Documentation for std::basic_filebuf::seekpos

Question

I have studied documentation page for std::basic_filebuf::seekpos provided by cppreference.com and got confused by the following sentence:

If the file is open for reading, updates the get area if necessary.

As far as I can see on my computer, seekpos ends up with eback()==gptr()==egptr(), i.e. function empties get area. Does this mean that get area has been updated? It would be very nice, if someone could clarify it. Thank you!

Answer 1

From further down the page:

Many implementations do not update the get area in seekpos(), delegating to underflow() that is called by the next sgetc()

and:

on some implementations, the get area is emptied by seekpos() and the second underflow() is necessary to observe the effects

Looks like that's what happened to you.

It is open to interpretation as to what "update" means. The standard is very flexible here (note the lack of explicit postconditions), basically saying "the implementation gets to do whatever it needs to do in order to carry out its business".

In this particular case, your filebuf implementation probably emptied the get area when you seeked, then will "fill" it on the next underflow() — arguably both operations in concert are the "update", but to be honest you can spin it whichever way you like.