preprocessor and parenthesis

[effbiae]

hi,

this code:

#define M(a,b) static int is[3]={a,b;
M(1,(2,3)})

produces this (using gcc -E):

static int is[3]={1,(2,3)};

which gcc does not complain about.

my question(s):
o is there a better way of passing a comma separated list as the 'b'
parameter
o is gcc being generous by
a) preprocessing this
b) compiling it
o how can i write a macro like this:

#define F(a,b) int a(obj*o b

so that i can write function declarations in which all functions have a
first param obj, but may optionally have more.

thanks,


jack
ivorykite.com

[Harald van Dijk]

Re: preprocessor and parenthesis

effbiae wrote:[color=blue]
> hi,[/color]

Hi,
[color=blue]
> this code:
>
> #define M(a,b) static int is[3]={a,b;
> M(1,(2,3)})[/color]

How about moving the } to the macro definition?

#define M(a,b) static int is[3]={a,b};
M(1,(2,3))

Isn't that more readable?
[color=blue]
> produces this (using gcc -E):
>
> static int is[3]={1,(2,3)};
>
> which gcc does not complain about.[/color]

gcc 3.4 and 4 do complain about that no matter what, and older versions
complain about it with -pedantic. Additionally, on those older
versions, it probably doesn't do what you expect: it initialises is[0]
with 1, and is[1] with (2,3), which evaluates to 3.
[color=blue]
> my question(s):
> o is there a better way of passing a comma separated list as the 'b'
> parameter[/color]

Variadic macros.

#define M(...) static int is[3]={__VA_ARGS__};
M(1,2,3)

Or, if you want to enforce a minimum of two arguments:

#define M(a, ...) static int is[3]={a, __VA_ARGS__};
M(1,2,3)
[color=blue]
> o is gcc being generous by
> a) preprocessing this[/color]

Nope.
[color=blue]
> b) compiling it[/color]

Yep.
[color=blue]
> o how can i write a macro like this:
>
> #define F(a,b) int a(obj*o b
>
> so that i can write function declarations in which all functions have a
> first param obj, but may optionally have more.[/color]

Again, why not put the ) in the macro definition?

#define F(a,b) int a(obj*o b)

gcc has a non-standard extension that makes this easy -- refer to the
documentation if you're interested -- but with standard C it, while
possible, is needlessly complex since you don't know in advance whether
you'll need a comma after o. The easiest route is to create two macros:

#define F_0(a) int a(obj* o)
#define F_1(a,...) int a(obj* o, __VA_ARGS__)

and choose the appropriate one by hand each time.

HTH

[effbiae]

Re: preprocessor and parenthesis

Harald wrote:

thanks for your help.
[color=blue]
> Variadic macros.[/color]

of course...
[color=blue]
> you'll need a comma after o. The easiest route is to create two macros:
>
> #define F_0(a) int a(obj* o)
> #define F_1(a,...) int a(obj* o, __VA_ARGS__)
>
> and choose the appropriate one by hand each time.[/color]

i think i could do something like this:
#define F(a) int a(obj*o,...)
and leave the argument-getting up to the function

do i pay an efficiency penalty for using va_list macros/functions?

thanks,



jack

[Harald van Dijk]

Re: preprocessor and parenthesis

effbiae wrote:[color=blue]
> i think i could do something like this:
> #define F(a) int a(obj*o,...)
> and leave the argument-getting up to the function[/color]

Yeah, that should work. You may now accidentally call the functions
with the wrong number of arguments, of course, so be careful about
that.
[color=blue]
> do i pay an efficiency penalty for using va_list macros/functions?[/color]

In theory, yes, but it's nothing likely to be noticeable. If it makes
things easier, go for it.

[tmp123]

Re: preprocessor and parenthesis


effbiae wrote:[color=blue]
> hi,
>
> this code:
>
> #define M(a,b) static int is[3]={a,b;
> M(1,(2,3)})
>
> produces this (using gcc -E):
>
> static int is[3]={1,(2,3)};
>
> which gcc does not complain about.
>
> my question(s):
> o is there a better way of passing a comma separated list as the 'b'
> parameter
> o is gcc being generous by
> a) preprocessing this
> b) compiling it
> o how can i write a macro like this:
>
> #define F(a,b) int a(obj*o b
>
> so that i can write function declarations in which all functions have a
> first param obj, but may optionally have more.
>
> thanks,
>
>
> jack
> ivorykite.com[/color]

Hello,

If all the others posibilities fails, you can always use an external
and more powerful preprocessor, like "m4".

Kind regards.

[Jordan Abel]

Re: preprocessor and parenthesis

On 2006-02-06, effbiae <effbiae@ivoryk ite.com> wrote:[color=blue]
> hi,
>
> this code:
>
> #define M(a,b) static int is[3]={a,b;
> M(1,(2,3)})
>
> produces this (using gcc -E):
>
> static int is[3]={1,(2,3)};
>
> which gcc does not complain about.[/color]

I believe this statement is equivalent to
static int is[3] = {1,3,0};