Variable Arguement With Class Reference As 1st Parameter

Question

I have the following code :

#include <cstdarg>
#include <iostream>

using namespace std;

class a {
};

void fun1(a& aa, ...)
{
    va_list argp;
    va_start(argp, aa);
    char *p = 0;
    while ((p = va_arg(argp, char *)) != 0) {
        cout << p << endl;
    }
    va_end(argp);
}

void fun2(char *aa, ...)
{
    va_list argp;
    va_start(argp, aa);
    char *p = 0;
    while ((p = va_arg(argp, char *)) != 0) {
        cout << p << endl;
    }
    va_end(argp);
}

int main()
{
    cout << "fun2" << endl;
    fun2("a", "1", "2", (char *)0);
    cout << "fun1" << endl;
    fun1(a(), "1", "2", (char *)0);
    getchar();
}

Everything works fine with fun2. However, fun1 will just crash.

May I know how can I prevent from crashing, at the same time able to use class reference as 1st parameter.

Currently, it prints :

fun2
1
2
fun1

then crash.

I wish

fun2
1
2
fun1
1
2

Answer 1

You can't use a reference parameter as the last named parameter with va_start. The reason is because va_start takes the address of the named parameter to find the location of the rest of the arguments. However, taking the address of a reference gives the address of the variable pointed at by the reference, not the address of the parameter itself. Your options are:

1) change the variable type from a reference to a pointer (or a non-reference if you are OK with a copy of the passed in variable).

2) Add an additional required parameter so that the reference isn't the last named parameter. The additional parameter can be a useful parameter, such as one of the char* you are going to pass to your particular function, or it can be a dummy variable you just ignore.

3) Change the definition of va_start. It's not recommended, but you can do it. See http://support.microsoft.com/kb/119394 for a non-portable redefinition.

Answer 2

It looks to me like you're crashing in fun2.

Because you're calling va_arg too many times and screwing up the stack.

You must only call va_arg the same number of times as there are parameters.

Answer 3

Both fun1 and fun2 terminate the loop when they encounter a NULL or 0 parameter. You are never passing one. Change main to:

int main()
{
    cout << "fun2" << endl;
    fun2("a", "1", "2", NULL);
    cout << "fun1" << endl;
    fun1(a(), "1", "2", NULL);
    getchar();
    return 0;
}

Note I haven't compiled this, but it should work. You may have to follow janm's advice as well.

Update: I set down and thought about this again. You have to either:

1. Instantiate an object of type a inside of main and pass it or...
2. Following janm's advice and change a& aa in fun2 to a const& aa

When I tried to compile the original under g++, I was greeted with the following error:

error: invalid initialization of non-const reference of type 'a&' from a temporary of type 'a'
error: in passing argument 1 of 'void fun1(a&, ...)'

Essentially, you cannot pass a temporary variable as a non-const reference. See this SO question and this Herb Sutter GotW for some of the gory details.

Answer 4

You are passing a non-const reference to a temporary. Change the fun1 prototype to:

void fun1(a const& aa, ...)

Update:

Haven't used varargs for a long time, missed that a terminating parameter wasn't being passed. See D. Shawley's answer; you must pass a terminating parameter if you're going to use that as your interface.

Answer 5

I'm sorry to hear this code doesnt work. I notice if fun2(aa is a ptr instead of a ref the code works. I also notice while trying to compile on gcc (via http://codepad.org/) you pass "a" into fun2 which is a char*. codepad/gcc complained about it not being a const char*. In codepad this code works. In my copy of VS2008 it crashes and 2010b2 as well.

My recommendation is to avoid va params but i'll assume you can't so i suggest not to use ref and use pointers. Or switch to gcc but i wouldnt do that unless there are no other (reasonable) option.

#include<cstdlib>
#include <cstdio>
#include <ios>
#include <iostream>
using namespace std;
class a {
};

void fun1(a& aa, ...)
{
    //cout<< "sizeof" << sizeof(aa) << "&aa == " << &aa;
    va_list argp;
    va_start(argp, aa);
    char *p = 0;
    while ((p = va_arg(argp, char *)) != 0) {
        cout << p << endl;
    }
    va_end(argp);
}

void fun2(const char *aa, ...)
{
    va_list argp;
    va_start(argp, aa);
    char *p = 0;
    while ((p = va_arg(argp, char *)) != 0) {
        cout << p << endl;
    }
    va_end(argp);
}

int main()
{
    cout << "fun2" << endl;
    fun2("a", "1", "2", 0);
    cout << "fun1" << endl;
    a aa;
    //cout<< "sizeof" << sizeof(aa) << "&aa == " << &aa;
    fun1(aa, "1", "2", 0);
    getchar();
}