Is it possible to expand a macro which accepts multiple arguments to a different macro if first argument is not the expected value
E.g
int main()
{
PRINT(2, "%d%d\n", i, j); //should expand to syslog(2, "%d%d\n", i, j)
PRINT("%d%d\n", i, j); //arg1 which is expected to be an int is not preset.
/* This should expand differently may be to a default level say 3. syslog(3, "%d%d\n", i,j); */
}
I would have tried this kind of over loading if I knew total number of args.
I really recommend to write two separate macros for this, just as you would write two differently named functions for the two signatues in C. (I would rather write macros that tell you what level they are explicitly, like ERROR(...), WARNING(..) etc. than introduce a default argument.)
That said, there are two possibilities to achieve what you want.
C11 _Generic selections
The _Generic keyword was introduced with C11. It allows to expand macros in a switch-like manner according to the type of an argument; Robert Gamble has a good introduction.
You want to distinguish two cases: First argument is a string and first argument is an integer. A drawback is that in _Generic, a string literal isn't treated as char * or const char *, but as char[size]. For example, "%d" is a char[3].
In your case, we can get around this by treating a string as anything that isn't an integer. The compiler will sort out all non-string, non-integer arguments later. So:
#define PRINT(fmt, ...) \
_Generic(fmt, \
int: syslog(fmt, __VA_ARGS__), \
default: syslog(3, fmt, __VA_ARGS__))
There are drawbacks: You can't have a single-argument call, because that would leave a comma in the call. (gcc's ##__VA_ARGS__ gets around that.) And the _Generic keyword is not yet widely implemented; this solution will make your code highly unportable.
String introspection hack
Ordinary C99 macros have no information on their type. C code can make a guess, however. Here's an example that checks whether a macro argument is a string literal:
#define PRINT(sev, ...) \
if (#sev[0] == '"') syslog(3, sev, __VA_ARGS); \
else syslog(sev, __VA_ARGS__);
This works -- almost. The compiler will probably compile the constant condition away and only gererate code for one of the branches. But it will parse the branches anyway and the dead branch will have a wrong function signature, which will generate warnings.
You can get around this by writing a variadic front-end function in C. Here's an example that works:
#include <stdlib.h>
#include <stdio.h>
#include <stdarg.h>
#define HEAD(X, ...) X
#define STR_(x) #x
#define STR(x) STR_(x)
#define PRINT(...) \
msg(*STR(HEAD(__VA_ARGS__)) == '"', __VA_ARGS__)
int msg(int dflt, ...)
{
va_list va;
int sev = 3;
const char *fmt;
va_start(va, dflt);
if (!dflt) sev = va_arg(va, int);
fmt = va_arg(va, const char *);
fprintf(stderr, "[%d] ", sev);
vfprintf(stderr, fmt, va);
fprintf(stderr, "\n");
va_end(va);
return 0;
}
int main()
{
PRINT(1, "Incompatible types %s and %s", "Apple", "Orange");
PRINT("Microphone test: %d, %d, %d, ...", 1, 2, 3);
return 0;
}
This solution is dangerous, because the msg function is only safe if it is generated by the macro. And the macro is only safe if the format string is a string literal beginning with a double quote. The macro expands the arguments by one boolean argument to the left and hides the argument incompatibility in a variadic argument list.
It may be a nice trick, but you'll be better off having separate, clearly named macros.
C macros do not have the ability to inspect their arguments. As noted in the answer you posted, there is a sneaky way to do different things based on the number of arguments, but that's the extent of it. If you already have a variable number of arguments outside of the overload you are trying to do, it will not be possible. If all you need is a default level:
#define PRINTNORM(...) PRINT(3, __VA_ARGS__)
or whatever you'd like to call it. IMHO, cleaner code than overloading PRINT.
Simply use another value for your need. And perhaps a bit of magic with variadic macro would help.
something like:
#define PRINT( print_level , print_string , ... )\
switch( print_level ) \
/* as many syslog cas as needed */
case( 5 ):\
case( 4 ):\
case( 3 ):\
case( 2 ):\
case( 2 ):\
case( 1 ):\
syslog( print_level , __VA_ARGS__ );\
break ; \
default: \
case( 0 ): \
printf( __VA_ARGS__ ); \ /* else we simply want to print it */
break ;
Edit:
Doc on variadic macro: https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
P99 has conditional macro evaluation. Here you could probably use something like P99_IF_EMPTY for something like
#define PRINT(LEV, ...) my_print(P99_IF_EMPTY(LEV)(3)(LEV), __VA_ARGS__)
this would still have you insert a , for the case of the empty argument but comes probably close to what you want to achieve.
Optional arguments coming before other mandatory arguments can potentially be handled by folding them together in parentheses:
PRINT((2, "%d%d\n"), i, j);
PRINT("%d%d\n", i, j);
Define PRINT like this:
#define PRINT(SL, ...) PRINT_LEVEL(APPLY(CAT(LEVEL, IS_SPLIT(SL)), IDENTITY SL), APPLY(CAT(FSTRING, IS_SPLIT(SL)), IDENTITY SL), __VA_ARGS__)
#define PRINT_LEVEL(LEVEL, ...) syslog(LEVEL, __VA_ARGS__)
PRINT detects whether the first argument is an atom (just the format string) or a parenthesized list of two elements (printlevel + string), and expands into the real implementation PRINT_LEVEL accordingly, either extracting the level from the first argument, or supplying a default value.
Definitions for IS_SPLIT and the other helpers are as follows:
#define LEVEL_0(_S) 3
#define LEVEL_1(L, S) L
#define FSTRING_0(S) K_##S
#define FSTRING_1(L, S) S
#define CAT(A, B) CAT_(A, B)
#define CAT_(A, B) A ## B
#define APPLY(F, ...) F(__VA_ARGS__)
#define IDENTITY(...) __VA_ARGS__
#define K_IDENTITY
#define IS_SPLIT(...) IS_SPLIT_1(IDENTITY __VA_ARGS__)
#define IS_SPLIT_1(...) IS_SPLIT_2(__VA_ARGS__, _1, _0, _)
#define IS_SPLIT_2(_X, _Y, R, ...) R
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.