[Solved] Memory (de)allocation within slots and public functions

VikMorroHun

Hi!

I have a form which has some buttons on it. Let's say I press ButtonA which clicked() signal is connected to its on_buttonA_clicked() slot. In this slot some memory is allocated with calloc() then deallocated with free(). Program works fine. But if I call a public member function from here (and I'd like to) called function1() and I put free() into this function, the program becomes unstable, crashes at exit. Clearly the cause of this behaviour is that free() cannot recognize the public variable I'm using. Is there a way to make this bug-free?

Here is a sample class:
@
#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
Q_OBJECT

public:
explicit Widget(QWidget *parent = 0);
void function1();
char *pchar; //to allocate a char array on the heap
~Widget();

private slots:
void on_buttonA_clicked();

private:
Ui::Widget *ui;
};

#endif // WIDGET_H
@

mlong

I won't go into commentary on your design, except to say that I feel strongly that it's mostly likely a series of poor choices (using calloc rather than new, using character arrays instead of Qt's native container classes, etc.) and should probably be written to be more C++-like rather than C-like.

However, with that briefly said:

Make sure you always initialize your pointers to 0 initially.

If you call free on a pointer, be sure and set it's value to 0. If you try to free memory twice, you'll have a bad time. Calling free() on a null pointer is harmless.

VikMorroHun

It seems the problem occurs with a more complex data structure:

@
struct rek {
QByteArray ID, Offset;
unsigned int length;
bool trans;
char * string;
};
rek * record;

void Widget::function1() {
QMessageBox::information(this, tr("Ok."), record[0].string);
}

void Widget::on_buttonA_clicked() {
rek * record=new rek[20]; //let's assume allocation was successful
record[0].string=(char *)calloc(1000,sizeof(char)); //detto
strcpy(record[0].string,"some text...");
QMessageBox::information(this, tr("Ok."), record[0].string); //"some text..." appear in a messagebox
function1(); //empty string or garbage appear in a messagebox - data structure unavailable, trying to free() it causes crash
}
@

I'm not sure what would be more C++ like code, so if you could tell me I would be thankful. On the other hand I have to handle non-latin characters which are not stored in Unicode format so it seemed appropriate to stick to the old character arrays and string functions to prevent data loss.

mlong

You're redeclaring @rek *record@ in line 14, which is masking the declaration at line 7.

VikMorroHun

Thank you very much. Now I can use the debugger again. It couldn't handle this line, but it didn't tell me what the problem was either.