C++ Abstract Class: constructor yes or no

abstractcclassconstructor

A class with one (or more) virtual pure functions is abstract, and it can't be used to create a new object, so it doesn't have a constructor.

I'm reading a book that provides the following example:

class Employee {
   public:
       Employee(const char*, const char*);
       ~Employee();
       const char* getFirstName() const;
       const char* getLastName() const;


       virtual double earnings() const=0  // pure virtual => abstract class
       virtual void print() const

  private:
       char* firstName, lastName;
};

If the class is abstract why we have a constructor? It uses this class later (Boss is public derived from Employee):

void Boss::Boss (const char* first, const char* last, double s)
     : Employee (first, last)

Best Answer

You're correct when you say that a class that has a pure virtual function is abstract and can't be instantiated. But you're wrong when you say that it can't have a constructor.

Indeed, as your example show, an abstract class can have private members, that may be used by member functions of this class. And these members must be initialized. A constructor is a way to do that (e.g. with an initialization list in the derived class, as your second sample shows), better in my opinion than an init() function for example.

Editing my comment in the answer: An abstract class can have member variables and potentially non-virtual member functions, so that every derived class from the former implements specific features.

Then, the responsibility for the initialization of these members variables may belong to the abstract class (at least always for private members, because the derived class wouldn't be able to initialize them, yet could use some inherited member functions that may use/rely on these members). Thus, it makes it perfectly reasonable for abstract classes to implement constructors.

Related Solutions

C# – Calling the base constructor in C#

Modify your constructor to the following so that it calls the base class constructor properly:

public class MyExceptionClass : Exception
{
    public MyExceptionClass(string message, string extrainfo) : base(message)
    {
        //other stuff here
    }
}

Note that a constructor is not something that you can call anytime within a method. That's the reason you're getting errors in your call in the constructor body.

Python – Are static class variables possible in Python

Variables declared inside the class definition, but not inside a method are class or static variables:

>>> class MyClass:
...     i = 3
...
>>> MyClass.i
3

As @millerdev points out, this creates a class-level i variable, but this is distinct from any instance-level i variable, so you could have

>>> m = MyClass()
>>> m.i = 4
>>> MyClass.i, m.i
>>> (3, 4)

This is different from C++ and Java, but not so different from C#, where a static member can't be accessed using a reference to an instance.

See what the Python tutorial has to say on the subject of classes and class objects.

@Steve Johnson has already answered regarding static methods, also documented under "Built-in Functions" in the Python Library Reference.

class C:
    @staticmethod
    def f(arg1, arg2, ...): ...

@beidy recommends classmethods over staticmethod, as the method then receives the class type as the first argument, but I'm still a little fuzzy on the advantages of this approach over staticmethod. If you are too, then it probably doesn't matter.

Best Answer

Related Solutions

C# – Calling the base constructor in C#

Python – Are static class variables possible in Python

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