CODEWITHSUNDEEP
c
Kundan Kumar
Kundan Kumar

Reputation: 2002

Understanding the nature of #define

If I write something like 

#define INT_PTR int*
INT_PTR ptr4, ptr5, ptr6;

In this case only ptr4 is pointer to an integer, rest of the values (ptr5 and ptr6) are integers. How they are taking the integer value ? Either it should give some compilation error.

Why is it this way that compiler is treating ptr5 and ptr6 as integers.

Upvotes: 2

Views: 119

Answers (3)

paxdiablo
paxdiablo

Reputation: 882726

This actually has nothing to do with the #define, which is simply a textual replacement.

After the preprocessor phase (when the substitution takes place), you end up with:

int* ptr4, ptr5, ptr6;

and, because the * binds to the variable rather than the type, you create one integer pointer and two integers.

This is why I prefer to write:

int *xyzzy;

rather than:

int* xyzzy;

since the former makes it clearer that the * belongs to the variable. If you want to define a new type in C, the command is, surprisingly enough, typedef :-)

typedef int * INTPTR;
INTPTR ptr4, ptr5, ptr6;

That defines a new type that will apply to all variables that follow it, rather than just substituting text, as per the macro. In other words, the type INTPTR (int *) applies to all three of ptr4, ptr5 and ptr6.

Upvotes: 4

user529758
user529758

Reputation:

Your code expands to

int* ptr4, ptr5, ptr6;

And since the asterisk (*) declaration is applied to only the identifier follwing it, it's natural that only ptr4 will actually be a pointer. (By the way, that's why it's conceptually wrong to write

int* ptr;

as you should indicate that the pointer declaration is only for one variable, as such:

int *ptr.

)

So, instead of a #define preprocessor directive (which is also wrong for type definitions), use the C typedef keyword:

typedef int *inptr;
intptr ptr4, ptr5, ptr6;

Upvotes: 1

ouah
ouah

Reputation: 145919

Because to declare pointer objects you would do:

int *ptr4, *ptr5, *ptr6;

In C, the token * does not belong to the type information, so you have to repeat it when you declare several objects. Note that this is a frequent mistake in C.

What you can do is to typedef instead:

typedef int * INTPTR;

Upvotes: 11

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