Concepts of Programming Languages

424 Chapter 9 Subprograms

Although this process could be defined as a macro, a macro would have the

disadvantage of not operating correctly if the parameters were expressions with

side effects. For example, suppose the macro were defined as

#define max(a, b) ((a) > (b))? (a) : (b)

This definition is generic in the sense that it works for any numeric type.

However, it does not always work correctly if called with a parameter that has

a side effect, such as

max(x++, y)

which produces

((x++) > (y)? (x++) : (y))

Whenever the value of x is greater than that of y, x will be incremented

twice.

C++ template functions are instantiated implicitly either when the func-

tion is named in a call or when its address is taken with the & operator. For

example, the example template function defined would be instantiated twice

by the following code segment—once for int type parameters and once for

char type parameters:

int a, b, c;

char d, e, f;

...
c = max(a, b);
f = max(d, e);

The following is a C++ generic sort subprogram:

template <class Type>

void generic_sort(Type list[], int len) {

int top, bottom;

Type temp;

for (top = 0; top < len - 2; top++)

for (bottom = top + 1; bottom < len - 1; bottom++)

if (list[top] > list[bottom]) {

temp = list[top];

list[top] = list[bottom];

list[bottom] = temp;

} //** end of if (list[top]...

} //** end of generic_sort

The following is an example instantiation of this template function: