Why does calling istream :: tellg () affect the behavior of my program?

Question

I am trying to convert a 24 bit bitmap to grayscale.

#include<iostream> #include<fstream> #include<conio.h> #include<stdio.h> using namespace std; class pixel{ public: unsigned char b; unsigned char g; unsigned char r; void display() { cout<<r<<" "<<g<<" "<<b<<" "; } }p1; using namespace std; int main(){ unsigned char avg; fstream file("image.bmp",ios::binary|ios::in|ios::out); int start; file.seekg(10); file.read((char*)&start,4); file.seekg(start); int i=0; while(!file.eof()){ cout<<file.tellg();//Remove this and the program doesn't work! file.read((char*)&p1,3); avg=(p1.b+p1.g+p1.r)/3; p1.b=avg; p1.g=avg; p1.r=avg; file.seekg(-3,ios::cur); file.write((char*)&p1,3); } file.close(); getch(); return 0; } 

When I delete the cout tellg statement, the loop only works twice!

I don't understand what is the difference when deleting a cout statement?

Result: only one pixel changes to grayscale.

I found a simpler version of my problem here

Reading and writing to files at the same time?

But did not find a solution ...

Answer 1

When reading and writing std::fstream you need to search when switching between reading and writing. The reason for this is because file streams share a common input and output position. To also maintain effective buffering, it is necessary to inform the corresponding other buffer about the current position. This is part of what the search does. tellg() searches for the current position.

Note that it is very difficult to switch between reading and writing, especially when the implementation is well optimized. You will be much better off writing another file or updating values ​​in reasonable size groups.