When a function, like f(), is fully default-argumented, the invocations without any argument to that function work.

For example, if the function prototype is:
void f(int = 1, int = 1);

then one may call f in this form:
f();

Why is not this the case for default constructors, though they are but functions?

class cls {
public:
    int i = 10;
    cla(int i = 10): i(i){}
};

int main() {
    cls c();
    cout << c.i << endl;
}

Defining the default constructor, I expect the statement “cls c();” invokes the default constructor and the program compiles smoothly. However, the compiler returns this error message:

error: request for member ‘i’ in ‘c’, which is of non-class type ‘cls()’

Of course, the code compiles correctly in this form: cls c;

Why do not things go according to my expectations?