C++ file streams: which std::ios open mode flag tells the library to open a file for input (reading)?

Introduction / Context:When working with std::fstream, std::ifstream, or std::ofstream, you specify open modes using std::ios flags. Correct selection of flags controls whether the file is opened for reading, writing, appending, truncation, or binary access. This question focuses on the flag that enables input operations.

Given Data / Assumptions:

• Standard C++ iostreams and fstreams are used.
• We distinguish between reading (input) and writing (output) modes.
• Flags may be combined with bitwise OR for multiple behaviors.

Concept / Approach:

• ios::in opens for reading.
• ios::out opens for writing.
• ios::app appends all output to the end of the file.
• Other strings like “add::ios” or “in::file” are not valid std::ios flags.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• ios::app: Appending output; not input.
• ios::out: Output (writing) mode.
• add::ios / in::file: Not valid C++ flags.

Common Pitfalls:

• Forgetting to combine ios::in | ios::binary for binary input.
• Assuming ios::out implies read access; it does not.

Final Answer: