ostream

ostream is the standard output stream class in the C++ standard library. It is defined as part of the iostream facilities and represents the destination for character output. In standard terminology, std::ostream is an alias for std::basic_ostream<char>, with std::wostream serving as the wide-character counterpart. The class participates in the iostream hierarchy and uses a stream buffer (std::streambuf) to perform the actual I/O operations.

The primary interface of ostream is the insertion operators, commonly written as operator<<. These operators are overloaded to handle a variety of built-in types, as well as many library types, and can be extended by user-defined types through appropriate overloads. The operator<< calls return a reference to the stream, enabling chaining of output expressions, such as std::cout << "Value: " << x << '\n'.

An ostream maintains formatting state that affects how data is written. This includes format flags, field width, precision, and the fill character. Manipulators, many defined in <iomanip> (for example std::setw, std::setprecision) or standard ones like std::endl and std::flush, modify this state and influence subsequent output operations. The state can be queried and modified, and the stream can be checked for errors using good(), fail(), and related member functions.

Common instances include std::cout for standard output, std::cerr for unbuffered error output, and std::clog for buffered logging. The standard library also provides derived stream classes such as std::ostringstream, std::istringstream, and std::ofstream, which extend ostream for string-based or file-based I/O. User code can extend ostream by overloading operator<< for custom types to customize how objects are rendered to streams.