#define in C Structures

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While browsing some code on net I came across #define macro used inside structure:
For example:

C++

typedef struct
{
#define XYZ                 \
   LONG           x;         \
   INT            y;        \
   CHAR           z;

   XYZ

} Data;

I guess it does not matter whether the macro is defined inside the structure or outside.
What is the benefit of declaring structure members through a macro?

Posted 15-Apr-13 8:07am

rupeshkp728

Updated 15-Apr-13 8:27am

skydger

v2

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Sergey Alexandrovich Kryukov 15-Apr-13 14:31pm

Sounds complete gibberish to me. Preprocessor is evil.
And please, try to ask the questions only your own code. You could bring enormous amount of trash from the Wen, but why?
—SA

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OriginalGriff · Accepted Answer · 2013-04-15T08:43:00

Solution 2

It has "advantages" if you are going to include the same data in a number of different structures, and want to declare the common data in a single place - but generally speaking that is a poor way to do things anyway.

The only real "use" of this is when you want to define a set of related structures:

C#

typedef struct
{
#define XYZ                 \
   LONG           x;         \
   INT            y;        \
   CHAR           z;

   XYZ
} Base;

typedef struct
{
   XYZ
   int            Count;
} Derived;

To emulate inheritance in more OOP oriented languages, where things like

C++

typedef struct
{
   LONG           x;         
   INT            y;        
   CHAR           z;
} Base;
 
typedef struct
{
   Base           A;
   int            Count;
} Derived;

Could cause following data to be aligned differently (as the structure size will be rounded up to a machine word). But I'd want it well and truly commented before I let it into my code base.

But for general use, it's a waste of time, and confusing to read.

Posted 15-Apr-13 8:43am

OriginalGriff

Comments

rupeshkp728 15-Apr-13 14:55pm

Thanks Griff for the reply.
That OOP example clears my doubt.
The intel code is also trying to implement some oop(base/derived) kind of structure, but using macros this way is really confusing.

OriginalGriff 15-Apr-13 14:58pm

It is indeed - but it is slightly better than declaring the same variables in eight places, then forgetting to modify one of them when you make a change...:laugh:

That's why I'd want it well commented!

rupeshkp728 15-Apr-13 15:05pm

But the same can also be acheived by using a common struct like "Base" in your example instead of using a macro.
Will using macro instead of a common struct make any difference?

OriginalGriff 15-Apr-13 15:16pm

It can, yes. The size of a structure is not just the sum of the bytes in the variables it contains - it is always padded to align on machine words. So if you define three characters in a structure on a 16 bit system, the structure is 4 bytes long. Same for a 32 bit system, and 8 bytes for a 64 bit OS. If you include an instance of the padded base structure as part of the derived structure, you do not end up with the same result as if you had included all the variables individually. This can have nasty effects if you expect two different applications (or even teams working on the same application) using the same base structure to interact correctly.

Including them via a #DEFINE means that no such padding happens.

CPallini 15-Apr-13 15:37pm

Good point. I didn't consider that.