Well I just noticed that by changing the position -in microsoft visual studio- through "seekp" I implicitelly also change the read-position, when handling files.
I am wondering however if this is "portable" behaviour? Can I expect the position of reading & writing to be always the same? And consequently: will tellp
& tellg
always return the same value?
For file positions they are the same. In other words there is only one pointer maintained.
From 27.9.1.1p3
:
A joint file position is maintained for both the input sequence and
the output sequence.
So, seekg
and seekp
are interchangeable for file streams. However, this is not true for other types of streams, as they may hold separate pointers for the put and get positions.
Update: So from all the comments and everything, it seems that for fstream, seekp and seekg use the same pointer. But for stringstream and probably other non-file based streams, they are separate.
Original Post:
Doesn't work for me on linux with g++ 4.7.2. They seem to be independent:
#include <sstream>
#include <iostream>
int main(int, char**) {
std::stringstream s("0123456789");
std::cout << "put pointer: " << s.tellp() << std::endl;
std::cout << "get pointer: " << s.tellg() << std::endl;
std::cout << std::endl;
s.seekp(2);
std::cout << "put pointer: " << s.tellp() << std::endl;
std::cout << "get pointer: " << s.tellg() << std::endl;
std::cout << std::endl;
s.seekg(4);
std::cout << "put pointer: " << s.tellp() << std::endl;
std::cout << "get pointer: " << s.tellg() << std::endl;
std::cout << std::endl;
}
Output:
put pointer: 0
get pointer: 0
put pointer: 2
get pointer: 0
put pointer: 2
get pointer: 4
Also the behaviour you describe sounds like it doesn't comply with the quotes here:
Sets the position of the get pointer. The get pointer determines the
next location to be read in the source associated to the stream.
and here:
Sets the position of the put pointer. The put pointer determines the location in the
output sequence where the next output operation is going to take
place.