foo()-with the parenthesis- is a function. foo-without the parenthesis-is a variable. That means that: Foo foo(); is a function pointer declaration within t

foo()-with the parenthesis- is a function. foo-without the parenthesis-is a variable. That means that: Foo foo(); is a function pointer declaration within the scope of the main function. Since it's a function, which does not have member types, a is not within the scope of Foo(). Now, if there was a 0 argument constructor for struct Foo, this would be valid: Foo foo = Foo();