How can I wrap a function with variable length arguments?

Prakash_123 0
Viewed 31515

I am looking to do this in C/C++. I came across Variable Length Arguments, but this suggests a solution with Python and C using libffi.

Now, if I want to wrap the printf function with myprintf.

I do it like below:

void myprintf(char* fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    printf(fmt, args);
    va_end(args);
}

int _tmain(int argc, _TCHAR* argv[])
{
    int a = 9;
    int b = 10;
    char v = 'C';
    myprintf("This is a number: %d and \nthis is a character: %c and \n another number: %d\n", a, v, b);
    return 0;
}

But the results are not as expected!

This is a number: 1244780 and
this is a character: h and
another number: 29953463

What did I miss?

variadic-functions c c++
7 Answers

The problem is that you cannot use 'printf' with va_args. You must use vprintf if you are using variable argument lists. vprint, vsprintf, vfprintf, etc. (there are also 'safe' versions in Microsoft's C runtime that will prevent buffer overruns, etc.)

You sample works as follows:

void myprintf(char* fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    vprintf(fmt, args);
    va_end(args);
}

int _tmain(int argc, _TCHAR* argv[])
{
    int a = 9;
    int b = 10;
    char v = 'C';
    myprintf("This is a number: %d and \nthis is a character: %c and \n another number: %d\n", a, v, b);
    return 0;
}
Mark_44050

I am also unsure what you mean by pure.

In C++ we use:

#include <cstdarg>
#include <cstdio>

class Foo
{   
    void Write(const char* pMsg, ...);
};

void Foo::Write( const char* pMsg, ...)
{
    char buffer[4096];
    std::va_list arg;
    va_start(arg, pMsg);
    std::vsnprintf(buffer, 4096, pMsg, arg);
    va_end(arg);
    ...
}
David_2590

Are you using C or C++? The next C++ version, C++0x, will support variadic templates which provide a solution to that problem.

Another workaround can be achieved by clever operator overloading to achieve a syntax like this:

void f(varargs va) {
    BOOST_FOREACH(varargs::iterator i, va)
        cout << *i << " ";
}

f(args = 1, 2, 3, "Hello");

In order to get this to work, the class varargs has to be implemented to override operator = that returns a proxy object which, in turn, overrides operator ,. However, making this variant type safe in current C++ isn't possible as far as I know since it would have to work by type erasure.

user_19680
Mark_Ingram_9860
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