How to insert text in an existing file?

Question

Example, I have a sample.txt file with content:

1 2 3 7 8 9 10

and I want to insert 4 5 6in file to have

1 2 3 4 5 6 7 8 9 10

so that numbers are inserted in the right place.

Answer 1

Files generally don't support inserting text in the middle. You should read the file, update the contents and overwrite the file.

Use a sorted container, e.g. std::set to hold the contents of the file in memory.

std::set<int> contents;

// Read the file
{
    std::ifstream input("file");
    int i;
    while (input >> i)
        contents.insert(i);
}

// Insert stuff
contents.insert(4);
contents.insert(5);
contents.insert(6);

// Write the file
{
    std::ofstream output("file");
    for (int i: contents)
        output << i << ' ';
}