How to initialize dynamicly array from struct C++

Question

I have an assignment to write a function that dynamicly initialize an array from struct that is in the header file. and for some resone I am keep getting the same error "uninitialized local variable 'columnData' used this is the header file

#ifndef QUEUE_H
#define QUEUE_H


/* a queue contains positive integer values. */
typedef struct queue
{
    int arraySize;
    int* column;
} queue;

void initQueue(queue* q, unsigned int size);
void cleanQueue(queue* q);

void enqueue(queue* q, unsigned int newValue);
int dequeue(queue* q); // return element in top of queue, or -1 if empty

#endif /* QUEUE_H */

this is my code:

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

int main()
{
    queue* columnData;
    unsigned int size = 0;
    std::cout << "Please enter column size: ";
    std::cin >> size;
    initQueue(columnData, size);
    printf("%d", &columnData->column[0]);

}

void initQueue(queue* q, unsigned int size) {
    q->column = new int[size];
    q->column[0] = 5;
}

void cleanQueue(queue* q) {

}

void enqueue(queue* q, unsigned int newValue) {

}

int dequeue(queue* q) {
    return 1;
}

If someone can help me it will be great.

You never assign columnData a value in main, then you read that uninitialized value to pass to a function. You might want to have queue columnData and pass it as &columnData to your functions. Better would be to take all those functions and make them members of queue. — 1201ProgramAlarm
– 1201ProgramAlarm, Commented Oct 24, 2021 at 22:18

Vlad from Moscow · Accepted Answer · 2021-10-24 22:33:24Z

You declared an uninitialized pointer

queue* columnData;

that has an indeterminate value. So calling the function initQueue

initQueue(columnData, size);

invokes undefined behavior because within the function this pointer is dereferenced.

q->column = new int[size];
q->column[0] = 5;

Also the function does not set the data member arraySize.

You need in main to declare an object of the type queue

queue columnData;

and call the function like

initQueue( &columnData, size);

and within the function you have to set also the data member arraySize like

columnData->arraySize = size;

Pay attention to that this call

printf("%d", &columnData->column[0]);

is also wrong. You are trying to output a pointer using the incorrect conversion specifier %d.

After changing the declaration of the object columnData shown above the call of printf will look like

printf("%d", columnData.column[0]);

Though it will be more consistent to use the overloaded operator <<.

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