Is there a constructor in an abstract class?

Yes, an abstract class can have a constructor. An abstract class is a class that is not intended to be instantiated directly, but it can contain implementations, including a constructor.

The constructor of an abstract class is usually used to initialize common fields or perform shared logic that will be inherited by subclasses.

Example in C++:

class AbstractBase {
public:
    AbstractBase(int value) : data(value) {}
    virtual void doSomething() = 0; // pure virtual function
protected:
    int data;
};

class Derived : public AbstractBase {
public:
    Derived(int value) : AbstractBase(value) {}
    void doSomething() override {
        // implementation
    }
};

Here, the constructor AbstractBase is called from the constructor of the derived class to initialize the data field. Thus, having a constructor in an abstract class is normal practice.