What are protect dll functions with key solutions?

Question

I searched a bit through stackoverflow and can't find the exact examples of approaches of how to prevent someone to use dll.

Consider a dll with some exported function, that creates com-like object:

extern "C" _declspec(dllexport)void factory(void **obj)
{
    Object *object = new Object();
    *obj = object;
}

Obvious idea that comes to mind is to add second argument as a key:

extern "C" _declspec(dllexport)void factory(void **obj, const char *key)
{
    if(strcmp(key, "just a secret here as a const string"))
         return;
    Object *object = new Object();
    *obj = object;
}

But I think it would be extremely easy to decompile it and get the secret. Well, I can manually obfuscate it a bit:

extern "C" _declspec(dllexport)void factory(void **obj, const char *key)
{
    if(key[0] != 256 - 59 || key[1] != sqrt(16) || key[2] != 'g')
         return;
    Object *object = new Object();
    *obj = object;
}

Yet, I think this is also easy to disassembler.

Does anyone one more secure way to license the dll?