basicistream

basic_istream is a class template in the C++ Standard Library that provides the input side of iostreams. It is defined as template<class Elem, class Traits = std::char_traits<Elem>> class basic_istream and serves as the common base for input streams such as std::istream and std::wistream. The Elem parameter denotes the character type, and Traits defines how characters are handled; by default Traits is std::char_traits<Elem>.

As part of the iostreams framework, basic_istream operates in conjunction with a stream buffer (std::basic_streambuf<Elem, Traits>) to read data. It offers a variety of input operations, including formatted extraction via operator>> and unformatted reads such as get, read, getline, and ignore. It also provides access to unformatted peeking and character-by-character reading through functions like peek and get. The extraction operators work together with locale and traits to convert characters into values of arithmetic types, strings, and user-defined types, typically using operator>> overloads.

The class uses a sentry object to streamline common input setup tasks, perform initial state checks, and manage whitespace-skipping and synchronization with the buffer. This sentry helps establish a valid reading state before actual extraction begins and influences the stream’s error state in case of failures.

basic_istream also exposes state and positioning facilities inherited from the iostream framework, including access to the underlying stream buffer via rdbuf(), and position interfaces such as tellg and seekg through the associated stream. It maintains and exposes the usual ios_base state flags (eof, fail, bad, good).

In practice, basic_istream is the foundational base for input streams in C++. std::istream is an instantiation of basic_istream<char> and std::wistream is an instantiation of basic_istream<wchar_t>, providing the primary interfaces used when reading from standard input, files, or other input sources.