Variable argument lists in C++ - c++

Question

I have this method (by using code from vsnprintf man page):
MYSQL_RES *nsDatabase::queryf(const char *fmt,...){
int n, size = 1024;
char *query=NULL,*np;
va_list ap;
if (this->dbLink == NULL){
return NULL;
}
query = (char *) malloc (size);
if (query == NULL) return NULL;//memory error
while (1) {
va_start(ap, fmt);
n=vsnprintf(query,size,fmt,ap);
va_end(ap);
if (n > -1 && n < size){// format string succeeded
break;
}
if (n > -1) /* glibc 2.1 */
size = n+1; /* precisely what is needed */
else /* glibc 2.0 */
size *= 2; /* twice the old size */
np = (char *) realloc (query, size);
if (np == NULL) {
printf("memory error\n");
FREE(query);
return NULL;//again memory error
} else {
query = np;
}
}
MYSQL_RES *r = this->query(query);
FREE(query);
if(r == NULL){
return NULL; //mysql error
}
return mysql_store_result(this->dbLink);
}
I want to make general function queryFormat which will calculate the size needed for formatted string, allocate memory, print format in it and return the string (not real code, pseudo code):
char *queryFormat(const char *fmt, va_list ap){
// allocate memory
// problem - can I use vsnprintf multiple times here (to determine the size of formatted string and allocate memory)
return <formatted string>;
}
Then I need to call it from all functions that format queries (not real code):
queryRow(const char *fmt,...){
va_start(ap, fmt);
// I need to call vsnprintf many times in queryFormat ... Do I need to call va_start() before every call to fsnprintf ?
char * formattedQuery = queryFormat(query, size, fmt, ap);
va_end(ap);
MYSQL_RES *r = this->query(formattedQuery);
free(formattedQuery);
...
}
I write for GCC under linux, but the code should be runnable under MinGW and CygWin.

Answer 1

See this - http://publications.gbdirect.co.uk/c_book/chapter9/stdarg.html.There is a good example in the book of Kernighan and Ritchie book.
EDIT
Its the same in C++ too . you can have a look at this too. - http://www.cplusplus.com/reference/clibrary/cstdarg/va_start/
EDIT 2
http://www.cplusplus.com/reference/clibrary/cstdio/vprintf/. I think this shall help you.

Answer 2

I believe this should help anyone who googled here in search of solution to the problem of how to "reuse" a va_list in a function that had it already passed as parameter:
// Incorrect:
/*void g(int foo, va_list ap) {
vprintf("%s %s %s\n", ap);
vprintf("%s %s %s\n", ap);
}*/
void g(int foo, va_list ap) {
va_list copy;
va_copy(copy, ap);
vprintf("%s %s %s\n", copy);
va_copy(copy, ap);
vprintf("%s %s %s\n", copy);
}
void f(int foo, ...) {
va_list ap;
va_start(ap, foo);
g(foo, ap);
va_end(ap);
}
int main() {
f(42, "aaa", "bbb", "ccc");
return 0;
}