Ambiguous declarations

Question

What is the difference between the following two declarations:

 1. int foo(int); 
 2. int foo(int());

I am not sure if both the declarations are equivalent. What makes (2) different from (1)?

Answer 1

int foo(int); is the declaration of a function taking an integer as an argument and returning an integer as well

int foo(int()); declares a function taking as an argument "a pointer to a function returning int and taking {no arguments[in C++] and unspecified number of arguments[in C]} " and returning an integer.

(2) is equivalent to int foo(int (*pf)()) and int foo(int f())