CODEWITHSUNDEEP
c++visual-studiogccc-preprocessorvariadic-macros
riv
riv

Reputation: 7343

Portable variadic macro

I'm looking to make a variadic macro that simply calls a certain function for every argument (say, up to 6). So far I've been using the following code in MSVC:

#define do_write2(x,y) do{do_write(x);do_write(y);}while(0)
#define do_write3(x,y,z) do{do_write(x);do_write(y);do_write(z);}while(0)
#define do_write4(x,y,z,w) do{do_write(x);do_write(y);do_write(z);do_write(w);}while(0)
#define do_write5(x,y,z,w,t) do{do_write(x);do_write(y);do_write(z);do_write(w);do_write(t);}while(0)
#define do_write6(x,y,z,w,t,u) do{do_write(x);do_write(y);do_write(z);do_write(w);do_write(t);do_write(u);}while(0)
#define expand(x) x
#define _get_write(_1,_2,_3,_4,_5,_6,name,...) name
#define dumpval(...) expand(_get_write(__VA_ARGS__,do_write6,do_write5,do_write4,do_write3,do_write2,do_write))expand((__VA_ARGS__))

The expand is needed due to a peculiar handling of __VA_ARGS__ in MSVC, otherwise I get

error C2660: 'do_write' : function does not take 6 arguments

However, now I need to build the same code in GCC, and its having trouble with it:

error: ‘do_write3’ was not declared in this scope

Simply removing the expand wrapper does the trick. However, is there any "correct" way to make the code compile in both cases without using #ifdef?

Upvotes: 1

Views: 631

Answers (1)

dwks
dwks

Reputation: 602

There are various techniques for variable-argument macros, discussed here: Variadic recursive preprocessor macros - is it possible?

Here are a few implementations that you might be interested in. Personally, I think call_vla2 is the best assuming you have C++11 support. If this is not possible, tell us more about your problem.

#include <iostream>

// target function
void z(int i) {
    std::cout << "[" << i << "]\n";
}

// 1. manually specifying the argument count
#define call1(a)        do{z(a);}while(0)
#define call2(a,b)      do{z(a);z(b);}while(0)
#define call3(a,b,c)    do{z(a);z(b);z(c);}while(0)
#define call_n(n, ...)  call ## n (__VA_ARGS__)

// 2. using a variable-length array (GCC compatible)
// thanks to https://stackoverflow.com/a/824769/6096046
#define call_vla(...) do { \
        int args[] = { __VA_ARGS__ }; \
        for(size_t i = 0; i < sizeof(args)/sizeof(*args); i ++) \
            z(args[i]); \
    } while(0)

// 3. using a variable-length array and C++11
#define call_vla2(...) for(auto x : { __VA_ARGS__ }) z(x);

// 4. using C++11 variadic templates
template <typename... T>
void call_n_times() {}

template <typename... T>
void call_n_times(int a, T... other) {
    z(a), call_n_times(other...);
}

#define call_template(...) call_n_times(__VA_ARGS__)

// tests
int main() {
    call_n(1, 88);      call_n(3, 1,2,3);
    call_vla(88);       call_vla(1,2,3);
    call_vla2(88);      call_vla2(1,2,3);
    call_template(88);  call_template(1,2,3);
    return 0;
}

Upvotes: 1

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