The iostream library contains two basic types, istream and ostream, representing input streams and output streams, respectively.
Four IO objects are defined.

std::cin // standard input
std::cout // standard output
std::cerr // standard error
std::clog // standard logging(some information during program runtime)

e.g.

#include <iostream>
int main(){
    std::cout << "Enter two numbers:" << std::endl;
    int v1, v2;
    std::cin >> v1 >> v2;
    std::cout << "The sum of " << v1 << " and " << v2
              << " is " << v1 + v2 << std::endl;
    return 0;
}