Passing variable arguments to another function that accepts a variable argument list

So I have 2 functions that both have similar arguments

void example(int a, int b, ...);
void exampleB(int b, ...);

Now example calls exampleB, but how can I pass along the variables in the variable argument list without modifying exampleB (as this is already used elsewhere too).

Solution 1:

You can't do it directly; you have to create a function that takes a va_list:

#include <stdarg.h>

static void exampleV(int b, va_list args);

void exampleA(int a, int b, ...)    // Renamed for consistency
{
    va_list args;
    do_something(a);                // Use argument a somehow
    va_start(args, b);
    exampleV(b, args);
    va_end(args);
}

void exampleB(int b, ...)
{
    va_list args;
    va_start(args, b);
    exampleV(b, args);
    va_end(args);
}

static void exampleV(int b, va_list args)
{
    ...whatever you planned to have exampleB do...
    ...except it calls neither va_start nor va_end...
}

Solution 2:

Maybe throwin a rock in a pond here, but it seems to work pretty OK with C++11 variadic templates:

#include <stdio.h>

template<typename... Args> void test(const char * f, Args... args) {
  printf(f, args...);
}

int main()
{
  int a = 2;
  test("%s\n", "test");
  test("%s %d %d %p\n", "second test", 2, a, &a);
}

At the very least, it works with g++.

Solution 3:

you should create versions of these functions which take a va_list, and pass those. Look at vprintf as an example:

int vprintf ( const char * format, va_list arg );

Solution 4:

I also wanted to wrap printf and found a helpful answer here:

How to pass variable number of arguments to printf/sprintf

I was not at all interested in performance (I'm sure this piece of code can be improved in a number of ways, feel free to do so :) ), this is for general debugprinting only so I did this:

//Helper function
std::string osprintf(const char *fmt, ...)
{
    va_list args;
    char buf[1000];
    va_start(args, fmt);
    vsnprintf(buf, sizeof(buf), fmt, args );
    va_end(args);
    return buf;
}

which I then can use like this

Point2d p;

cout << osprintf("Point2d: (%3i, %3i)", p.x, p.y);
instead of for example:
cout << "Point2d: ( " << setw(3) << p.x << ", " << p.y << " )";

The c++ ostreams are beautiful in some aspects, but practically the become horrific if you want to print something like this with some small strings such as parenthesis, colons and commas inserted between the numbers.