Question

According to docs for vswprintf: This function Write formatted output using a pointer to a list of arguments.

Does anyone know of functions for native iOS API that can achieve similar effect?

Or, how would you do this?

I know NSString Initwithformat can emulate sprint, but what is the function to emulate vswprintf or even vsprintf?

Thanks

Answer 1

The NSString equivalent of vsprintf is initWithString:arguments:, like this:

// format has type NSString *
// ap has type va_list
NSString *formattedString = [[NSString alloc] initWithFormat:format arguments:ap];

There is no separate wide-character version of NSString. Every NSString stores Unicode characters, not bytes. If you have an array of UTF-16 characters, you can use +[NSString stringWithCharacters:length:] to convert it to an NSString.

NSString thinks the %s format string means an array of one-byte character codes, terminated by a zero byte.

NSString thinks the %S format string means an array of two-byte character codes, terminated by two zero bytes.

On iOS, a wchar_t is 4 bytes. I don’t think NSString supports the %ls format string for an array of wchar_t. This documentation doesn’t mention using the l modifier with the s format. I looked at the CFFormatCharsType case in __CFStringAppendFormatCore and I don’t think it supports %ls.

If you have an array of wchar_t, and it contains UTF-32 character codes, you can create an NSString containing the same characters like this:

wchar_t *wchars = ...;
unsigned int wcharCount = ...; // number of characters pointed to by wchars

NSString *string = [[NSString alloc] initWithBytes:(const void *)wchars
    length:wcharCount * sizeof *wchars
    encoding:NSUTF32StringEncoding];

If you want to just use vswprintf, you can; iOS supports it. You need to #include <wchar.h> to get its declaration.