Hello all,
I'm trying to write a macro that will let me access the fields of a
struct variable, given the field name as a parameter. I tried the
follwing which produces the desired result when run thru gnu cpp, but
it complains and stops.
#define ref(var,field) var. ## field
struct mystruct {
int i;
char c;
};
int main (void) {
struct mystruct aaa;
ref(aaa,i) = 1;
}
Any help is greatly appreciated.
___
Regards,
Ziyan Maraikar
Vrije Universiteit, Amsterdam. 5 1766
In article <11*********************@o13g2000cwo.googlegroups. com>,
ziyan <zi****@gmail.com> wrote: I'm trying to write a macro that will let me access the fields of a struct variable, given the field name as a parameter. I tried the follwing which produces the desired result when run thru gnu cpp, but it complains and stops.
#define ref(var,field) var. ## field
The gcc message is along the lines of,
pasting "." and "i" does not give a valid preprocessing token
which is accuate. ## is used to create new tokens, and period is
not a possible character in a preprocessor token.
For your purposes, just use
#define ref(var,field) var.field
--
Studies show that the average reader ignores 106% of all statistics
they see in .signatures.
Hi Walter
Thanx for the tip. For your purposes, just use
#define ref(var,field) var.field
Hmm...seems obvious enough. Guess I assumed macro replacement only
works on whitespace separated tokens.
BTW I came up with a nice hack for a hashtable I implemented using this
technique:
typedef struct {
enum {INT, FLOAT, STRING} type;
union {
int INT;
float FLOAT;
char *STRING;
} value;
} Hashobject;
typedef struct {
char *key;
Hashobject object;
short chain;
} Hashentry;
#define hash_add(_ht, _key, _value, _type){\
Hashentry _he; \
_he.key = _key; \
_he.object.type = _type; \
_he.object.value._type = _value;\
hash_addentry (_ht, _he); \
}
int hash_addentry (Hashtable *ht, Hashentry ent);
"ziyan" <zi****@gmail.com> writes: For your purposes, just use
#define ref(var,field) var.field
Hmm...seems obvious enough. Guess I assumed macro replacement only works on whitespace separated tokens.
[snip]
As far as the compiler is concerned, tokens are tokens, and spaces
between them are often optional. Just as "x + y" and "x+y" are
equivalent, "var . field" is equivalent to "var.field". (As a matter
of style, of course, "var . field" is ugly.)
--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
Keith Thompson wrote: "ziyan" <zi****@gmail.com> writes:
For your purposes, just use
#define ref(var,field) var.field
Hmm...seems obvious enough. Guess I assumed macro replacement only works on whitespace separated tokens.
[snip]
As far as the compiler is concerned, tokens are tokens, and spaces between them are often optional. Just as "x + y" and "x+y" are equivalent, "var . field" is equivalent to "var.field". (As a matter of style, of course, "var . field" is ugly.)
... which raises the question of why anyone would
want to write `ref(var,field)' instead of `var.field'.
Maybe I'm blind, but I don't see the attraction ...
-- Er*********@sun.com
Eric Sosman <er*********@sun.com> writes: Keith Thompson wrote: "ziyan" <zi****@gmail.com> writes:
For your purposes, just use
#define ref(var,field) var.field
Hmm...seems obvious enough. Guess I assumed macro replacement only works on whitespace separated tokens.
[snip]
As far as the compiler is concerned, tokens are tokens, and spaces between them are often optional. Just as "x + y" and "x+y" are equivalent, "var . field" is equivalent to "var.field". (As a matter of style, of course, "var . field" is ugly.)
... which raises the question of why anyone would want to write `ref(var,field)' instead of `var.field'. Maybe I'm blind, but I don't see the attraction ...
By itself, it doesn't make much sense, but if ziyan's article upthread
shows a hash table macro that uses it. The macro has several
arguments, and it's not obvious from a call that one of them happens
to be a member name
--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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