Objects and member functions in C++

Overloading member functions
Objects and member functions

The declaration

ch_stack s, t, u;

creates three separate ch_stack objects of sizeof(ch_stack) bytes.
Each of these objects has its own data members:

char  s[max_len];
int   top;

A member function is conceptually part of the type.
There is no distinct member function for any of these three ch_stack objects.