function typedefs

[Graham Lee]

Given a typedef like this:

typedef double LoadsOfArgs(double arg1, double arg2, double arg3, double
arg4);

I can *declare* a function using the typedef:

LoadsOfArgs geoMean;

int main(void)
{
printf("G.M.(1,2,3,4) = %g\n",geoMean(1.0,2.0,3.0,4.0));
exit(EXIT_SUCCESS);
}

but then don't appear to be able to *define* the function in the same way:

LoadsOfArgs geoMean
{
return pow(arg1*arg2*arg3*arg4,0.25);
}

is use of the full function prototype mandated, or is my function
definition wrong?

 

[Ian Collins]

Given a typedef like this:

typedef double LoadsOfArgs(double arg1, double arg2, double arg3, double
arg4);

The parameter names are not part of the type, only the parameter types.

I can *declare* a function using the typedef:
Correct;

but then don't appear to be able to *define* the function in the same way:
Correct.

is use of the full function prototype mandated, or is my function
definition wrong?

As I said, the parameter names are not part of the type, so you can use
the typedef in this way.

 

[Richard Heathfield]

Graham Lee said:

Given a typedef like this:

typedef double LoadsOfArgs(double arg1, double arg2, double arg3, double
arg4);

I can *declare* a function using the typedef:

LoadsOfArgs geoMean;

Yes. I do that too.

but then don't appear to be able to *define* the function in the same way:

LoadsOfArgs geoMean

I, too, was a bit annoyed by that, but c'est la vie.

is use of the full function prototype mandated, or is my function
definition wrong?

Alas, you have to define the function in longhand. Grrrr.

 

[lovecreatesbeauty]

typedef double LoadsOfArgs(double arg1, double arg2, double arg3, double
arg4);

LoadsOfArgs geoMean;

LoadsOfArgs geoMean
wrong

{
return pow(arg1*arg2*arg3*arg4,0.25);
}

This is not a good idea. The prototype can not be directly shown as you
define (compose) the function in a .c file while the function prototype
maybe is placed in another .h file.

lovecreatesbeauty

 

[Ian Collins]

This is not a good idea. The prototype can not be directly shown as you
define (compose) the function in a .c file while the function prototype
maybe is placed in another .h file.

What do you mean 'not a good idea'? It's plain (as in won't compile) wrong.

 

[August Karlstrom]

Given a typedef like this:

typedef double LoadsOfArgs(double arg1, double arg2, double arg3, double
arg4);

I can *declare* a function using the typedef:

LoadsOfArgs geoMean;

int main(void)
{
printf("G.M.(1,2,3,4) = %g\n",geoMean(1.0,2.0,3.0,4.0));
exit(EXIT_SUCCESS);
}

but then don't appear to be able to *define* the function in the same way:

LoadsOfArgs geoMean
{
return pow(arg1*arg2*arg3*arg4,0.25);
}

is use of the full function prototype mandated, or is my function
definition wrong?

You probably want something like

#include <stdio.h>
#include <math.h>

typedef double LoadsOfArgs(double a, double b, double c, double d);

double f(double a, double b, double c, double d)
{
return pow(a * b * c * d, 0.25);
}

LoadsOfArgs *geoMean = f;

int main(void)
{
printf("G.M.(1,2,3,4) = %g\n", geoMean(1.0, 2.0, 3.0, 4.0));
return 0;
}


August

 

[Dave Thompson]

I can *declare* a function using the typedef:

but then don't appear to be able to *define* the function in the same way:

is use of the full function prototype mandated, or is my function
definition wrong?

Nonuse of the typedef is indeed mandated; C90 6.7.1 or C99 6.9.1p2.

You must use either the longhand prototype or the equally longhand
oldstyle definition (although this is labelled 'obsolescent') e.g.
double LoadsOfArgs (a,b,c,d)
double a,b,c,d;
{ body }

You _could_ do (at least the argument portion of) either of these as a
macro, and for the prototype case could reuse that macro in the
typedef. Personally I can't decide whether that's cute or disgusting.

- David.Thompson1 at worldnet.att.net