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    #include <iostream>
 
    struct IAudible {
        virtual void sound() const = 0;
    };
 
    struct Explosion : public IAudible {
        virtual void sound() const override { std::cout << "Boom\n"; }
    };
 
    struct Word : public IAudible {
    };
 
    void announce(const IAudible& audible) {
        audible.sound();
    }
 
    int main() {
        Explosion e;
        announce(e);
        Word w;
        announce(w);
    }
 

prog.cpp: In function 'int main()':
prog.cpp:21:14: error: cannot declare variable 'w' to be of abstract type 'Word'
         Word w;
              ^
prog.cpp:11:12: note:   because the following virtual functions are pure within 'Word':
     struct Word : public IAudible {
            ^
prog.cpp:4:22: note: 	virtual void IAudible::sound() const
         virtual void sound() const = 0;
                      ^

Standard output is empty