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#include <iostream>
using namespace std;
 
struct Interf {
    virtual void foo(const int& a) = 0;
};
struct Impl : Interf {
	virtual void foo(int& a) override {}
};
 
 
int main() {
	Impl i;
	int b;
	i.foo(b);
	// your code goes here
	return 0;
}

prog.cpp:8:15: error: 'virtual void Impl::foo(int&)' marked 'override', but does not override
  virtual void foo(int& a) override {}
               ^
prog.cpp: In function 'int main()':
prog.cpp:13:7: error: cannot declare variable 'i' to be of abstract type 'Impl'
  Impl i;
       ^
prog.cpp:7:8: note:   because the following virtual functions are pure within 'Impl':
 struct Impl : Interf {
        ^
prog.cpp:5:18: note: 	virtual void Interf::foo(const int&)
     virtual void foo(const int& a) = 0;
                  ^

Standard output is empty