Passing function pointer with string parameter from Swift to Obj C++

I'm currently at an impasse in calling a function in my C++ dynamic library that requires a function pointer to be passed, where that function pointer has a parameter of a pointer to a string, from a Swift test harness I've built. I have a feeling that either my function declaration in my Obj C++ header in my test harness is incorrect, or I'm improperly formatting the call in Swift.

The header in my Obj C++ library (implementation is in my .mm file):

@interface Objective_CPP : NSObject
- (void)InitCppDriver:(customCallbackType)readCompleteCallback;
@end

The corresponding file my bridging header links to in my test harness (unsure if this is equivalent to the declaration in my library project) :

@interface Objective_CPP : NSObject
- (void) InitCppDriver:(void (*)(const wchar_t* data))readCompleteCallback;
@end

The declaration of my customCallbackType in my library project (C++):

typedef void(*customCallbackType)(const wchar_t* json);

extern customCallbackType frReadCompleteCallback;

My implementation of my InitCppDriver in my library:

@implementation Objective_CPP
- (void)InitCppDriver:(customCallbackType)readMeterComplCallback
   {
   ...
   }
@end

When I go to call my InitCppDriver function in my test harness (Swift):

var ptrStr: UnsafePointer<wchar_t>?;
Objective_CPP().InitCppDriver(callBack(outString: ptrStr));
...
func callBack (outString:UnsafePointer<wchar_t>?){
   //code manipulating string here
}

I've tried passing a closure and a number of other function definitions, but the error I end up getting is a variant of this for everything I have tried:

Cannot convert value of type '()' to expected argument type '(@convention(c) (UnsafePointer?) -> Void)!'

I've been able to call my InitCppDriver function successfully without the parameter of having a function pointer passed into it, but I cannot seem to figure out the syntax/setup of how to call that same function with a function pointer parameter. Any insight on how to do this would be great - I don't work in Obj-C or Swift often and have been stuck on this for a little too long now.

If there is any other context I can give to the issue I'll be happy to post up more code.

Thanks!!