Question

I am trying to get the current user name in c++ using the NSFullUserName function but it returns an NSString. Therefore, how can I convert an NSString to a CString in c++ without Obj-C code?

Answer 1

NSString is toll-free bridged with CFString (ie, you can just cast the pointer — the thing it points to can be interpreted as either type), and you can manipulate CFString using vanilla C calls.

So you’d typically attempt CFStringGetCStringPtr to try to get a pointer directly to the NSString contents as a C string without any copying, then fall back on allocating a suitably sized C-style array and CFStringGetCString if the direct pointer isn’t available (typically because the encoding you want doesn’t match the NSString‘s internal encoding).

CFStringGetMaximumSizeForEncoding will return the maximum size of buffer you need to allocate for a given encoding and string length, CFStringGetLength the length of a given string.

So e.g.

void Class::DoSomethingWithObjCString(NSString *objcString)
{
    CFStringRef string = (CFStringRef)objcString;

    const char *bytes = CFStringGetCStringPtr(string, kCFStringEncodingUTF8);

    if(bytes)
        DoSomethingWithUTF8String(bytes);
    else
    {
        size_t sizeToAllocate = 
                CFStringGetMaximumSizeForEncoding(
                     CFStringGetLength(string), kCFStringEncodingUTF8);

        char *allocatedBytes = new char[sizeToAllocate];

        CFStringGetCString(string, 
                           allocatedBytes, 
                           sizeToAllocate, 
                           kCFStringEncodingUTF8);
        DoSomethingWithUTF8String(allocatedBytes);

        delete[] allocatedBytes;
    }
}