Pointers...

[salmanmanekia]

Hi.
Can someone shed a a bit og light on the differences between the following

QVector <int*> *rect;

QVector <int*> *rect;

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QVector <int> *rect;

QVector <int> *rect;

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QVector <int*> rect;

QVector <int*> rect;

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QVector <int> rect;

QVector <int> rect;

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[Zlatomir]

What don't you understand about pointers?

They are not that hard to understand, if you think about "storing" the address of a variable (a good analogy is the address of a house, on an envelope you put the pointer (address) not the actual object(house))

I'll tell you a little about the code you post (but i don't think that will help you understand pointers) so please ask what you don't understand

QVector <int*> *rect; /* rect is a pointer to a QVector <witch stores pointers to ints> carefull with uninitialized pointers, here you don't have the actual QVector, only a pointer*/
QVector <int*> rect;   // rect is a QVector <in witch you can add pointers to int>
QVector <int> *rect;   // rect is a pointer to a QVector <witch stores integers> carefull with uninitialized pointers... same as first case

QVector <int*> *rect; /* rect is a pointer to a QVector <witch stores pointers to ints> carefull with uninitialized pointers, here you don't have the actual QVector, only a pointer*/
QVector <int*> rect;   // rect is a QVector <in witch you can add pointers to int>
QVector <int> *rect;   // rect is a pointer to a QVector <witch stores integers> carefull with uninitialized pointers... same as first case

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[salmanmanekia]

Thanks for the reply...
In my project i am using QGraphicsView/Scene and Items .Since i have to have many items which i dont know at runtime so i decided to have a vector of it ..
so after this consideration i wrote something like

H_file : QVector <QGraphicsRectItem> rect;
then i tried to setRect the rect like
CPP_file : rect.at(0).setRect(QReal)....

H_file : QVector <QGraphicsRectItem> rect;
then i tried to setRect the rect like
CPP_file : rect.at(0).setRect(QReal)....

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but this throws a runtime error..
I am not good at pointers but i searched for QVector in this forum and found something similar to what i had been doing so i tried to implement that code which was

H_file : QVector <QGraphicsRectItem *> *rect;
CPP_file : rect = new QVector<QGraphicsRectItem>();
and the rect->at(0)->setRect...

H_file : QVector <QGraphicsRectItem *> *rect;
CPP_file : rect = new QVector<QGraphicsRectItem>();
and the rect->at(0)->setRect...

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but again a run time error follow..

[Zlatomir]

That is because, this code:

QVector <QGraphicsRectItem> rect; //rect has "place" for 0 elements

QVector <QGraphicsRectItem> rect; //rect has "place" for 0 elements

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create an empty QVector, and you need to use:

QVector <QGraphicsRectItem> rect;
rect.push_back(someInstanceOf_QGraphicRectItem); //this add one element (at the end) to the empty QVector

QVector <QGraphicsRectItem> rect;
rect.push_back(someInstanceOf_QGraphicRectItem); //this add one element (at the end) to the empty QVector

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or

QVector <QGraphicsRectItem> rect(10); //10 is the number of elements, it can grow, with push_back or resize...
rect[0] = someInstanceOf_QGraphicRectItem; //or rect[0].set....
... // this way (operator[]) the position is not checked, so carefull

QVector <QGraphicsRectItem> rect(10); //10 is the number of elements, it can grow, with push_back or resize...
rect[0] = someInstanceOf_QGraphicRectItem; //or rect[0].set....
... // this way (operator[]) the position is not checked, so carefull

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rect.at(positon) it's for accessing elements, not for set the elements, so it returns the const element at "position" (with check that the position is valid)

QVector reference