How can my library source code instantiate an object from another library? I'm having trouble explaining my issue. What I'm doing is creating a library to work easily with the RFID reader/writer RC522 module. I need to create an object of the library MFRC522 in my .cpp file to be able to communticate with the module itself. I want my constructor end up like this:UltimateRFID rfid(53, 5);, the parameters being the slave select and the reset pins, respectively. 
The problem is that I don't know how to make my constructor so that it is within the scope of the functions that require the MFRC522 object to be called. Below's my code.

#include "Arduino.h"
#include "UltimateRFID.h"
#include &lt;MFRC522.h&gt;


MFRC522::MIFARE_Key key;


UltimateRFID::UltimateRFID(int ss, int rst)
{
   MFRC522 mfrc522(ss, rst); //this would be ideal, but how can my functions access the instance?
}


void UltimateRFID::initialize() //sample of a function that uses the MFRC522 object
{
  //digitalWrite(49, HIGH);
  digitalWrite(2, LOW);
  for (byte i = 0; i &lt; 6; i++) {
    key.keyByte[i] = 0xFF;
  }     
  mfrc522.PCD_Init();
  if ( ! mfrc522.PICC_IsNewCardPresent()) 
  {
    return;
  }
  if ( ! mfrc522.PICC_ReadCardSerial()) 
  {
    return;          
  }

}


Now comes my header file, although I'm not sure it matters much.

#ifndef UltimateRFID_h
#define UltimateRFID_h

#include "Arduino.h"
#include &lt;MFRC522.h&gt;

class UltimateRFID
{
  public:
    UltimateRFID(int ss, int rst);
    void initialize();
  private:
    MFRC522::MIFARE_Key key;
};

#endif


Please be patient, it's my first library.
Why not have your class just extend the MFRC522 class?

#include &lt;MFRC522.h&gt;

class UltimateRFID : public MFRC522 {
    private:
        MIFARE_Key key;

    public: 
        UltimateRFID(uint8_t ss, uint8_t rst) : MFRC522(ss, rst) {}
        // your other methods
        void initialize();
};

void UltimateRFID::initialize() {
    //digitalWrite(49, HIGH);
    digitalWrite(2, LOW);
    for (byte i = 0; i &lt; 6; i++) {
        key.keyByte[i] = 0xFF;
    }     
    PCD_Init();
    if ( ! PICC_IsNewCardPresent()) {
        return;
    }
    if ( ! PICC_ReadCardSerial()) {
        return;          
    }
}

UltimateRFID rfid(53, 5);

void setup() {
    rfid.initialize();
}

void loop() {}


Your class then becomes a "flavour" of MFRC522 with additional functionality. It can access all the public functions of the MFRC522 class, since they are now a part of your class. 

Anything you can do with an MFRC522 object you can now do with your object. As well as your extra functions.