QUESTION 33 · Qualifiers and Preprocessor

Why should macro parameters be parenthesised?

2 marks

Answer

Parentheses help preserve the intended grouping after token replacement.

Write #define SQUARE(x) ((x) * (x)) rather than x * x.
Parentheses do not prevent repeated evaluation of an argument.

Explain object-like and function-like macros with precautions.

5 marks

Answer in brief

A macro is a name with replacement tokens processed by the preprocessor. It may be object-like or function-like.

Object-like macro

#define LIMIT 100

Function-like macro

#define SQUARE(x) ((x) * (x))

Expansion example

SQUARE(2 + 3) expands to ((2 + 3) * (2 + 3)), giving 25. Without the parameter parentheses, an expansion such as 2 + 3 * 2 + 3 would give 11. The outer parentheses also protect the full result when it is placed inside another expression.

Repeated evaluation

The parameter x occurs twice in SQUARE. Therefore its argument expression also appears twice after replacement. Do not write SQUARE(i++): the resulting expression modifies i without the ordering required by C and has undefined behaviour. Parentheses do not solve that problem. A typed function such as int square(int x) evaluates an argument once before using its parameter.

Comparison with functions

A function has a declared interface, parameter objects and a return type. A macro is token replacement and does not provide the same parameter type checking. A compiler may inline a real function, so do not assume a macro must be faster. Prefer a function for ordinary calculations unless a macro is specifically useful.

Other points

There must be no space between a function-like macro name and the opening parenthesis in its definition. #undef SQUARE removes the definition for later source. Avoid an accidental trailing semicolon in a simple expression macro.

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
#define SQUARE(x) ((x) * (x))
int main(void)
{
    printf("Square = %d\n", SQUARE(2 + 3));
    return 0;
}

Sample input and output

This example uses values already written in the program.

Output

Square = 25