Can C/C++ preprocessor macros have default parameter values?

Question

Can we specify default parameter values for macro parameters?

I know there isn't any type-checking, so I expect the default value to be nothing more than just some text used by the preprocessor for macro expansion in instances where the parameter value is not specified.

Answer 1

You are looking for a macro overload mechanism which is provided in e.g. Boost.PP's facilities.

#define MACRO_2(a, b) std::cout << a << ' ' << b;

#define MACRO_1(a) MACRO_2(a, "test") // Supply default argument

// Magic happens here:

#define MACRO(...) BOOST_PP_OVERLOAD(MACRO_, __VA_ARGS__)(__VA_ARGS__)

Demo. The number of arguments is concatenated with the macro name, which can easily be implemented without Boost as follows:

#define VARGS_(_10, _9, _8, _7, _6, _5, _4, _3, _2, _1, N, ...) N 
#define VARGS(...) VARGS_(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)

#define CONCAT_(a, b) a##b
#define CONCAT(a, b) CONCAT_(a, b)

And

#define MACRO_2(a, b) std::cout << a << ' ' << b;

#define MACRO_1(a) MACRO_2(a, "test") // Supply default argument

#define MACRO(...) CONCAT(MACRO_, VARGS(__VA_ARGS__))(__VA_ARGS__)

Demo.

Answer 2

There is a way to do it, if you permit an unusual syntax:

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>

void TestInternal( int n )
{
    printf("%d\n" , n ) ;
}
#define TestGet( f , ... )  f
#define Test( ... ) TestInternal( TestGet( __VA_ARGS__ DEFAULT , DEFAULT ) )
#define DEFAULT 100

int main( void )
{ 
    Test( ) ;
    Test( 12345 , ) ;

return 0 ;
}

This is type safe and can be expanded to any number of arguments.

Note that the comma must be present if you pass a variable. You can treat it as an indicator that this function/macro has default parameters.

Using a similar version you can omit the comma, but then your functions must have at least one non-default parameter which must be first.

Answer 3

It seems like the simplest solution is to define an additional macro that is missing the default parameter.

#define MACRO1( PARAM1 , PARAM2 ) &( PARAM1 + PARAM2 ) // or whatever logic

#define MACRO1_DEFAULT( PARAM1 ) MACRO1 ( PARAM1 , 12 ) // PARAM2 default of 12

Also, as mentioned by 40two, the language does support using an ellipsis for variadic macros, which may also be a viable solution in some instances.

Contents of the linked article:

Variadic macros are function-like macros that contain a variable number of arguments.

To use variadic macros, the ellipsis may be specified as the final formal argument in a macro definition, and the replacement identifier __VA_ARGS__ may be used in the definition to insert the extra arguments.
__VA_ARGS__ is replaced by all of the arguments that match the ellipsis, including commas between them.

The C Standard specifies that at least one argument must be passed to the ellipsis, to ensure that the macro does not resolve to an expression with a trailing comma.

<Microsoft specific>
The Visual C++ implementation will suppress a trailing comma if no arguments are passed to the ellipsis.
</Microsoft Specific>

Example

// variadic_macros.cpp
#include <stdio.h>
#define EMPTY

#define CHECK1(x, ...) if (!(x)) { printf(__VA_ARGS__); }
#define CHECK2(x, ...) if ((x)) { printf(__VA_ARGS__); }
#define CHECK3(...) { printf(__VA_ARGS__); }
#define MACRO(s, ...) printf(s, __VA_ARGS__)

int main() {
    CHECK1(0, "here %s %s %s", "are", "some", "varargs1(1)\n");
    CHECK1(1, "here %s %s %s", "are", "some", "varargs1(2)\n");   // won't print

    CHECK2(0, "here %s %s %s", "are", "some", "varargs2(3)\n");   // won't print
    CHECK2(1, "here %s %s %s", "are", "some", "varargs2(4)\n");

    // always invokes printf in the macro
    CHECK3("here %s %s %s", "are", "some", "varargs3(5)\n");

    MACRO("hello, world\n");

    MACRO("error\n", EMPTY); // would cause error C2059, except VC++ 
                             // suppresses the trailing comma
}

Output

here are some varargs1(1)
here are some varargs2(4)
here are some varargs3(5)
hello, world
error