CODEWITHSUNDEEP
c
GyuMin Han
GyuMin Han

Reputation: 101

I can not understand Macro function`s result in C language

-- Hello, I can`t understand why "#define max(a,b) (a>b)?(a):(b)" does not work that i expected.

#include<cstdio>
#define max(a,b) (a>b)?(a):(b)
int main(void){   
  printf("%d\n",max(3,5)); // result = 5 (work correctly)
  int a = 3 + max(3,5);
  printf("%d\n",a); // result = 3 (work incorrect and i don`t know why..)
}

i can`t why variable + macro does not work.

but, In this case "#define max(a,b) ((a>b)?(a):(b))" work correctly. even that var + macro situation.

plz anybody clarify this.. thx.

Upvotes: 1

Views: 63

Answers (2)

larsks
larsks

Reputation: 311606

When you have this:

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

And you write this:

int a = 3 + max(3,5);

That expands to:

int a = 3 + (a>b)?(3):(5)

Because + has a higher operator precendence than ?, this is effectively:

int a = (3 + (a>b)) ? (3) : (5)

And since 3 + (a>b) is always going to evaluate to a true-ish value, the result will always be 3.

By putting the whole expansion in parentheses:

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

You get instead:

int a = 3 + ((a>b)?(3):(5))

Because the outer parentheses group the ternary operator expression, you get the result you expect.

Note that as @zwol writes in a comment, you should actually write your macro like:

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

The extra parentheses around a and b in (a)>(b) protect against exactly this sort of operator precedence problem.

Upvotes: 6

Elliott Frisch
Elliott Frisch

Reputation: 201447

Operator precedence. I get a warning with clang.

test.cc:7:15: warning: operator '?:' has lower precedence than '+'; '+' will be evaluated first [-Wparentheses]
  int a = 3 + max(3,5);

Easy to fix with parenthesis.

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

Upvotes: 2

Related Questions