Quote from 6.7.6.3.14 of the C++11 spec (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf):

> An identifier list declares only the identifiers of the parameters of the function. An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters. The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of the parameters is supplied.

So, my interpretation is that the following two function definitions define the same function of the exact same type

  void func() {}
  void func(void) {}

although the following two function declarations declare two functions of different types

  void func();
  void func(void);