C++ :: Void Pointer As A Parameter

I want to have a function that has a pointer to an int/double/string so I thought I'd use a void pointer like so:

int someFnc(int a, int b, const void *key){
// take care of converting key into appropriate type in here
}

And when I want to use this function I'd like to be able to do something like this:

main{
...
int myKey;
someFnc(1,2,myKey);
]

But I get a compiler error telling me:

invalid conversion from 'int' to 'const void' -[fpermissive]

Do I need to convert myKey into a void pointer before passing it as an argument?

Why does passing myKey like this work?

someFnc(1,2,&myKey);