I'd like to make _Generic evaluate another macro. In the example below, db_put_u8 will insert a uint8_t into the buffer and while at it, it will also leave a log message. The log helps me understand if I missed a field. Hence the macro. I have reduced the functionality to illustrate the problem here, but in practice it's supposed to have for various data types.
#include <stdio.h>
#include <stdint.h>
#define db_put_u8(con, name, val) \
do { \
fputs(name "<- " #val, con); \
} while(0)
#define db_put(con, name, val) _Generic(val, uint8_t: db_put_u8)(con, name, val)
int main(void)
{
uint8_t v = 10;
db_put(stdout, "some-field", v);
return 0;
}
When I try compiling this, I get an error:
test.c:15:5: error: use of undeclared identifier 'db_put_u8'
db_put(stdout, "some-field", v);
^
test.c:9:55: note: expanded from macro 'db_put'
#define db_put(con, name, val) _Generic(val, uint8_t: db_put_u8)(con, name, val)
^
1 error generated.
Am I doing _Generic right? The generic selection syntax does not explicitly restrict calling macros in it. Can someone explain what I am doing wrong and how to fix it?
This line:
#define db_put(con, name, val) _Generic(val, uint8_t: db_put_u8)(con, name, val)
First of all has to change to:
#define db_put(con, name, val) _Generic(val, uint8_t: db_put_u8(con, name, val))
Because _Generic works at compile time, not preprocessor time. And after preprocessing, db_put_u8 will not expand because it doesn't exist as a macro. db_put_u8(arg1, arg2, arg3), however, does, so you have to put the brackets in the generic. However, there's another error then: _Generic expects an eression, so a do while loop would not work. In this case though, you don't even need to encase your macro in a loop because it's just one statement, so it's free from the potential issues of macros with many statements. Your final program should look something like this:
#include <stdio.h>
#include <stdint.h>
#define db_put_u8(con, name, val) \
fputs(name "<- " #val, con)
#define db_put(con, name, val) _Generic(val, uint8_t: db_put_u8(con, name, val))
int main(void)
{
uint8_t v = 10;
db_put(stdout, "some-field", v);
return 0;
}
EDIT: In response to the comment, there is a solution. So long as the second thing to do in db_put_u8 is also a statement, you could use the comma operator and a few more brackets. Here's an example program to put it to the test:
#include <stdio.h>
#include <stdint.h>
#define db_put_u8(con, name, val) \
(fputs(name "<- " #val, con), \
fputs("\nAnother test line\n", con))
#define db_put(con, name, val) \
(_Generic(val, uint8_t: db_put_u8(con, name, val)))
int main(void)
{
uint8_t v = 10;
db_put(stdout, "some-field", v);
return 0;
}
When I run this, it outputs:
some-field<- v
Another test line
Macro db_put_u8 would expand as do ... while(...) statement which is not an expression. _Generic expects expression thus error is observed. You should provide a function named db_put_u8. The function name decays to pointer to the function which is an an expression.
Other way around is to replace db_put_u8 with 'fputs(name "<- " #val, con)' inside _Generic and remove final '(name, val,con)'
#define db_put(con, name, val) \
_Generic(val, uint8_t: fputs(name "<- " #val, con))
If db_put_u8 cannot be changed then one could use an C extension named statement expression. It is supported by GCC and CLANG. Basically it allows to put arbitrary statement info ({ ... }). The last expression evaluated is returned as result.
#define db_put(con, name, val) \
_Generic(val, uint8_t: ({db_put_u8(con, name, val);}))
I have found myself in strange situation trying to add some syntactic prettyness to some C code. I have triplets of numbers that are just structs.
typedef struct {int x, y, z} coord;
Now I have some functions that take 2 such structs as arguments. The simplest one counts the number of coordinates inside the 3D space defined two structs:
static inline int boxes_inside(const coord min, const coord max)
{
return (1 + max.x - min.x) * (1 + max.y - min.y) * (1 + max.z - min.z);
}
I find myself pretty regularly calling it with fixed arguments, which I find to be quite ugly
coord foo;
/* initialize foo with something */
int n = boxes_inside((coord){.x = 0, .y = 0, .z = 0}, foo);
Nevermind this example being silly, it makes more sense for more complicated functions.
I thought I would use _Generic to pass either triplets of ints, or structs.
int boxes_inside_cc(const coord min, const coord max);
int boxes_inside_ci(const coord min, const int maxx, const int maxy, const int maxz);
int boxes_inside_ic(const int minx, const int miny, const int minz, const coord max);
int boxes_inside_ii(const int minx, const int miny, const int minz, const int maxx, const int maxy, const int maxz);
#define arg1(a, ...) (a)
#define arg2(a, b ...) (b)
#define arg4(a, b, c, d, ...) (d)
#define boxes_inside(...) _Generic(arg1(__VA_ARGS__), \
coord: _Generic(arg2(__VA_ARGS__), coord: boxes_inside_cc, int: boxes_inside_ci) \
int: _Generic(arg4(__VA_ARGS__), coord: boxes_inside_ic, int: boxes_inside_ii) \
)(__VA_ARGS__)
I thought this would be fine since "the expressions of the selections that are not chosen are are never evaluated." (ref) but it turns out that since this is done after preprocessing, all macros are still expanded even in selections that are not chosen.
In particular, if I now do the following call:
coord min, max;
/* stuff */
int n = boxes_inside(min, max);
I get the problem that arg4(__VA_ARGS__) tries to expand more parameters than it actually has, even though this branch of the _Generic will never be evaluated later on.
So I then tried to expand the structs to, always have enough arguments:
#define boxes_inside_(a, b, c, d, ...) _Generic((a), \
coord: boxes_inside_ii(a, b, c, d.x, d.y, d.z), \
int: boxes_inside_ii(a, b, c, d, __VA_ARGS__) \
)
#define boxes_inside(a, ...) _Generic((a), \
coord: boxes_inside_(a.x, a.y, a.z, __VA_ARGS__) \
int: boxes_inside_(a, __VA_ARGS__) \
)
However this unsurprisingly fails with the same reason: both branches expand the other macro, in particular boxes_inside(min, max) still expands to boxes_inside_(min max) on the branch we already know won't be used.
So is there a way around this? Or are _Generic expressions basically useless if you want to test a parameter that's beyond the minimal number of parameters you might use?
Well, here goes what we discussed in the comments, even though it is not really satisfying, since it's not really an elegant solution.
First, define boxes_inside_X for every X being an acceptable number of arguments, using _Generic when necessary.
Then overload the macro by pasting the number of arguments after it (or using Boost.PP as suggested by Quentin).
/* macros that can be reused (possibly with more arguments) */
#define paste2(a, b) a ## b
#define paste(a, b) paste2(a, b)
#define get_seventh(_1, _2, _3, _4, _5, _6, this_one, ...) this_one
#define get_suffix(...) get_seventh(__VA_ARGS__, _6, _5, _4, _3, _2, _1)
/* define all variants with number of arguments suffix */
int boxes_inside_2(const coord min, const coord max);
int boxes_inside_6(const int minx, const int miny, const int minz, const int maxx, const int maxy, const int maxz);
/* make it a _Generic, if several functions have the same number of arguments */
int boxes_inside_ci(const coord min, const int maxx, const int maxy, const int maxz);
int boxes_inside_ic(const int minx, const int miny, const int minz, const coord max);
#define boxes_inside_4(a, ...) _Generic((a),\
coord: boxes_inside_ci) \
int: boxes_inside_ic) \
)(__VA_ARGS__)
/* make macro call itself with the number of arguments pasted after it */
#define boxes_inside(...) paste(boxes_inside, get_suffix(__VA_ARGS__))(__VA_ARGS__)
The upside of this approach is that you get reasonably readable error messages, for example
warning: implicit declaration of function ‘boxes_inside_3’ for the wrong number of arguments, or
expected ‘coord {aka const struct <anonymous>}’ but argument is of type ‘int’ if the types are wrong.
Is there a way to specify default arguments to a function in C?
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.
Yes. :-) But not in a way you would expect.
int f1(int arg1, double arg2, char* name, char *opt);
int f2(int arg1, double arg2, char* name)
{
return f1(arg1, arg2, name, "Some option");
}
Unfortunately, C doesn't allow you to overload methods so you'd end up with two different functions. Still, by calling f2, you'd actually be calling f1 with a default value. This is a "Don't Repeat Yourself" solution, which helps you to avoid copying/pasting existing code.
Not really. The only way would be to write a varargs function and manually fill in default values for arguments which the caller doesn't pass.
We can create functions which use named parameters (only) for default values. This is a continuation of bk.'s answer.
#include <stdio.h>
struct range { int from; int to; int step; };
#define range(...) range((struct range){.from=1,.to=10,.step=1, __VA_ARGS__})
/* use parentheses to avoid macro subst */
void (range)(struct range r) {
for (int i = r.from; i <= r.to; i += r.step)
printf("%d ", i);
puts("");
}
int main() {
range();
range(.from=2, .to=4);
range(.step=2);
}
The C99 standard defines that later names in the initialization override previous items. We can also have some standard positional parameters as well, just change the macro and function signature accordingly. The default value parameters can only be used in named parameter style.
Program output:
1 2 3 4 5 6 7 8 9 10
2 3 4
1 3 5 7 9
OpenCV uses something like:
/* in the header file */
#ifdef __cplusplus
/* in case the compiler is a C++ compiler */
#define DEFAULT_VALUE(value) = value
#else
/* otherwise, C compiler, do nothing */
#define DEFAULT_VALUE(value)
#endif
void window_set_size(unsigned int width DEFAULT_VALUE(640),
unsigned int height DEFAULT_VALUE(400));
If the user doesn't know what he should write, this trick can be helpful:
No.
Not even the very latest C99 standard supports this.
No, that's a C++ language feature.
Probably the best way to do this (which may or may not be possible in your case depending on your situation) is to move to C++ and use it as 'a better C'. You can use C++ without using classes, templates, operator overloading or other advanced features.
This will give you a variant of C with function overloading and default parameters (and whatever other features you chose to use). You just have to be a little disciplined if you're really serious about using only a restricted subset of C++.
A lot of people will say it's a terrible idea to use C++ in this way, and they might have a point. But's it's just an opinion; I think it's valid to use features of C++ that you're comfortable with without having to buy into the whole thing. I think a significant part of the reason for the sucess of C++ is that it got used by an awful lot of programmers in it's early days in exactly this way.
Short answer: No.
Slightly longer answer: There is an old, old workaround where you pass a string that you parse for optional arguments:
int f(int arg1, double arg2, char* name, char *opt);
where opt may include "name=value" pair or something, and which you would call like
n = f(2,3.0,"foo","plot=yes save=no");
Obviously this is only occasionally useful. Generally when you want a single interface to a family of functionality.
You still find this approach in particle physics codes that are written by professional programs in c++ (like for instance ROOT). It's main advantage is that it may be extended almost indefinitely while maintaining back compatibility.
Yet another option uses structs:
struct func_opts {
int arg1;
char * arg2;
int arg3;
};
void func(int arg, struct func_opts *opts)
{
int arg1 = 0, arg3 = 0;
char *arg2 = "Default";
if(opts)
{
if(opts->arg1)
arg1 = opts->arg1;
if(opts->arg2)
arg2 = opts->arg2;
if(opts->arg3)
arg3 = opts->arg3;
}
// do stuff
}
// call with defaults
func(3, NULL);
// also call with defaults
struct func_opts opts = {0};
func(3, &opts);
// set some arguments
opts.arg3 = 3;
opts.arg2 = "Yes";
func(3, &opts);
Another trick using macros:
#include <stdio.h>
#define func(...) FUNC(__VA_ARGS__, 15, 0)
#define FUNC(a, b, ...) func(a, b)
int (func)(int a, int b)
{
return a + b;
}
int main(void)
{
printf("%d\n", func(1));
printf("%d\n", func(1, 2));
return 0;
}
If only one argument is passed, b receives the default value (in this case 15)
No.
No, but you might consider using a set of functions (or macros) to approximate using default args:
// No default args
int foo3(int a, int b, int c)
{
return ...;
}
// Default 3rd arg
int foo2(int a, int b)
{
return foo3(a, b, 0); // default c
}
// Default 2nd and 3rd args
int foo1(int a)
{
return foo3(a, 1, 0); // default b and c
}
Yes, with features of C99 you may do this. This works without defining new data structures or so and without the function having to decide at runtime how it was called, and
without any computational overhead.
For a detailed explanation see my post at
http://gustedt.wordpress.com/2010/06/03/default-arguments-for-c99/
Jens
I improved Jens Gustedt’s answer so that:
inline functions aren’t employed
defaults are computed during preprocessing
modular reuseable macros
possible to set compiler error that meaningfully matches the case of insufficient arguments for the allowed defaults
the defaults aren’t required to form the tail of the parameter list if the argument types will remain unambiguous
interopts with C11 _Generic
vary the function name by the number of arguments!
variadic.h:
#ifndef VARIADIC
#define _NARG2(_0, _1, _2, ...) _2
#define NUMARG2(...) _NARG2(__VA_ARGS__, 2, 1, 0)
#define _NARG3(_0, _1, _2, _3, ...) _3
#define NUMARG3(...) _NARG3(__VA_ARGS__, 3, 2, 1, 0)
#define _NARG4(_0, _1, _2, _3, _4, ...) _4
#define NUMARG4(...) _NARG4(__VA_ARGS__, 4, 3, 2, 1, 0)
#define _NARG5(_0, _1, _2, _3, _4, _5, ...) _5
#define NUMARG5(...) _NARG5(__VA_ARGS__, 5, 4, 3, 2, 1, 0)
#define _NARG6(_0, _1, _2, _3, _4, _5, _6, ...) _6
#define NUMARG6(...) _NARG6(__VA_ARGS__, 6, 5, 4, 3, 2, 1, 0)
#define _NARG7(_0, _1, _2, _3, _4, _5, _6, _7, ...) _7
#define NUMARG7(...) _NARG7(__VA_ARGS__, 7, 6, 5, 4, 3, 2, 1, 0)
#define _NARG8(_0, _1, _2, _3, _4, _5, _6, _7, _8, ...) _8
#define NUMARG8(...) _NARG8(__VA_ARGS__, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define _NARG9(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, ...) _9
#define NUMARG9(...) _NARG9(__VA_ARGS__, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define __VARIADIC(name, num_args, ...) name ## _ ## num_args (__VA_ARGS__)
#define _VARIADIC(name, num_args, ...) name (__VARIADIC(name, num_args, __VA_ARGS__))
#define VARIADIC(name, num_args, ...) _VARIADIC(name, num_args, __VA_ARGS__)
#define VARIADIC2(name, num_args, ...) __VARIADIC(name, num_args, __VA_ARGS__)
// Vary function name by number of arguments supplied
#define VARIADIC_NAME(name, num_args) name ## _ ## num_args ## _name ()
#define NVARIADIC(name, num_args, ...) _VARIADIC(VARIADIC_NAME(name, num_args), num_args, __VA_ARGS__)
#endif
Simplified usage scenario:
const uint32*
uint32_frombytes(uint32* out, const uint8* in, size_t bytes);
/*
The output buffer defaults to NULL if not provided.
*/
#include "variadic.h"
#define uint32_frombytes_2( b, c) NULL, b, c
#define uint32_frombytes_3(a, b, c) a, b, c
#define uint32_frombytes(...) VARIADIC(uint32_frombytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
And with _Generic:
const uint8*
uint16_tobytes(const uint16* in, uint8* out, size_t bytes);
const uint16*
uint16_frombytes(uint16* out, const uint8* in, size_t bytes);
const uint8*
uint32_tobytes(const uint32* in, uint8* out, size_t bytes);
const uint32*
uint32_frombytes(uint32* out, const uint8* in, size_t bytes);
/*
The output buffer defaults to NULL if not provided.
Generic function name supported on the non-uint8 type, except where said type
is unavailable because the argument for output buffer was not provided.
*/
#include "variadic.h"
#define uint16_tobytes_2(a, c) a, NULL, c
#define uint16_tobytes_3(a, b, c) a, b, c
#define uint16_tobytes(...) VARIADIC( uint16_tobytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
#define uint16_frombytes_2( b, c) NULL, b, c
#define uint16_frombytes_3(a, b, c) a, b, c
#define uint16_frombytes(...) VARIADIC(uint16_frombytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
#define uint32_tobytes_2(a, c) a, NULL, c
#define uint32_tobytes_3(a, b, c) a, b, c
#define uint32_tobytes(...) VARIADIC( uint32_tobytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
#define uint32_frombytes_2( b, c) NULL, b, c
#define uint32_frombytes_3(a, b, c) a, b, c
#define uint32_frombytes(...) VARIADIC(uint32_frombytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
#define tobytes(a, ...) _Generic((a), \
const uint16*: uint16_tobytes, \
const uint32*: uint32_tobytes) (VARIADIC2( uint32_tobytes, NUMARG3(a, __VA_ARGS__), a, __VA_ARGS__))
#define frombytes(a, ...) _Generic((a), \
uint16*: uint16_frombytes, \
uint32*: uint32_frombytes)(VARIADIC2(uint32_frombytes, NUMARG3(a, __VA_ARGS__), a, __VA_ARGS__))
And with variadic function name selection, which can't be combined with _Generic:
// winternitz() with 5 arguments is replaced with merkle_lamport() on those 5 arguments.
#define merkle_lamport_5(a, b, c, d, e) a, b, c, d, e
#define winternitz_7(a, b, c, d, e, f, g) a, b, c, d, e, f, g
#define winternitz_5_name() merkle_lamport
#define winternitz_7_name() winternitz
#define winternitz(...) NVARIADIC(winternitz, NUMARG7(__VA_ARGS__), __VA_ARGS__)
Generally no, but in gcc You may make the last parameter of funcA() optional with a macro.
In funcB() i use a special value (-1) to signal that i need the default value for the 'b' parameter.
#include <stdio.h>
int funcA( int a, int b, ... ){ return a+b; }
#define funcA( a, ... ) funcA( a, ##__VA_ARGS__, 8 )
int funcB( int a, int b ){
if( b == -1 ) b = 8;
return a+b;
}
int main(void){
printf("funcA(1,2): %i\n", funcA(1,2) );
printf("funcA(1): %i\n", funcA(1) );
printf("funcB(1, 2): %i\n", funcB(1, 2) );
printf("funcB(1,-1): %i\n", funcB(1,-1) );
}
YES
Through macros
3 Parameters:
#define my_func2(...) my_func3(__VA_ARGS__, 0.5)
#define my_func1(...) my_func2(__VA_ARGS__, 10)
#define VAR_FUNC(_1, _2, _3, NAME, ...) NAME
#define my_func(...) VAR_FUNC(__VA_ARGS__, my_func3, my_func2, my_func1)(__VA_ARGS__)
void my_func3(char a, int b, float c) // b=10, c=0.5
{
printf("a=%c; b=%d; c=%f\n", a, b, c);
}
If you want 4th argument, then an extra my_func3 needs to be added. Notice the changes in VAR_FUNC, my_func2 and my_func
4 Parameters:
#define my_func3(...) my_func4(__VA_ARGS__, "default") // <== New function added
#define my_func2(...) my_func3(__VA_ARGS__, (float)1/2)
#define my_func1(...) my_func2(__VA_ARGS__, 10)
#define VAR_FUNC(_1, _2, _3, _4, NAME, ...) NAME
#define my_func(...) VAR_FUNC(__VA_ARGS__, my_func4, my_func3, my_func2, my_func1)(__VA_ARGS__)
void my_func4(char a, int b, float c, const char* d) // b=10, c=0.5, d="default"
{
printf("a=%c; b=%d; c=%f; d=%s\n", a, b, c, d);
}
Only exception that float variables cannot be given default values (unless if it is the last argument as in the 3 parameters case), because they need period ('.'), which is not accepted within macro arguments. But can figure out a work around as seen in my_func2 macro (of 4 parameters case)
Program
int main(void)
{
my_func('a');
my_func('b', 20);
my_func('c', 200, 10.5);
my_func('d', 2000, 100.5, "hello");
return 0;
}
Output:
a=a; b=10; c=0.500000; d=default
a=b; b=20; c=0.500000; d=default
a=c; b=200; c=10.500000; d=default
a=d; b=2000; c=100.500000; d=hello
Yes you can do somthing simulair, here you have to know the different argument lists you can get but you have the same function to handle then all.
typedef enum { my_input_set1 = 0, my_input_set2, my_input_set3} INPUT_SET;
typedef struct{
INPUT_SET type;
char* text;
} input_set1;
typedef struct{
INPUT_SET type;
char* text;
int var;
} input_set2;
typedef struct{
INPUT_SET type;
int text;
} input_set3;
typedef union
{
INPUT_SET type;
input_set1 set1;
input_set2 set2;
input_set3 set3;
} MY_INPUT;
void my_func(MY_INPUT input)
{
switch(input.type)
{
case my_input_set1:
break;
case my_input_set2:
break;
case my_input_set3:
break;
default:
// unknown input
break;
}
}
you don't need to use VARARGS with just C. Here is an example.
int funcA_12(int a1, int a2) { ... }
#define funcA(a1) funcA_12(a1, 0)
This answer is very similar to the two functions method above but in this case, you're using a macro for the function name that defines the arguments.
https://github.com/cindRoberta/C/blob/master/structure/function/default_parameter.c
#include<stdio.h>
void f_impl(int a, float b) {
printf("%d %g\n", a, b);
}
#define f_impl(...) f_macro(__VA_ARGS__, 3.7)
#define f_macro(a, b, ...) f_impl(a, b)
int main(void) {
f_impl(1);
f_impl(1, 2, 3, 4);
return 0;
}
I know how to do this in a better manner.
You simply assign NULL to a parameter, so, you will have no value. Then you check if the parameter value is NULL, you change it to the default value.
void func(int x){
if(x == NULL)
x = 2;
....
}
Though, it will cause warnings. a better choice would be to assign a value that will do nothing if the parameter value is that:
void func(int x){
if(x == 1)
x = 2;
....
}
In the example above, if x is 1 the function changes it to 2;
Thanks to #user904963, EDIT:
if you have to cover all ranges of numbers, it's not hard to add another argument only to say to the function whether it would set the parameter to default or not
void func(int x, bool useDefault){
if(useDefault) //useDefault == true
x = 2;
....
}
However, remember to include stdbool.h
There's a trick I've occasionally used, which has been available since C99, using variadic macros, compound literals and designated initializers. As with any macro solution, it is cumbersome and generally not recommended other than as a last resort...
My method is built in the following way:
Wrap the actual function in a function-like, variadic macro:
void myfunc (int x, int y) // actual function
#define myfunc(...) myfunc(params) // wrapper macro
By using compound literals, copy down the parameters passed into a temporary object. This object should be a private struct corresponding directly to the function's expected parameter list. Example:
typedef struct
{
int x;
int y;
} myfunc_t;
#define PASSED_ARGS(...) (myfunc_t){__VA_ARGS__}
This means that the same type safety ("as per assignment") rules used when passing parameters to a function is also used when initializing this struct. We don't lose any type safety. Similarly, this automatically guards against providing too many arguments.
However, the above doesn't cover the case of an empty argument list. To counter this, add a dummy argument so that the initializer list is never empty:
typedef struct
{
int dummy;
int x;
int y;
} myfunc_t;
#define PASSED_ARGS(...) (myfunc_t){0,__VA_ARGS__}
Similarly, we can count the number of arguments passed, assuming that every parameter passed can get implicitly converted to int:
#define COUNT_ARGS(...) (sizeof(int[]){0,__VA_ARGS__} / sizeof(int) - 1)
We define a macro for the default arguments #define DEFAULT_ARGS (myfunc_t){0,1,2}, where 0 is the dummy and 1,2 are the default ones.
Wrapping all of this together, the outermost wrapper macro may look like:
#define myfunc(...) myfunc( MYFUNC_INIT(__VA_ARGS__).x, MYFUNC_INIT(__VA_ARGS__).y )
This assuming that the inner macro MYFUNC_INIT returns a myfunc_t struct.
The inner macro conditionally picks struct initializers based on the size of the argument list. In case the argument list is short, it fills up with default arguments.
#define MYFUNC_INIT(...) \
(myfunc_t){ 0, \
.x = COUNT_ARGS(__VA_ARGS__)==0 ? DEFAULT_ARGS.x : PASSED_ARGS(__VA_ARGS__).x, \
.y = COUNT_ARGS(__VA_ARGS__)<2 ? DEFAULT_ARGS.y : PASSED_ARGS(__VA_ARGS__).y, \
}
Full example:
#include <stdio.h>
void myfunc (int x, int y)
{
printf("x:%d y:%d\n", x, y);
}
typedef struct
{
int dummy;
int x;
int y;
} myfunc_t;
#define DEFAULT_ARGS (myfunc_t){0,1,2}
#define PASSED_ARGS(...) (myfunc_t){0,__VA_ARGS__}
#define COUNT_ARGS(...) (sizeof(int[]){0,__VA_ARGS__} / sizeof(int) - 1)
#define MYFUNC_INIT(...) \
(myfunc_t){ 0, \
.x = COUNT_ARGS(__VA_ARGS__)==0 ? DEFAULT_ARGS.x : PASSED_ARGS(__VA_ARGS__).x, \
.y = COUNT_ARGS(__VA_ARGS__)<2 ? DEFAULT_ARGS.y : PASSED_ARGS(__VA_ARGS__).y, \
}
#define myfunc(...) myfunc( MYFUNC_INIT(__VA_ARGS__).x, MYFUNC_INIT(__VA_ARGS__).y )
int main (void)
{
myfunc(3,4);
myfunc(3);
myfunc();
}
Output:
x:3 y:4
x:3 y:2
x:1 y:2
Godbolt: https://godbolt.org/z/4ns1zPW16 As you can see from the -O3 disassembly, there is zero overhead from the compound literals.
I noticed that my method reminds a bit of the current, top-voted answer. For comparison with other solutions here:
Pros:
Pure, portable standard ISO C, no dirty gcc extensions, no poorly-defined behavior.
Can handle empty argument lists.
Efficient, zero overhead, doesn't rely on function inlining getting carried out as expected.
No obscure designated initializers on the caller-side.
Cons:
Relies on every parameter being implicitly convertible to int, which often isn't the case. For example strict C does not allow implicit conversions from pointers to int - such implicit conversions is a non-conforming (but popular) compiler extension.
Default args and structs have to be generated per function. Although not covered by this answer, this could be automated with X macros. But doing so will also reduce readability even further.
Why can't we do this.
Give the optional argument a default value. In that way, the caller of the function don't necessarily need to pass the value of the argument. The argument takes the default value.
And easily that argument becomes optional for the client.
For e.g.
void foo(int a, int b = 0);
Here b is an optional argument.