Tellg

Tellg is a function in the input stream family (std::basic_istream and its derived types, such as std::ifstream and std::istringstream) that returns the current position of the get pointer in the associated stream. It yields a value of type std::basic_ios::pos_type (typically std::streampos) and is a const member function.

Return value semantics: On success, tellg provides the position indicator for the input sequence. If an error occurs or the position cannot be determined, it returns pos_type(-1). In text mode the reported position can be implementation-dependent due to character translations, while in binary mode it typically corresponds to a byte offset from the beginning. The result can be used with seekg to reposition the input pointer.

Usage notes: tellg is commonly used to inspect how much data has been read, to compute sizes by seeking to the end and then telling the position, or to save and restore a position for non-destructive lookahead. After performing reads, tellg reflects the current position of the get pointer, subject to any buffering and stream state.

Error handling: If the stream is in a failed or bad state, tellg may return pos_type(-1). If the stream’s exceptions mask includes ios_base::failure, an exception may be thrown on error. tellg does not change the stream’s state beyond what the underlying operations already did, aside from reporting the position.

See also: seekg, tellp, and the general behavior of std::istream position handling.