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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?
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The seekp(pos) method of ostream in C++ is used to set the position of the pointer in the output sequence with the specified position. This method takes the new position to be set and returns this ostream instance with the position set to the specified new position.
Whereasseekp() function is used to move/sets the put pointer at the particular/desired location to putting the data into a file (External file). And If seekp() function is used to move the put pointer at the particular/desired location.
Use seekp() to position the file position before writing. Use seekg() to position the file position before reading. If you use one, don't for get to use the other.
C++ tellg(), seekg() and seekp() Example tellg(), seekg() and seekp() functions are used to set/get the position of get and put pointers in a file while reading and writing.
In other words there is only one pointer maintained. 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.
We have a function called seekp which gets imported along with the other basic file handling operation in “fstream”. So what does seekp () do? The seekp () function moves the pointer to the desired location. When we create a text file or open a text file, our pointer is set to 0.
tellg (), seekg () and seekp () functions are used to set/get the position of get and put pointers in a file while reading and writing. pos – represents the new absolute position within the stream (from the beginning). off – represents the offset to seek. ios_base::beg / ios::beg – beginning of the stream.
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.
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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.
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