Question

Is there any easy way to convert an Objective-C holding class of NSStrings into parameters for a function accepting a variable list of char *? Specifically I have a function like:

-(void)someFunction:(NSSomething *) var

that I want to forward to a C function like

void someCFunction(char * var, …)

Is there an easy way to go about this?

Answer 1

No, you can only do what you want if the number of arguments you’re passing is known at compile time. If you just want to convert a single string, use the -UTF8String message:

// Example with two strings
NSString *str1 = ...;
NSString *str2 = ...;
someCFunction([str1 UTF8String], [str2 UTF8String]);  // etc.

But if the number of strings will vary at runtime, you’ll need to use a different API, if one is available. For example, if there’s an API that took an array of strings, you could convert the Objective-C array into a C array:

// This function takes a variable number of strings.  Note: in C/Objective-C
// (but not in C++/Objective-C++), it's not legal to convert 'char **' to
// 'char *const *', so you may sometimes need a cast to call this function
void someCFunction(const char *const *stringArray, int numStrings)
{
    ...
}

...

// Convert Objective-C array to C array
NSArray *objCArray = ...;

int numStrings = [objCArray count];
char **cStrArray = malloc(numStrings * sizeof(char*));
for (int i = 0; i < count; i++)
    cStrArray[i] = [[objCArray objectAtIndex:i] UTF8String];

// Call the function; see comment above for note on cast
someCFunction((const char *const *)cStrArray, numStrings);

// Don't leak memory
free(cStrArray);