operator()(MyClass const* o1, MyClass const* o2) const {
            return SortFunction(o1, o2, type_ );
      }
};

std::sort(Vec.begin(), Vec.end(), sorter(MY_TYPE) );

Answer 2

You're almost there, make type a template parameter and the signature is OK:

template<int type>
bool SortFunction(const MyClass* obj1, const MyClass* obj2) {
     if( type == VARA_ID )
         return obj1->varA < obj2->varA;
     else // if( type == VARB_ID ) -- A sort function must have a default.
         return obj1->varB < obj2->varB;
}

std::sort(Vec.begin(), Vec.end(), &SortFunction<VARA_ID> );

The optimizer will spot that ( type == VARA_ID ) is a compile-time constant.