c++ - Polymorphism: Instance class? -

is necessary instance class class has been derived from?
if dont, can access methods anyway, in example:

#include <iostream> struct class1 { public:     void func(); };  struct class2 : public class1 {     // class1 c1; <-- necessary? };  void class1::func(){     std::cout << "function called" << std::endl; } int main() {     class2 c2;     c2.func(); } 

i expected function called.
in examples see base class being instanced in derived class: class1 c1;. for?

is necessary instance class class has been derived from?

no, absolutely not necessary.

but in examples see base class being instanced in derived class: class1 c1;. for?

without knowing examples concretely deal - i.e. semantics of classes - it's not easy explain them in meaningful way. however, when create data member of type, object has sub-object of type. when do:

struct y { };  struct x {     int a;     std::string b;     y c; }; 

every object of type x have subobject a of type int, subobject b of type b, , subobject c of type y. accessing dot notation, done below:

y foo();  x x; std::cout << x.a; x.b = "hello, world!"; x.c = foo(); 

similarly, when class d derives base class b, contains sub-object of type b, , implicitly inherits data members of b:

struct b {     int a;     std::string b; };  struct d : b {     bool c; };  // ...  d d; d.c = true; d.b = "hello, base class!"; d.a = 42; 

now in case, class class2 apart deriving class1 has data member of type class1. notice, data member not base sub-object of class2, additional member sub-object. mean?

take grain of salt, in order develop intuition, can think inheritance models "is-a" relationship, while data membership models "has-a" relationship.

so fact class2 derives publicly class1 models fact "all class2 objects class1 objects", , therefore inherit of class1's data members. explains why objects of type class2 have base sub-object of type class1.

on other hand, fact class2 has data member of type class1 models fact "all class2 objects contain/embed/own object of type class1". explains why objects of type class2 have member sub-object of type class1.

now in particular case of examples mention both of above semantics modeled @ same time - i.e. class2 both is-a , has-a class1 - no means necessary. not every thingy is-a widget has-a widget.

i hope makes sense in abstract way. whether design correct or not depends on semantics of class1 , class2 objects, , on meant model, message is:

if class2 derives class1, there nothing forces add class1 data member class2.