CODEWITHSUNDEEP
c++vectorreferencestack
Tabrock
Tabrock

Reputation: 1169

Can't pass value by reference C++

I'm building a stack and queue class that are based on my Vector class. Each one, I need to call a function that will allow me to modify the value at that index.

For stack, that function is:

T &top()

However, I can't seem to modify this value, which is what my end goal is. Currently, I have removed the ampersands from the stack function, so the value can be displayed, but not modified.

When I include the ampersand on stack, or queue, the error is the same. So I assume the problem is the same, and I believe that it's syntax related

for ex.:

error C2440: 'return' : cannot convert from 'float' to 'float &'

Here is my Stack.h

template <class T>
class Stack{
private:
    Vector<T> stack;
public:
    void push(const T &x) {stack.push_back(x);}
    void pop(){stack.pop_back();}
    bool empty(){return stack.empty();}
    int currentCapacity() const {return stack.currentCapacity();}
    int size(){return stack.size();}
    T &top() {return stack.back();}

};

Here is my Vector header file, and the function I'm calling in Stack:

#include<iostream>
using namespace std;

const int SIZEFACTOR = 4;

template <class T>
class Vector{
private:
    unsigned int size_Of_Vector; // # of Items in list
    unsigned int total_Vector_Capacity;//Total Capacity
    T * vector_array;//Items themselves
public:
    Vector();
    ~Vector();  
    void push_back(const T &e);
    void pop_back();
    void pop_front();
    bool empty();
    int size() const;
    void growVector();
    void shrinkVector();
    void shrinkToSize();
    int currentCapacity() const;
    T back();
    T front();
    //Operator
    const T & operator [] (int index){

        if((index >= size_Of_Vector) || index < 0){
            cout << "ERROR! Index not used: " << index
                 << " (max = " << size_Of_Vector << ")" << endl;
            return EXIT_FAILURE;            
        }    

        return vector_array[index]; 
    };//End Operator    
};//End Header Definition

template <class T> 
T Vector<T>::back(){
    if(size_Of_Vector == 0){
        cout << "Vector is EMPTY" << endl;
        return 0;
    }
    return vector_array[size_Of_Vector-1];//returns top
}

Anyone able to help point me in the right direction? The end goal I'm trying to reach is to be able to call something like: (Assuming that the template is int)

x.top() += 5;

And then have a modified value stored on the stack.

Upvotes: 2

Views: 279

Answers (1)

Karl Nicoll
Karl Nicoll

Reputation: 16419

It is because you are passing back a copy of the object inside your vectors back and front methods, and not a reference.

void shrinkToSize();
int currentCapacity() const;
T back();      // < Copy returned from this method, should be T&
T front();     // < Same here!

Change the T in the lines above to T& and you should be good to go.

Upvotes: 5

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