Question

this may be an amateur question, but here it goes. I have three clases:

DrawableObject:

class DrawableObject
{
private:

    int x;
    //...
public:
    int getX();
        //...
}

FormElement which inheirts from DrawableObject:

class FormElement : public DrawableObject

FormElement has a method called wasPushed:

bool FormElement::wasPushed(SDL_Event event)
{
bool wasPushed =
        (
            ( event.motion.x >= getX()) //Inherited getX()
            && // Blah blah...
        ) ? true : false;

return wasPushed;
}

Finally, TextField, which inheirts from FormElement:

class TextField : public DrawableObject

I also have a class, named Form:

class Form {

public:

    list<FormElement*> getFormElements();
    void generateForm();

private:

    list<FormElement*> formElements;
}

Form adds some TextFields to its list, in its generateForm() method:

void Form::generateForm() {

TextField *aTextField = new TextField(10, 10, 120);
    this->formElements.push_back(aTextField);
}

Later, it tries to iterate it:

for(list<FormElement*>::iterator it = getFormElements().begin()
    ; it != getFormElements().end()
    ; ++it)
        {
          if ( (*it)->wasPushed(theEvent) )
            { //Etc.

Well, the program exits, when it tries to access getX() from wasPushed method.

Could anyone please tell me why? What am I defining wrong?

I thank you very much.
Martín.

Answer 1

You are returning the list by value:

list<FormElement*> getFormElements();

it should be by reference:

list<FormElement*> &getFormElements();

When you return by value, you are getting a temporary copy of the list.

So in this code:

for(list<FormElement*>::iterator it = getFormElements().begin()
    ; it != getFormElements().end()

Your begin and end iterators are pointing to two different copies of the list. Also, those temporary copies will be destroyed before you ever have a chance to iterate over them.

You could also use the formElements member directly:

for(list<FormElement*>::iterator it = formElements.begin()
    ; it != formElements.end()
    ; ++it)