C default arguments

Question

Is there a way to specify default arguments to a function in C?

Answer 1

Wow, everybody is such a pessimist around here. The answer is yes.

It ain't trivial: by the end, we'll have the core function, a supporting struct, a wrapper function, and a macro around the wrapper function. In my work I have a set of macros to automate all this; once you understand the flow it'll be easy for you to do the same.

I've written this up elsewhere, so here's a detailed external link to supplement the summary here: http://modelingwithdata.org/arch/00000022.htm

We'd like to turn

double f(int i, double x)

into a function that takes defaults (i=8, x=3.14). Define a companion struct:

typedef struct {
    int i;
    double x;
} f_args;

Rename your function f_base, and define a wrapper function that sets defaults and calls the base:

double var_f(f_args in){
    int i_out = in.i ? in.i : 8;
    double x_out = in.x ? in.x : 3.14;
    return f_base(i_out, x_out);
}

Now add a macro, using C's variadic macros. This way users don't have to know they're actually populating a f_args struct and think they're doing the usual:

#define f(...) var_f((f_args){__VA_ARGS__});

OK, now all of the following would work:

f(3, 8);      //i=3, x=8
f(.i=1, 2.3); //i=1, x=2.3
f(2);         //i=2, x=3.14
f(.x=9.2);    //i=8, x=9.2

Check the rules on how compound initializers set defaults for the exact rules.

One thing that won't work: f(0), because we can't distinguish between a missing value and zero. In my experience, this is something to watch out for, but can be taken care of as the need arises---half the time your default really is zero.

I went through the trouble of writing this up because I think named arguments and defaults really do make coding in C easier and even more fun. And C is awesome for being so simple and still having enough there to make all this possible.