Weird errors, please help.

Thanks for everyone's help so far but now I have a new error when I try to run my program.

18 [main] Driver2 5868 _cygtls::handle_exceptions: Exception: STATUS_ACCESS_VIOLATION

623 [main] Driver2 5868 open_stackdumpfile: Dumping stack trace to Driver2.exe.stackdump

Code:

#include "compTree.h"
#include <iostream>


int main(){
	CompTree t;
	t.add(10);
	t.add(9);
	t.add(8);
	
	cout<< t.remove();
	
	return 0;
}

Code:

#include "compTree.h"




CompTree::CompTree(){
	v[0] = -1;  //set rank 0 to null because the root of the tree starts at rank 1
}

CompTree::Position CompTree::add(int x){
	v.push_back(x);
	return last();
}

int CompTree::remove(){
	int temp = v.back();
	v.pop_back();
	return temp;
}

int CompTree::removeRoot(){
	v[1]=last().element();
	return remove();
}

CompTree::Position CompTree::last(){
	if(!isEmpty())
		return Position(v.size()-1,&v);
}

CompTree::Position CompTree::root(){
	if(!isEmpty())
		return Position(1,&v);
}

CompTree::Position CompTree::leftChild(Position &p){
	if(!isEmpty())
		return Position(p.rank*2 ,&v);
}

CompTree::Position CompTree::rightChild(Position &p){
	if(!isEmpty())
			return Position(p.rank*2+1 ,&v);
}

CompTree::Position CompTree::sibling(Position &p){
	int temp = parent(p).rank;
	if(v.at((int)floor(temp/2))== p.element())
		return Position((int)floor(temp/2+1),&v);
	else
		return Position((int)floor(temp/2),&v);
}

CompTree::Position CompTree::parent(Position &p){
	return Position((int)floor(p.rank/2) ,&v);
}

bool CompTree::isInternal(Position &p){
	int temp = (int)floor(p.rank*2);
		if ((int)v.size()>= temp)
			return true;
		else
			return false;
}

bool CompTree::isExternal(Position &p){
	int temp = (int)floor(p.rank*2);
	if ((int)v.size()>= temp)
		return false;
	else
		return true;
}	

bool CompTree::isEmpty(){
	if(v.at(1) != -1)
		return false;
	else
		return true;
}

void CompTree::swap(CompTree::Position &p1, CompTree::Position &p2){
	int temp = p1.element();
	p1.ptr->at(p1.rank) = p2.element();
	p2.ptr->at(p2.rank) = temp;
}

int CompTree::size(){
	return v.size()-1; //because v[0] is not considered part of the tree.
}

CompTree::~CompTree(){};

CompTree::Position::Position(int r, vector<int> *vp){
	rank = r; 
	ptr = vp;
}

int CompTree::Position::element(){
	return ptr->at(rank);
}

bool CompTree::Position::isNull(){
	if(rank == -1)
		return true;
	else
		return false;
}
CompTree::Position::~Position(){}

Code:

#ifndef COMPTREE_H_
#define COMPTREE_H_

#include <vector>
#include <math.h>

using namespace std;


class CompTree {
public:
	class Position;
	
	vector<int> v;
	
	CompTree();
	Position add(int x);
	Position last();
	Position root();
	Position leftChild(Position &p);
	Position rightChild(Position &p);
	Position sibling(Position &p);
	Position parent(Position &p);
	int remove();
	int removeRoot();
	bool isInternal(Position &p);
	bool isExternal(Position &p);
	bool isEmpty();
	void swap(Position &p1, Position &p2);
	int size();
	~CompTree();
	
	
	
	class Position {
	public:
		Position(int r, vector<int> *vp);
		int element();
		bool isNull();
		int rank;
		vector<int> *ptr;
		~Position();
	};
		
};
#endif /*COMPTREE2_H_*/

Your constructor accesses the vector out-of-bounds. You cannot access any item through indices in an empty vector.
You can use push_back or construct the vector with one item with value -1 to begin with:

Code:

CompTree::CompTree():
    v(1, -1)
{
	//v[0] = -1;  //was out of bounds
}