Question

I’ve been banging my head trying to write a proper #define function-like macro but am getting stuck. Here’s the example I’m working with:

#include <iostream>
#define pMAKE(x,y,z,dest)\
    (dest).(x) = (x);\
    (dest).(y) = (y);\
    (dest).(z) = (z);

struct pt {
    double x, y, z;
};

int main() {
    pt p;
    pMAKE(0,1,2,p);
    return 0;
}

And the errors I’m getting:

A.cpp: In function ‘int main()’:
A.cpp:13: error: expected unqualified-id before ‘(’ token
A.cpp:13: error: expected unqualified-id before ‘(’ token
A.cpp:13: error: expected unqualified-id before ‘(’ token

What do the errors mean and why am I getting them? I managed to get the following to work the way I wanted, but I honestly just got lucky and I seriously don’t understand what’s going on.

#define pMAKE(X,Y,Z,dest)\
    dest.x = (X);\
    dest.y = (Y);\
    dest.z = (Z);

I appreciate every bit of help!

Answer 1

With the first version of your macro, pMAKE(0,1,2,p); expands to

p.0 = 0;
p.1 = 1;
p.2 = 2;;

In other words, you’ve pre-processed away references to the x, y, z members of pt by using x, y, z as both variable names (on the left of the assignments) and labels to be replaced by the pre-processor (on the right of the assignments).

(As noted by Konrad Rudolph) Your macro still has another serious error unfortunately. Consider code of the form

pt p;
if (foo)
    pMAKE(0,1,2,p);

which expands to

pt p;
if (foo)
    p.x = 0;
p.y = 1;  // happens regardless of 'foo' test
p.z = 2;; // happens regardless of 'foo' test

You can fix this (and get rid of that annoying extra semi-colon) by changing your macro to

#define pMAKE(X,Y,Z,dest)\
    do { \
        (dest).x = (X);\
        (dest).y = (Y);\
        (dest).z = (Z);\
    } while (0)