How time consuming is a d-pointer access?

[pospiech]

I had a discussion with a college who argued that d-pointers should never be used in any time critical situations because it would include so much overhead. I use d-pointer a lot because it makes compilation of large projects simpler.

Now I have a very simple example where time is not very critical, but I would like to have an idea what the difference is and if it would be worth not to make the variables members of a d-pointer.

The class is used to store 10 bit and greater image data and I convert back to QColor:

QColor QRgbMatrix::operator()(int x, int y)
{    
    int r = int(d->red[ArrPos(x,y)]   * d->bitsPerPixelCorrection);
    int g = int(d->green[ArrPos(x,y)] * d->bitsPerPixelCorrection);
    int b = int(d->blue[ArrPos(x,y)]  * d->bitsPerPixelCorrection);
 
    if (d->isMonochrom) {
        return QColor(g,g,g);
    } else {
        return QColor(r,g,b);
    }
}

QColor QRgbMatrix::operator()(int x, int y)
{    
    int r = int(d->red[ArrPos(x,y)]   * d->bitsPerPixelCorrection);
    int g = int(d->green[ArrPos(x,y)] * d->bitsPerPixelCorrection);
    int b = int(d->blue[ArrPos(x,y)]  * d->bitsPerPixelCorrection);

    if (d->isMonochrom) {
        return QColor(g,g,g);
    } else {
        return QColor(r,g,b);
    }
}

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This has function calls for position, isMonochrome, red, blue and green array and the bitsPerPixelCorrection.
If I wanted to create a pixmap or an image I would loop over the whole array with this function.

Matthias

[tbscope]

This has zero impact.
Especially when the compiler can optimise this. If you write d->someThing or myClass->item->anotherItem->yetAntoher->someThing, the compiler just uses the memory adres of the thing you're pointing to.

[pospiech]

This has zero impact.
Especially when the compiler can optimise this. If you write d->someThing or myClass->item->anotherItem->yetAntoher->someThing, the compiler just uses the memory adres of the thing you're pointing to.

If this really has no measureable impact I would continue to rely on d-pointers.

[tbscope]

d pointers are usually used in public api's, like the Qt library to hide private functions from the user.