C++ overwriting data in a file at a particular position

Question

i m having problem in overwriting some data in a file in c++. the code i m using is

 int main(){
   fstream fout;
   fout.open("hello.txt",fstream::binary | fstream::out | fstream::app);
   pos=fout.tellp();
   fout.seekp(pos+5);
   fout.write("####",4);
   fout.close();
   return 0;

}

the problem is even after using seekp ,the data is always written at the end.I want to write it at a particular position. And if i dont add fstream::app , the contents of the file are erased. Thanks.

What is the exact position of the data you want to overwrite? — Flinsch, Sep 04 '11 at 15:43

Eli Iser · Accepted Answer · 2011-09-04T16:03:15.570

15

The problem is with the fstream::app - it opens the file for appending, meaning all writes go to the end of the file. To avoid having the content erased, try opening with fstream::in as well, meaning open with fstream::binary | fstream::out | fstream::in.

edited Sep 04 '11 at 16:03

answered Sep 04 '11 at 15:44

Eli Iser

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but isnt fstream::in for reading from stream ,how will i be able to write to the stream using that? – karyboy Sep 04 '11 at 15:58
2

If you use both `fstream::in` and `fsteam::out` you'll open the file for reading and writing - meaning it will be opened for writing without deleting the previous content. – Eli Iser Sep 04 '11 at 16:02

score 5 · Answer 2 · answered Sep 04 '11 at 16:17

You want something like

fstream fout( "hello.txt", fstream::in | fstream::out | fstream::binary );
fout.seek( offset );
fout.write( "####", 4 );

fstream::app tells it to move to the end of the file before each output operation, so even though you explicitly seek to a position, the write location gets forced to the end when you do the write() (that is seekp( 0, ios_base::end );).

cf. http://www.cplusplus.com/reference/iostream/fstream/open/

Another thing to note is that, since you opened the file with fstream::app, tellp() should return the end of the file. So seekp( pos + 5 ) should be trying to move beyond the current end of file position.

C++ overwriting data in a file at a particular position

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