VFSFileOutput

VFSFile file; //initialized somewhere before

auto output = file.output;
with(output)
{
    //Write to the file:
    write(cast(const void[])"The answer is ??");

    //Change the last two characters in the file:
    seek(-2, Seek.End);
    write(cast(const void[])"42");
}

//Appending:
//When appending to the file, every write writes to the end of file
//regardless of any calls to seek(), and sets the file position
//to end of file. This
//(This is to stay in line with the C standard so we can use C functions directly)
auto output = file.output(Yes.append);
with(output)
{
    //Append to the file:
    write(cast(const void[])"The answer is ??");

    //This will NOT change the last 2 characters: it will append anyway:
    seek(-2, Seek.End);
    write(cast(const void[])"42");
}