Create va_list dynamically - c++

Question

I have a function
void foo(int cnt, va_list ap);
I need to use it, but requirement is quite strict, number of va_list vary and it will change during run-time. What I would like to do is:
create a va_list (which expects char*) form
QList<Contact*>
where Contact is a defined class
class Contact
{
public:
QString getName();
private:
QString m_name;
};
and I would like to populate in the loop va_list for example:
for (int idx = 0; idx<contacts.count(); idx++)
{
contacts.at(idx)->getName(); // this i would like to pass to va_list
}
Does anybody have a clue about how I could do this?

Answer 1

What you're wanting to do is to simulate the call stack so you can pass a constructed va_list to foo(). This is rather specific to the compiler ( and warning, there are differences between even 32- and 64-bit compilers ). The following code is for ENTERTAINMENT PURPOSES ONLY!!! as (if it even works on your system) it is prone to breakage. With it, I use a flat memory buffer and the populate it with a count and a bunch of character strings. You could fill it as appropriate with pointers to your strings and hand them down.
It does seem to work on my system, Windows 7 w/ Visual Studio 2008, for 32-bit applications only.
* BAD IDEA CODE FOLLOWS!!! *
#define PSEUDOSTACKSIZE ( sizeof(int) + 999 * sizeof(const char*) )
#pragma pack( push,1 )
union PSEUDOSTACK
{
int count;
char data[PSEUDOSTACKSIZE];
};
#pragma pack( pop )
void foo( int count, va_list args )
{
for ( int i = 0; i < count; i++ )
{
char *s = va_arg( args, char* );
printf( "%s\n", s);
}
}
void bar( PSEUDOSTACK data, ... )
{
va_list args;
va_start(args, data.count);
foo( data.count, args);
va_end(args);
}
// And later on, the actual test case code.
PSEUDOSTACK barData;
barData.count = 999;
char *p = barData.data + sizeof(int);
for ( int i = 0; i < 999; i++, p += sizeof(char*) )
{
*reinterpret_cast<char**>(p) = "ThisIsABadIdea";
}
bar( barData );
I'll now go hang my head in shame for thinking of such an idea.

Answer 2

...hmmm...maybe not portable...for sure not nice...but may solve yor problem...
va_list is (at least for visual c++) just a #define for char*
→ arguments don't need to be on the stack
→ arguments are just required to be continuous in memory
→ no need to use assembler and/or copying (see my 'just for fun answer' :-)
→ no need to worry about cleanup
efficient!
tested on w2k3 sp2 32bit + vc++ 2010
#include <stdarg.h>
#include <string>
#include <vector>
#include <iostream>
#define N 6 // test argument count
void foo(int n, va_list args);
int main(int, char*[])
{
std::vector<std::wstring> strings;
std::wstring s(L"a");
int i(0);
// create unique strings...
for (; i != N; ++i)
{
strings.push_back(s);
++s.front();
}
foo(N, reinterpret_cast<va_list>(strings.data()));
return 0;
}
void foo(int n, va_list args)
{
int i(0);
for (; i != n; ++i)
std::wcout << va_arg(args, std::wstring) << std::endl;
}

Answer 3

Your question is tagged C++ and there are nice ways (like streams) to avoid varargs completely in C++.
This is a great example of why va_args can cause pain. If you have any chance at all to change the signature of foo, that's your best option. Taking a std::vector<std::string> instead of va_list would just solve your problem right there.
If foo is in an external library you can't change, my next suggestion would be to find a different library.
If none of those is an option it seems like there ought to be a way to recursively build up the call list using va_list, but I couldn't figure out how to make that work.

Answer 4

If the number of elements in the list is limited, I would go for manual dispatch depending on the number of elements.
void call_foo(int count, ...) {
va_list args;
va_start(args, count);
foo(count, args);
va_end(args);
}
switch (contacts.count()) {
case 0: return call_foo(contacts.count());
case 1: return call_foo(contacts.count(),
contacts.at(0)->getName());
case 2: return call_foo(contacts.count(),
contacts.at(0)->getName(),
contacts.at(1)->getName());
case 3: return call_foo(contacts.count(),
contacts.at(0)->getName(),
contacts.at(1)->getName(),
contacts.at(2)->getName());
default: /* ERROR HERE, ADD MORE CASES */ return call_foo(0);
}

Answer 5

What you are trying to use is alloca. A va_list object can not store variables, the function call stores them, and you can only access it via va_list. These variables are only valid during the call, and they get ovverwriten afterwards.
THIS WILL NOT WORK:
va_list func(int dummy, ...)
{
va_list result;
va_start(result, dummy);
return result;
}
To allocate memory on the stack, without having to write a variadic functions use alloca. It works more or less like malloc, but you don't have to call free, it automagically frees itself when you leave the scope.
int * local = (int *) alloca(3 * sizeof(int));
local[0] = 10;
local[1] = 20;
local[2] = 30;
It's fundamentally the same as writing
int local[3];
local[0] = 10;
local[1] = 20;
local[2] = 30;
But with alloca 3 does not need to be a constant. Again you can only use it inside the enclosing scope, so do not return it from the function.
if what you want from a va_list is the multiple types in one list consider writing a union like this:
union variant
{
int i;
unsigned int u;
float f;
double d;
const char * s;
void * v;
};

Answer 6

It depends on compiler what is the va_list type, what are the va_start and va_end macros. You cannot do this in a standard way. You would have to use compiler-specific construction.
Maybe you can alter the 'foo' function? If so, then make it inversely - convert va_list to QList and make 'foo' accept QList.
// EDIT
Then see what the va_list type is, what the va_start and va_end macros are in your specific compiler. Then build your va_list in such a way that these macros will work on it.

Answer 7

<just for fun>
allowing arbitrary argument count
luckily sizeof(std::wstring) is a multiple of sizeof(int)
tested on w2k3 sp2 32bit + visual c++ 2010
#include <stdarg.h>
#include <string>
#include <vector>
#include <iostream>
#define N 6 // test argument count
void foo(int n, ...);
int main(int, char*[])
{
std::vector strings;
std::wstring s(L"a");
int i(0);
// create unique strings...
for (; i != N; ++i)
{
strings.push_back(s);
++s.front();
}
int n_stack_strings(N*sizeof(std::wstring)), // space needed for strings
n_stack(sizeof(int)+n_stack_strings); // overall stack space...needed for cleanup
__asm sub esp, n_stack_strings ; reserve stack space
std::wstring* p_stack(0);
__asm mov p_stack, esp ; get stack pointer
std::wstring* p(p_stack);
std::vector<std::wstring>::iterator string(strings.begin());
// copy to stack
for (; string != strings.end(); ++string, ++p)
new (p) std::wstring(*string);
__asm push N ; argument count...arguments right to left (__cdecl)
__asm call foo
// cleanup
for (p = p_stack; p != p_stack+N; ++p)
p->~basic_string();
__asm add esp, n_stack ; caller has to cleanup the stack (__cdecl)
return 0;
}
void foo(int n, ...)
{
int i(0);
va_list marker;
va_start(marker, n);
for (; i != n; ++i)
std::wcout << va_arg(marker, std::wstring) << std::endl;
va_end(marker);
}
</just for fun>