void const f() vs void f() const

void const f() is equivilent to const void f(), which means the return type (in this case a void) is const. This is totally meaningless not only because it's a void (there is nothing there that needs a const qualifier), but also because it's a return type (returning something as const doesn't make a whole lot of sense).

void f() const makes the function itself const. This only really has meaning for member functions. Making a member function const means that it cannot call any non-const member functions, nor can it change any member variables. It also means that the function can be called via a const object of the class:

class A
{
public:
  void Const_No();   // nonconst member function
  void Const_Yes() const; // const member function
};


//-----------

A  obj_nonconst;  // nonconst object
obj_nonconst.Const_No();  // works fine
obj_nonconst.Const_Yes(); // works fine

const A obj_const = A(); // const object
obj_const.Const_Yes(); // works fine (const object can call const function)
obj_const.Const_No();  // ERROR (const object cannot call nonconst function)