How to write to middle of a file in C++?

I think this should be quite simple, but my googling didn't help so far... I need to write to an existing file in C++, but not necessarily at the end of the file.

I know that when I just want to append text to my file, I can pass the flag ios:app when calling open on my stream object. However, this only let's me write to the very end of the file, but not into its middle.

I made a short program to illustrate the issue:

#include <iostream> #include <fstream> using namespace std; int main () { string path = "../test.csv"; fstream file; file.open(path); // ios::in and ios::out by default const int rows = 100; for (int i = 0; i < rows; i++) { file << i << "\n"; } string line; while (getline(file, line)) { cout << "line: " << line << endl; // here I would like to append more text to certain rows } file.close(); }

-------------Problems Reply------------

You cannot insert in the middle of the file. You have to copy old file to a new file and insert whatever you want to be in middle during copying to new file.

Otherwise if you intend to overwrite data/lines in existing file that is possible by using std::ostream::seekp() to identify the position within the file.

Category:c# Views:1 Time:2018-10-10

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