Question

I recently was bitten by the fact that ios_base::width and/or the setw manipulator have to be reset with every item written to the stream.

That is, you must do this:

while(whatever)
{
    mystream << std::setw(2) << myval;
}

Rather than this:

mystream.width(2);
while(whatever)
{
    mystream << myval;
}

Ok, fine.

But does anyone know why this design decision was made?
Is there some rationale that I’m missing, or is this just a dark corner of the standard?

Other stream formatting modifiers (as mentioned in the linked SO question) are ‘sticky’, while setw is not.

Answer 1

The way i see it is : You can always do something like below if you want it to be applied uniformly.

int width =2;
while(whatever)
{
    mystream << std::setw(width) << myval;
}

but if it was sticky as you mention:

mystream.width(2);
while(whatever)
{
    mystream << myval;
}

and if i wanted a different width every line I have to keep setting width.

So essentially both approaches are almost the same, and i would like or dislike them depending on what i am doing now.