Friendship and the Attorney-Client Idiom

Figure 1: The Meyers algorithm.

Given a class T and a function f:

if (f needs to be virtual)
{
    f should be a member function of T;
}
else if ( (f is operator>>) or (f is operator<<) or 
	            (f needs type conversions on its left-most argument) )
{
    f should be a non-member function;
    if (f needs access to non-public members of T)
    {
        f should be a friend of T;
    }
}
else if (f can be implemented via T's public interface)
{
    f should be a non-member function;
}
else
{
    f should be a member function of T;
}