CODEWITHSUNDEEP
c++
user11287425
user11287425

Reputation:

Invalid new-expression of abstract class type 'A'

Why does this small example:

class A {
public:
    virtual void run() = 0;
};

class B : public A {
public:
    void run() override {
        std::cout << "Running...\n";
    }
};

int main() {
    std::unique_ptr<A> a = std::make_unique<A>(new B());
}

Gives me this error message:

error: invalid new-expression of abstract class type 'A'
     { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
because the following virtual functions are pure within 'A':
 class A {
'virtual void A::run()'
     virtual void run() = 0;

Is this because a unique_ptr cannot take an abstract class as argument?

Upvotes: 4

Views: 10559

Answers (1)

eerorika
eerorika

Reputation: 238351

Why does this small example: ... Gives me this error message: invalid new-expression of abstract

Is this because a unique_ptr cannot take an abstract class as argument?

No, it isn't. std::unique_ptr can take an abstract class as an argument.

It is because you're attempting to create an instance of A. This is not possible because A is abstract.

You may have intended instead to create an instance of B, and have the std::unique_ptr<A> take ownership of that pointer:

std::unique_ptr<A> a = std::make_unique<B>();

Note though however, that you mustn't let this pointer go out of scope, because upon destruction of a, the object would be deleted through a pointer to A, and since the pointed object is B, and destructor of A is not virtual, the behaviour would be undefined. Solution: declare A::~A virtual.

Upvotes: 8

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