Undefined symbols for architecture x86_64 when inheriting

I have a C++ code, where I'm trying to use inheritance to reuse my code. But when I make the code I get the following error:

Undefined symbols for architecture x86_64: "QuickUnion::QuickUnion(int)", referenced from: _main in main.o ld: symbol(s) not found for architecture x86_64

clang: error: linker command failed with exit code 1 (use -v to see invocation)

make: *** [a.out] Error 1

My Makefile:

CC = g++
CFLAGS = -std=c++11 -Wall -c
LFLAGS = -std=c++11 -Wall
DEBUG = -g
OBJS = main.o unionfind/UnionFind.o unionfind/QuickUnion.o

a.out: $(OBJS)
    $(CC) $(LFLAGS) $(OBJS) $(DEBUG)

main.o: main.cpp
    $(CC) $(CFLAGS) main.cpp $(DEBUG)

unionfind/QuickUnion.o: unionfind/UnionFind.h unionfind/QuickUnion.h unionfind/QuickUnion.cpp
    $(CC) $(CFLAGS) unionfind/QuickUnion.cpp $(DEBUG)

unionfind/UnionFind.o: unionfind/UnionFind.h unionfind/UnionFind.cpp
    $(CC) $(CFLAGS) unionfind/UnionFind.cpp $(DEBUG)

The UnionFind .h and .cpp:

#ifndef UNIONFIND_H
#define UNIONFIND_H

#include <vector>

class UnionFind {
protected:
    int connectedComponents;
    std::vector<int> parents;
public:
    UnionFind(int);

    virtual void connect(int, int) = 0;
    virtual int find(int) = 0;
    bool connected(int, int);
    int count();
};

#endif

...

#include "UnionFind.h"

UnionFind::UnionFind(int size): parents(size) {
    connectedComponents = size;
    for (int i = 0; i < size; i++) {
        parents[i] = i;
    }
}

bool UnionFind::connected(int p, int q) {
    return find(p) == find(q);
}

int UnionFind::count() {
    return connectedComponents;
}

The QuickUnion .h and .cpp:

#ifndef QUICKUNION_H
#define QUICKUNION_H

#include <vector>

#include "UnionFind.h"

class QuickUnion: public UnionFind {
public:
    QuickUnion(int);

    void connect(int, int);
    int find(int);
};

#endif

...

#include "QuickUnion.h"

QuickUnion::QuickUnion(int size): UnionFind(size) {

}

void QuickUnion::connect(int p, int q) {
    int pRoot = find(p);
    int qRoot = find(q);

    if (pRoot == qRoot) {
        return;
    }

    parents[pRoot] = qRoot;

    connectedComponents--;
}

int QuickUnion::find(int p) {
    while (p != parents[p]) {
        p = parents[p];
    }
    return p;
}

Maybe there is something to do with the constructor, but I don't know neither why this error is happening nor how to correct it.

PS.: I am using OS X El Capitan.