<ios> defines the base classes for input/output streams: std::ios_base, std::basic_ios. It handles stream state (good, fail, eof), format flags, and locale.

Most user code works with derived classes like std::cin, std::cout, std::fstream, and doesn't interact with this header directly.

This example sets format flags on cout to display output in hexadecimal and fixed-point notation.

// compile: g++ -std=c++11 -o iosexample iosexample.cpp
// run: ./iosexample
// description: check stream state and set format flags
 
#include <ios>
#include <iostream>
 
int main() {
    std::cout.setf(std::ios::hex, std::ios::basefield);
    std::cout << 42 << "\n";
 
    std::cout.setf(std::ios::fixed | std::ios::showpoint);
    std::cout << 3.14 << "\n";
 
    return 0;
}