are “seekp” & “seekg” interchangeable?

问题

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?

回答1:

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.

回答2:

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.

来源：https://stackoverflow.com/questions/14329261/are-seekp-seekg-interchangeable

标签

c++

position

binaryfiles

fstream