std::ios_base::openmode

member	constant	opening mode
app	(append)	Set the stream's position indicator to the end of the stream before each output operation.
ate	(at end)	Set the stream's position indicator to the end of the stream on opening.
binary	(binary)	Consider stream as binary rather than text.
in	(input)	Allow input operations on the stream.
out	(output)	Allow output operations on the stream.
trunc	(truncate)	Any current content is discarded, assuming a length of zero on opening.

write

std::ofstream out("data.txt", std::ios::app);
out << "This line will be appended to the end of the file";
out.close();

read

std::ofstream out2("data.txt", std::ios::in | std::ios::ate );
out2 << "This line will be ate to the end of the file";
out2.close();