Overloading Macro on Number of Arguments
CMacrosC PreprocessorC Problem Overview
I have two macros FOO2
and FOO3
:
#define FOO2(x,y) ...
#define FOO3(x,y,z) ...
I want to define a new macro FOO
as follows:
#define FOO(x,y) FOO2(x,y)
#define FOO(x,y,z) FOO3(x,y,z)
But this doesn't work because macros do not overload on number of arguments.
Without modifying FOO2
and FOO3
, is there some way to define a macro FOO
(using __VA_ARGS__
or otherwise) to get the same effect of dispatching FOO(x,y)
to FOO2
, and FOO(x,y,z)
to FOO3
?
C Solutions
Solution 1 - C
Simple as:
#define GET_MACRO(_1,_2,_3,NAME,...) NAME
#define FOO(...) GET_MACRO(__VA_ARGS__, FOO3, FOO2)(__VA_ARGS__)
So if you have these macros, they expand as described:
FOO(World, !) // expands to FOO2(World, !)
FOO(foo,bar,baz) // expands to FOO3(foo,bar,baz)
If you want a fourth one:
#define GET_MACRO(_1,_2,_3,_4,NAME,...) NAME
#define FOO(...) GET_MACRO(__VA_ARGS__, FOO4, FOO3, FOO2)(__VA_ARGS__)
FOO(a,b,c,d) // expands to FOO4(a,b,c,d)
Naturally, if you define FOO2
, FOO3
and FOO4
, the output will be replaced by those of the defined macros.
Solution 2 - C
To add on to netcoder's answer, you CAN in fact do this with a 0-argument macro, with the help of the GCC ##__VA_ARGS__
extension:
#define GET_MACRO(_0, _1, _2, NAME, ...) NAME
#define FOO(...) GET_MACRO(_0, ##__VA_ARGS__, FOO2, FOO1, FOO0)(__VA_ARGS__)
Solution 3 - C
Here is a more general solution:
// get number of arguments with __NARG__
#define __NARG__(...) __NARG_I_(__VA_ARGS__,__RSEQ_N())
#define __NARG_I_(...) __ARG_N(__VA_ARGS__)
#define __ARG_N( \
_1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
_11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
_21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
_31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
_41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
_51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
_61,_62,_63,N,...) N
#define __RSEQ_N() \
63,62,61,60, \
59,58,57,56,55,54,53,52,51,50, \
49,48,47,46,45,44,43,42,41,40, \
39,38,37,36,35,34,33,32,31,30, \
29,28,27,26,25,24,23,22,21,20, \
19,18,17,16,15,14,13,12,11,10, \
9,8,7,6,5,4,3,2,1,0
// general definition for any function name
#define _VFUNC_(name, n) name##n
#define _VFUNC(name, n) _VFUNC_(name, n)
#define VFUNC(func, ...) _VFUNC(func, __NARG__(__VA_ARGS__)) (__VA_ARGS__)
// definition for FOO
#define FOO(...) VFUNC(FOO, __VA_ARGS__)
Define your functions:
#define FOO2(x, y) ((x) + (y))
#define FOO3(x, y, z) ((x) + (y) + (z))
// it also works with C functions:
int FOO4(int a, int b, int c, int d) { return a + b + c + d; }
Now you can use FOO
with 2, 3 and 4 arguments:
FOO(42, 42) // will use makro function FOO2
FOO(42, 42, 42) // will use makro function FOO3
FOO(42, 42, 42, 42) // will call FOO4 function
Limitations
- Only up to 63 arguments (but expandable)
- Function for no argument only in GCC possible
Ideas
Use it for default arguments:
#define func(...) VFUNC(func, __VA_ARGS__)
#define func2(a, b) func4(a, b, NULL, NULL)
#define func3(a, b, c) func4(a, b, c, NULL)
// real function:
int func4(int a, int b, void* c, void* d) { /* ... */ }
Use it for functions with possible infinite number of arguments:
#define SUM(...) VFUNC(SUM, __VA_ARGS__)
#define SUM2(a, b) ((a) + (b))
#define SUM3(a, b, c) ((a) + (b) + (c))
#define SUM4(a, b, c) ((a) + (b) + (c) + (d))
// ...
PS: __NARG__
is copied from Laurent Deniau & Roland Illig here: https://groups.google.com/group/comp.std.c/browse_thread/thread/77ee8c8f92e4a3fb/346fc464319b1ee5?pli=1
Solution 4 - C
I was just researching this myself, and I came across this here. The author added default argument support for C functions via macros.
I'll try to briefly summarize the article. Basically, you need to define a macro that can count arguments. This macro will return 2, 1, 0, or whatever range of arguments it can support. Eg:
#define _ARG2(_0, _1, _2, ...) _2
#define NARG2(...) _ARG2(__VA_ARGS__, 2, 1, 0)
With this, you need to create another macro that takes a variable number of arguments, counts the arguments, and calls the appropriate macro. I've taken your example macro and combined it with the article's example. I have FOO1 call function a() and FOO2 call function a with argument b (obviously, I'm assuming C++ here, but you can change the macro to whatever).
#define FOO1(a) a();
#define FOO2(a,b) a(b);
#define _ARG2(_0, _1, _2, ...) _2
#define NARG2(...) _ARG2(__VA_ARGS__, 2, 1, 0)
#define _ONE_OR_TWO_ARGS_1(a) FOO1(a)
#define _ONE_OR_TWO_ARGS_2(a, b) FOO2(a,b)
#define __ONE_OR_TWO_ARGS(N, ...) _ONE_OR_TWO_ARGS_ ## N (__VA_ARGS__)
#define _ONE_OR_TWO_ARGS(N, ...) __ONE_OR_TWO_ARGS(N, __VA_ARGS__)
#define FOO(...) _ONE_OR_TWO_ARGS(NARG2(__VA_ARGS__), __VA_ARGS__)
So if you have
FOO(a)
FOO(a,b)
The preprocessor expands that to
a();
a(b);
I would definitely read the article that I linked. It's very informative and he mentions that NARG2 won't work on empty arguments. He follows this up here.
Solution 5 - C
Here is a more compact version of the answer above. With example.
#include <iostream>
using namespace std;
#define OVERLOADED_MACRO(M, ...) _OVR(M, _COUNT_ARGS(__VA_ARGS__)) (__VA_ARGS__)
#define _OVR(macroName, number_of_args) _OVR_EXPAND(macroName, number_of_args)
#define _OVR_EXPAND(macroName, number_of_args) macroName##number_of_args
#define _COUNT_ARGS(...) _ARG_PATTERN_MATCH(__VA_ARGS__, 9,8,7,6,5,4,3,2,1)
#define _ARG_PATTERN_MATCH(_1,_2,_3,_4,_5,_6,_7,_8,_9, N, ...) N
//Example:
#define ff(...) OVERLOADED_MACRO(ff, __VA_ARGS__)
#define ii(...) OVERLOADED_MACRO(ii, __VA_ARGS__)
#define ff3(c, a, b) for (int c = int(a); c < int(b); ++c)
#define ff2(c, b) ff3(c, 0, b)
#define ii2(a, b) ff3(i, a, b)
#define ii1(n) ii2(0, n)
int main() {
ff (counter, 3, 5)
cout << "counter = " << counter << endl;
ff (abc, 4)
cout << "abc = " << abc << endl;
ii (3)
cout << "i = " << i << endl;
ii (100, 103)
cout << "i = " << i << endl;
return 0;
}
Run:
User@Table 13:06:16 /c/T
$ g++ test_overloaded_macros.cpp
User@Table 13:16:26 /c/T
$ ./a.exe
counter = 3
counter = 4
abc = 0
abc = 1
abc = 2
abc = 3
i = 0
i = 1
i = 2
i = 100
i = 101
i = 102
Note that having both _OVR
and _OVR_EXPAND
may look redundant, but it's necessary for the preprocessor to expand the _COUNT_ARGS(__VA_ARGS__)
part, which otherwise is treated as a string.
Solution 6 - C
Here's a spin off from Evgeni Sergeev's answer. This one supports zero argument overloads as well!
I tested this with GCC and MinGW. It ought to work with old and new versions of C++. Note that I wouldn't guarantee it for MSVC... But with some tweaking, I'm confident it could be made to work with that too.
I also formatted this to be pasted into a header file (which I called macroutil.h). If you do that, you can just include this header whatever you need the feature, and not look at the nastiness involved in the implementation.
#ifndef MACROUTIL_H
#define MACROUTIL_H
//-----------------------------------------------------------------------------
// OVERLOADED_MACRO
//
// used to create other macros with overloaded argument lists
//
// Example Use:
// #define myMacro(...) OVERLOADED_MACRO( myMacro, __VA_ARGS__ )
// #define myMacro0() someFunc()
// #define myMacro1( arg1 ) someFunc( arg1 )
// #define myMacro2( arg1, arg2 ) someFunc( arg1, arg2 )
//
// myMacro();
// myMacro(1);
// myMacro(1,2);
//
// Note the numerical suffix on the macro names,
// which indicates the number of arguments.
// That is the REQUIRED naming convention for your macros.
//
//-----------------------------------------------------------------------------
// OVERLOADED_MACRO
// derived from: https://stackoverflow.com/questions/11761703/overloading-macro-on-number-of-arguments
// replaced use of _COUNT_ARGS macro with VA_NUM_ARGS defined below
// to support of zero argument overloads
#define OVERLOADED_MACRO(M, ...) _OVR(M, VA_NUM_ARGS(__VA_ARGS__)) (__VA_ARGS__)
#define _OVR(macroName, number_of_args) _OVR_EXPAND(macroName, number_of_args)
#define _OVR_EXPAND(macroName, number_of_args) macroName##number_of_args
//#define _COUNT_ARGS(...) _ARG_PATTERN_MATCH(__VA_ARGS__, 15,14,13,12,11,10,9,8,7,6,5,4,3,2,1)
#define _ARG_PATTERN_MATCH(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,_11,_12,_13,_14,_15, N, ...) N
// VA_NUM_ARGS
// copied from comments section of:
// http://efesx.com/2010/07/17/variadic-macro-to-count-number-of-arguments/
// which itself was derived from:
// https://gustedt.wordpress.com/2010/06/08/detect-empty-macro-arguments/
#define _ARG16(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, ...) _15
#define HAS_COMMA(...) _ARG16(__VA_ARGS__, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0)
#define HAS_NO_COMMA(...) _ARG16(__VA_ARGS__, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1)
#define _TRIGGER_PARENTHESIS_(...) ,
#define HAS_ZERO_OR_ONE_ARGS(...) \
_HAS_ZERO_OR_ONE_ARGS( \
/* test if there is just one argument, eventually an empty one */ \
HAS_COMMA(__VA_ARGS__), \
/* test if _TRIGGER_PARENTHESIS_ together with the argument adds a comma */ \
HAS_COMMA(_TRIGGER_PARENTHESIS_ __VA_ARGS__), \
/* test if the argument together with a parenthesis adds a comma */ \
HAS_COMMA(__VA_ARGS__ (~)), \
/* test if placing it between _TRIGGER_PARENTHESIS_ and the parenthesis adds a comma */ \
HAS_COMMA(_TRIGGER_PARENTHESIS_ __VA_ARGS__ (~)) \
)
#define PASTE5(_0, _1, _2, _3, _4) _0 ## _1 ## _2 ## _3 ## _4
#define _HAS_ZERO_OR_ONE_ARGS(_0, _1, _2, _3) HAS_NO_COMMA(PASTE5(_IS_EMPTY_CASE_, _0, _1, _2, _3))
#define _IS_EMPTY_CASE_0001 ,
#define _VA0(...) HAS_ZERO_OR_ONE_ARGS(__VA_ARGS__)
#define _VA1(...) HAS_ZERO_OR_ONE_ARGS(__VA_ARGS__)
#define _VA2(...) 2
#define _VA3(...) 3
#define _VA4(...) 4
#define _VA5(...) 5
#define _VA6(...) 6
#define _VA7(...) 7
#define _VA8(...) 8
#define _VA9(...) 9
#define _VA10(...) 10
#define _VA11(...) 11
#define _VA12(...) 12
#define _VA13(...) 13
#define _VA14(...) 14
#define _VA15(...) 15
#define _VA16(...) 16
#define VA_NUM_ARGS(...) VA_NUM_ARGS_IMPL(__VA_ARGS__, PP_RSEQ_N(__VA_ARGS__) )
#define VA_NUM_ARGS_IMPL(...) VA_NUM_ARGS_N(__VA_ARGS__)
#define VA_NUM_ARGS_N( \
_1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
_11,_12,_13,_14,_15,_16,N,...) N
#define PP_RSEQ_N(...) \
_VA16(__VA_ARGS__),_VA15(__VA_ARGS__),_VA14(__VA_ARGS__),_VA13(__VA_ARGS__), \
_VA12(__VA_ARGS__),_VA11(__VA_ARGS__),_VA10(__VA_ARGS__), _VA9(__VA_ARGS__), \
_VA8(__VA_ARGS__),_VA7(__VA_ARGS__),_VA6(__VA_ARGS__),_VA5(__VA_ARGS__), \
_VA4(__VA_ARGS__),_VA3(__VA_ARGS__),_VA2(__VA_ARGS__),_VA1(__VA_ARGS__), \
_VA0(__VA_ARGS__)
//-----------------------------------------------------------------------------
#endif // MACROUTIL_H
Solution 7 - C
Maybe you can use this macro to count the number of arguments.
#define VA_NUM_ARGS(...) VA_NUM_ARGS_IMPL(__VA_ARGS__, 5,4,3,2,1)
#define VA_NUM_ARGS_IMPL(_1,_2,_3,_4,_5,N,...) N
Solution 8 - C
This seems to work fine on GCC, Clang and MSVC. It's a cleaned up version of some of the answers here
#define _my_BUGFX(x) x
#define _my_NARG2(...) _my_BUGFX(_my_NARG1(__VA_ARGS__,_my_RSEQN()))
#define _my_NARG1(...) _my_BUGFX(_my_ARGSN(__VA_ARGS__))
#define _my_ARGSN(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,N,...) N
#define _my_RSEQN() 10,9,8,7,6,5,4,3,2,1,0
#define _my_FUNC2(name,n) name ## n
#define _my_FUNC1(name,n) _my_FUNC2(name,n)
#define GET_MACRO(func,...) _my_FUNC1(func,_my_BUGFX(_my_NARG2(__VA_ARGS__))) (__VA_ARGS__)
#define FOO(...) GET_MACRO(FOO,__VA_ARGS__)
Solution 9 - C
Based on @netcoder's answer and @vexe's suggestion about the Visual Studio compiler support, I found this code working pretty well across various platforms:
#define FOO1(a) func1(a)
#define FOO2(a, b) func2(a, b)
#define FOO3(a, b, c) func3(a, b, c)
#define EXPAND(x) x
#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define FOO(...) EXPAND(GET_MACRO(__VA_ARGS__, FOO3, FOO2, FOO1)(__VA_ARGS__))
, where func1()
, func2()
, func3()
are just normal functions that accept different number of parameters.