CODEWITHSUNDEEP
c++cmacros
user1002288
user1002288

Reputation: 5050

some macro statements may have unexpected results in C++?

Which macro statement may cause an unexpected results ? 

#define YEAR_LENGTH   365
#define MONTH_LENGTH  30
 #define DAYCALC(y, m, d) ((y * YEAR_LENGTH) + (m * MONTH_LENGTH) + d)

 int main()
 {
    int x = 5, y = 4 , z = 1;
    cout << DAYCALC(x *3 , y %3 , z) << endl ;
    cout << DAYCALC(x +12 , y  , 300) << endl ;
    cout << DAYCALC(x , 40 - y , 3+z) << endl ;
    cout << DAYCALC(x  , y  , (z+50)) << endl ;
    cout << DAYCALC(x  , y %3 , z) << endl ;
    cout << DAYCALC(4 % x , y++ , z) << endl;
    return 0;
 }

I run the program very well w/o any unexpected results.

Are there some hidden exceptions ?

Upvotes: 2

Views: 427

Answers (3)

Bill Forster
Bill Forster

Reputation: 6297

You need to change DAYCALC;

#define DAYCALC(y, m, d) ( ((y) * YEAR_LENGTH) + ((m) * MONTH_LENGTH) + (d) )

That way, if m is say 3+z, then the inner term will be the correct;

(3+z) * MONTH_LENGTH

not the incorrect;

3 + z*MONTH_LENGTH

Upvotes: 2

bashrc
bashrc

Reputation: 4835

you can check what will happen after macro expansion by g++ -E .

int main()
{
int x = 5, y = 4 , z = 1;
cout << ((x *3 * 365) + (y %3 * 30) + z) << endl ; 
cout << ((x +12 * 365) + (y * 30) + 300) << endl ;
cout << ((x * 365) + (40 - y * 30) + 3+z) << endl ; //precedence problem
cout << ((x * 365) + (y * 30) + (z+50)) << endl ; 
cout << ((x * 365) + (y %3 * 30) + z) << endl ;
cout << ((4 % x * 365) + (y++ * 30) + z) << endl;
return 0;
}

Upvotes: 7

Mysticial
Mysticial

Reputation: 471569

You have an operator precendence problem. Macros are literally expanded as text copy and paste.

For example:

DAYCALC(x , 40 - y , 3+z)

gets expanded to:

((40 - y * YEAR_LENGTH) + (x * MONTH_LENGTH) + 3+z)

Note that 40 - y * YEAR_LENGTH, is not what you want due to operator precedence.

So you need to put () around your parameters in the macro:

#define DAYCALC(y, m, d)     (((y) * YEAR_LENGTH) + ((m) * MONTH_LENGTH) + (d))

In general, if a macro parameter appears more than once in the macro, side effects such as y++ (in your last statement) will also be applied more than once. So it's something to be careful of.

Upvotes: 8

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