Question

I’m having an issue with the wrong method being called. In my program, I have 3 classes: symbol, nonTerminal, and terminal. nonTerminal and terminal are both subclasses of symbol.

In my program, I only ever create a terminal or nonTerminal. Here are some condensed versions of each class:

#ifndef SYMBOL_H
#define SYMBOL_H

#include <vector>

class terminal;

using namespace std;

class symbol {
   public:
        virtual vector<terminal> getFirstSet();
};

#endif

_

#ifndef NONTERMINAL_H
#define NONTERMINAL_H

#include "symbol.h"
#include "terminal.h"
#include <vector>

using namespace std;

class terminal;

class nonTerminal: public symbol {
    public:
        vector<terminal> getFirstSet();
};

#endif

_

#ifndef TERMINAL_H
#define TERMINAL_H

#include "symbol.h"
#include <vector>

using namespace std;

class terminal: public symbol {
    public:
        vector<terminal> getFirstSet();
};

#endif

I have this function:

bool addFirst(symbol s) {
    vector<terminal> first = s.getFirstSet();
    //....
}

However, anytime I call it with a terminal or nonTerminal, it always uses the symbol::getFirstSet method. How can I get it to call the correct nonTerminal::getFirstSet or terminal::getFirstSet method?

Answer 1

Your function addFirst is receiving your object by value. What this means is that:

1. Every time you call addFirst() a new symbol object is created and copied from existing terminal or nonTerminal.

2. Whenever said copy occurs, the resulting object is neither terminal nor nonTerminal, but their base class – symbol.

To counter it, and to avoid copying the object, try passing your object by reference:

bool addFirst (symbol &_s) {
    vector<terminal> first = s.getFirstSet();
    //....
}