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)
 
类成员函数中const的使用  
一般放在函数体后，形如：void   fun()   const;  
任何不会修改数据成员的函数都因该声明为const类型。如果在编写const成员函数时，不慎修改了数据成员，或者调用了其他非const成员函数，编译器将报错，这大大提高了程序的健壮性。