vsnprintf() on Mac gives EXC_BAD_ACCESS?

Question

I'm not sure why I'm seeing an EXC_BAD_ACCESS in the following code:

template <typename ArgType>
String Format(ArgType args, ...) const {
    va_list argList;
    va_start(argList, args);

    // determine num of chars needed, don't store anything anywhere though!
    size_t charsNeeded = vsnprintf(NULL, 0, ToStdString().c_str(), argList);

    va_end(argList);

    // print formatted string into buf
    va_start(argList, args); // EDIT: new va_list here
    char buf[charsNeeded];
    vsprintf(buf, ToStdString().c_str(), argList);

    va_end(argList);

    return String(buf);
}

EDIT: I should probably mention that the code is supposed to be used like this:

String str = String("Hello, %s!").Format("World");

I should mention String is a small wrapper class which has a ToStdString() method which returns a std::string member var.

It's segfaulting on the vsnprintf() call, but I have no idea why!

Answer 1

va_start initialises the va_list to get arguments after the one specified, since in your example you are only sending one argument to the method, the va_list is empty.

You also need to reinitialise the va_list after calling vsnprintf.

You need to rethink the signature of your method, could do a static function:

class String : public std::string {
public:
    String(std::string str) : std::string(str) {}

    String(char *str) : std::string(str) {}

    static String Format(String format, ...) {
        va_list argList;
        va_start(argList, format);

        // determine num of chars needed, don't store anything anywhere though!      
        size_t charsNeeded = vsnprintf(NULL, 0, format.c_str(), argList);

        va_end(argList);
        va_start(argList, format);

        // print formatted string into buf                                           
        char buf[charsNeeded];
        vsprintf(buf, format.c_str(), argList);

        va_end(argList);

        return String(buf);
    }
};

EDIT:

If you are looking to format strings the best solutions is probably to use a std::stringstream: http://www.cplusplus.com/reference/sstream/stringstream/stringstream/