#include <iostream>
#include <type_traits>
 
 
class A
{
public:
virtual void met() = 0; 
};
class B : public A
{
public:
virtual void met() const; 
}; 
class C : public A
{
public:
virtual void met(); 
};
class D : public A
{
public:
void met(){}; 
};
 
 
int main() {
   std::cout << std::is_abstract<A>::value << std::endl;
   std::cout << std::is_abstract<B>::value << std::endl;
   std::cout << std::is_abstract<C>::value << std::endl;
   std::cout << std::is_abstract<D>::value << std::endl;
}

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