Object-Oriented Programming With ANSI-C (pdf)

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3 Programming Savvy — Arithmetic Expressions
struct Node {
enum tokens token;
struct Node * left, * right;
};
This is not very flexible, however. We need to introduce a union to create a node
in which we can store a numerical value and we waste space in nodes representing
unary operators. Additionally, process() and delete() will contain switch statements
which grow with every new token which we invent.
3.5 Information Hiding
Applying what we have learned thus far, we do not reveal the structure of a node at
all. Instead, we place some declarations in a header file value.h:
const void * Add;
...
void * new (const void * type, ...);
void process (const void * tree);
void delete (void * tree);
Now we can code sum() as follows:
#include "value.h"
static void * sum (void)
{ void * result = product();
const void * type;
for (;;)
{ switch (token) {
case ’+’:
type = Add;
break;
case ’—’:
type = Sub;
break;
default:
return result;
}
scan(0);
result = new(type, result, product());
}
}
product() has the same architecture as sum() and calls on a function factor() to
recognize numbers, signs, and a sum enclosed in parentheses:
static void * sum (void);
static void * factor (void)
{ void * result;
switch (token) {
case ’+’:
scan(0);
return factor();