C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 33:
[33.9] What is the difference between the .* and ->* operators?

You won't need to understand this if you use a macro for member-function-pointer calls. Oh yea, please use a macro in this case. And did I mention that you should use a macro in this case??!?

But if you really want to avoid the macro, sigh, groan, okay, here it is: use .* when the left-hand argument is a reference to an object, and ->* when it is a pointer to an object.

For example:

class Fred { ... };

typedef  int (Fred::*FredMemFn)(int i, double d);  // use a typedef!!! please!!!

void sample(Fred x, Fred& y, Fred* z, FredMemFn func)
{
  x.*func(42, 3.14);
  y.*func(42, 3.14);
  z->*func(42, 3.14);
}
BUT please consider using a macro instead:
void sample(Fred x, Fred& y, Fred* z, FredMemFn func)
{
  CALL_MEMBER_FN(x,func)(42, 3.14);
  CALL_MEMBER_FN(y,func)(42, 3.14);
  CALL_MEMBER_FN(*z,func)(42, 3.14);
}
As discussed earlier, real-world invocations are often much more complicated than the simple ones here, so using a macro will typically improve your code's writability and readability.