What is argument evaluation?

Question

Herbert Schildt says:

In some situations, real function should be used in place of function-like-macro, for example: where code size is to be minimized or where an argument must not be evaluated more than once.

What does he mean by "an argument must not be evaluated more than once?"

Answer 1

Let's take a macro to calculate the maximum of two values:

#define MAX(a, b) ((a) < (b) ? (a) : (b)) 

Then we use it like this:

int x = 5; int y = 10; int max = MAX(x++, y++); 

Then the macro is expanded to

int max = ((x++) < (y++) ? (x++) : (y++)); 

As you can see, the increment operation on either x or y will happen twice, not what would happen if you had a function where each argument you pass is evaluated only once.

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Another important point is the use of parentheses in the macro. Let's take another simple macro:

#define MUL(a, b) a * b 

Now if you invoke the macro as

int sum = MUL(x + 3, y - 2); 

then the expansion becomes

int sum = x + 3 * y - 2; 

Which due to operator precedence is equal to

int sum = x + (3 * y) - 2; 

Often not quite what was expected, if one expects (x + 3) * (y - 2).

This problem is also "solved" by using functions.