What happens when ios::in mode is specified with an output file stream?

Question

What is the purpose of ios::in and ios::out modes while creating file streams in c++? I have created an output file stream as ofstream myout("file.txt",ios::in); This is an output file stream but ios::in mode is specified. What is the effect of ios::in mode in output stream and similarly ios::out mode in input file stream?

I have looked here, but couldn't understand.

Answer 1

Per std::basic_filebuf<CharT,Traits>::open(), which better explains what the various flag combinations actually do:

The file is opened as if by calling std::fopen with the second argument (mode) determined as follows:

mode	openmode & ~ate	Action if file already exists	Action if file does not exist
"r"	in	Read from start	Failure to open
"w"	out, out|trunc	Destroy contents	Create new
"a"	app, out|app	Append to file	Create new
"r+"	out|in	Read from start	Error
"w+"	out|in|trunc	Destroy contents	Create new
"a+"	out|in|app, in|app	Write to end	Create new
"rb"	binary|in	Read from start	Failure to open
"wb"	binary|out, binary|out|trunc	Destroy contents	Create new
"ab"	binary|app, binary|out|app	Write to end	Create new
"r+b"	binary|out|in	Read from start	Error
"w+b"	binary|out|in|trunc	Destroy contents	Create new
"a+b"	binary|out|in|app, binary|in|app	Write to end	Create new

If openmode is not one of the modes listed, the open() fails.

If the open operation succeeds and openmode & std::ios_base::ate != 0 (the ate bit is set), repositions the file position to the end of file, as if by calling std::fseek(file, 0, SEEK_END), where file is the pointer returned by calling fopen. If the repositioning fails, calls close() and returns a null pointer to indicate failure.

If the associated file was already open, returns a null pointer right away.