how to use string in va_start?

Question

for some reason, i cant get this working:

    void examplefunctionname(string str, ...){
...
    va_start(ap, str.c_str());

nor do i get this work:

    void examplefunctionname(string str, ...){
...
    int len = str.length();
    char *strlol = new char[len+1];
    for(int i = 0; i < len; i++){
        strlol[i] = str[i];
    }
    strlol[len] = 0;
    va_start(ap, strlol);

but this does:

    void examplefunctionname(const char *str, ...){
...
    va_start(ap, str);

could someone show me how i can use string instead of const char * there?

its outputting random numbers when i call examplefunctionname("%d %d %d", 1337, 1337, 1337)

Answer 1

From the documentation:

va_start(va_list ap, last) ... The parameter last is the name of the last parameter before the variable argument list, i.e., the last parameter of which the calling function knows the type.

You've done this correctly in your working example: va_start(ap, str), and str is the last known argument. But in the other two examples, you're passing odd things to va_start.

Answer 2

va_start requires the previous parameter. That means that you have to pass in str directly, regardless of it's type. It does not take a const char* and it does not parse the string for you.