C++ file stream open modes ambiguity

Question

For performing file IO in C++ we use the ofstream, ifstream and fstream classes.

• ofstream: Stream class to write on files
  
• ifstream: Stream class to r

Answer 1

Because the mode is not limited to input/output. The constructor of ifstream, for example, looks like:

explicit ifstream ( const char * filename, ios_base::openmode mode = ios_base::in );

Note the default value is ios_base::in, so you don't have to specify it yourself. However, the mode sets the streams flags which are not limited to in/out, but include:

• 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.