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#include <random>
#include <iostream>
 
struct Base
{
    virtual void f()=0;
};
 
struct Derived: Base
{
    // void f() override; // NEED DECLARATION
};
 
void Derived::f(){}
 
int main()
{
    Derived d;
}

prog.cpp:14:17: error: no 'void Derived::f()' member function declared in class 'Derived'
 void Derived::f(){}
                 ^
prog.cpp: In function 'int main()':
prog.cpp:18:13: error: cannot declare variable 'd' to be of abstract type 'Derived'
     Derived d;
             ^
prog.cpp:9:8: note:   because the following virtual functions are pure within 'Derived':
 struct Derived: Base
        ^
prog.cpp:6:18: note: 	virtual void Base::f()
     virtual void f()=0;
                  ^

Standard output is empty