The error means there are some methods of the class that aren’t implemented. You cannot instantiate such a class, so there isn’t anything you can do, other than implement all of the methods of the class. On the other hand, a common pattern is to instantiate a concrete class and assign it to a pointer … Read more
Say you have three printers that you need to write drivers for, Lexmark, Canon, and HP. All three printers will have the print() and getSystemResource() methods. However, print() will be different for each printer, and getSystemResource() remains the same for all three printers. You also have another concern, you would like to apply polymorphism. Since … Read more
There are two costs to a virtual function call: The vtable lookup and the function call. The vtable lookup is already taken care of by the hardware. Modern CPUs (assuming you’re not working on a very simple embedded CPU) will predict the address of the virtual function in their branch predictor and speculatively execute it … Read more
Ok, I’ve just learned something. A pure virtual function must be declared as follows: class Abstract { public: virtual void pure_virtual() = 0; }; It may have a body, although it is illegal to include it at the point of declaration. This means that to have a body the pure virtual function must be defined … Read more
This depends on your goals. You can use version 1 if you Intend to replace brakes of a car (at runtime) Intend to pass Car around to non-template functions I would generally prefer version 1 using the runtime polymorphism, because it is still flexible and allows you to have the Car still have the same … Read more
What we want First, let’s define the final class and the base abstract class. public class MyControl : AbstractControl … public abstract class AbstractControl : UserControl // Also works for Form … Now all we need is a Description provider. public class AbstractControlDescriptionProvider<TAbstract, TBase> : TypeDescriptionProvider { public AbstractControlDescriptionProvider() : base(TypeDescriptor.GetProvider(typeof(TAbstract))) { } public override … Read more
invalid new-expression of abstract class type ‘box’ There is nothing unclear about the error message. Your class box has at least one member that is not implemented, which means it is abstract. You cannot instantiate an abstract class. If this is a bug, fix your box class by implementing the missing member(s). If it’s by … Read more
Use the abc module to create abstract classes. Use the abstractmethod decorator to declare a method abstract, and declare a class abstract using one of three ways, depending upon your Python version. In Python 3.4 and above, you can inherit from ABC. In earlier versions of Python, you need to specify your class’s metaclass as … Read more
import inspect print(inspect.isabstract(object)) # False print(inspect.isabstract(MessageDisplay)) # True print(inspect.isabstract(FriendlyMessageDisplay)) # True print(inspect.isabstract(FriendlyMessagePrinter)) # False This checks that the internal flag TPFLAGS_IS_ABSTRACT is set in the class object, so it can’t be fooled as easily as your implementation: class Fake: __abstractmethods__ = ‘bluh’ print(is_abstract(Fake), inspect.isabstract(Fake)) # True, False