C++ In a Nutshell

<iostream>

The <iostream> header declares the eight standard stream objects: cerr, cin, clog, cout, wcerr, wcin, wclog, and wcout. These objects are initialized when the first instance of ios_base::Init is constructed (or earlier), or before the main program starts. They are not destroyed during normal program execution, so any static object’s destructor or other function can use the standard I/O objects.

Each of the standard I/O objects is associated with a standard C FILE pointer (see <cstdio>). (You can sever the connection by calling ios_base::sync_with_stdio(false), as described in <ios>.) You can use narrow or wide I/O objects, but once you have performed any I/O on an underlying C stream, you cannot switch from narrow to wide or wide to narrow. For example, after writing to cerr, you cannot write to wclog because both objects use the same C stream, stderr. See Chapter 9 for more information about I/O.

Many C++ programmers assume that <iostream> automatically #includes <istream> and <ostream>, but the standard does not guarantee that behavior. Always #include every header you need, for example:

#include <cstdlib>
#include <iostream>
#include <istream>
#include <ostream>
   
// Copy standard input to standard output.
int main(  )
{
  std::cout << std::cin.rdbuf(  );
  return std::cout ? EXIT_SUCCESS : EXIT_FAILURE;
}