Default callback function in a library

Projects Programming
2 
typedef void (*callbackFunc) (int arg1);  //is int right ?

for typing a function signature you can just use:

typedef void (*callbackFunc) (int);

but prefer to use your C++, because you do not need to create a new type, it already exists:

using callbackFunc = void (*)(int);

uninitialized class member pointers will be nullptr by default, but I like that you explicitly initialize it.

you don't need all this:

void TestCallback::update(callbackFunc func = nullptr)
{
  if (func == nullptr)
  {
    _callbackRequired = false;
  }
  else
  {
    cb1 = func;
    _callbackRequired = true;
  }

pointers can be truth tested:

if (func) {
  cb1 = func;
}

likewise when you call it... not like this:

if (_callbackRequired)
    {
      cb1(_currentCount);  //execute the callback function
    }

but this:

if (cb1)
  {
    cb1(_currentCount);  //execute the callback function
  }

so you save a variable, _callbackRequired isn't necessary.