C++ – Why Define a Pure Virtual Function?

c++classpolymorphismvirtual-functions

Scott said on Effective C++, 3rd Edition, pg. 43 that to make an abstract class, we just need to give it a pure virtual destructor:

class AWOV {                  // AWOV = "Abstract w/o Virtuals"
public:
  virtual ~AWOV() = 0;        // declare pure virtual destructor
};

Then, he went on said that there is one twist: we must provide a definition for the pure virtual destructor:

AWOV::~AWOW() {}              // definition of pure virtual dtor

My question is, by specifiying = 0, for pure virtual functions, we are saying that the function cannot have any definition for the class where this pure virtual function is declared.

Why is it OK to provide a definition (even it is empty) for the pure virtual destructor here?

Best Answer

"we are saying that the function cannot have any definition for the class where this pure virtual function is declared."

That's not what pure virtual means. Pure virtual only means that the containing class cannot be instantiated (is abstract), so it has to be subclassed, and subclasses must override the method. E.g.,

struct A {
    virtual ~A() = 0;
};

A::~A() {}

struct B : A {};

int main()
{
    A a;  // error
    B b;  // ok
}

Here, the B destructor is implicitly defined. If it was another method that is pure virtual, you'd have to explicitly override it:

struct A {
    virtual void foo() = 0;
};

void A::foo() {}

struct B : A {};

int main()
{
    B b;  // error
}

Providing a definition for a pure virtual method is desirable when the base class must be abstract but still provide some default behavior.

In the specific case of a destructor, it has to be provided because it will be called automatically when subclass instances are destroyed. A program that tries to instantiate a subclass of a class with a pure virtual destructor without a definition will not pass the linker.

Related Solutions

C++ – Why Virtual Functions Are Needed

Here is how I understood not just what virtual functions are, but why they're required:

Let's say you have these two classes:

class Animal
{
    public:
        void eat() { std::cout << "I'm eating generic food."; }
};

class Cat : public Animal
{
    public:
        void eat() { std::cout << "I'm eating a rat."; }
};

In your main function:

Animal *animal = new Animal;
Cat *cat = new Cat;

animal->eat(); // Outputs: "I'm eating generic food."
cat->eat();    // Outputs: "I'm eating a rat."

So far so good, right? Animals eat generic food, cats eat rats, all without virtual.

Let's change it a little now so that eat() is called via an intermediate function (a trivial function just for this example):

// This can go at the top of the main.cpp file
void func(Animal *xyz) { xyz->eat(); }

Now our main function is:

Animal *animal = new Animal;
Cat *cat = new Cat;

func(animal); // Outputs: "I'm eating generic food."
func(cat);    // Outputs: "I'm eating generic food."

Uh oh... we passed a Cat into func(), but it won't eat rats. Should you overload func() so it takes a Cat*? If you have to derive more animals from Animal they would all need their own func().

The solution is to make eat() from the Animal class a virtual function:

class Animal
{
    public:
        virtual void eat() { std::cout << "I'm eating generic food."; }
};

class Cat : public Animal
{
    public:
        void eat() { std::cout << "I'm eating a rat."; }
};

Main:

func(animal); // Outputs: "I'm eating generic food."
func(cat);    // Outputs: "I'm eating a rat."

Done.

C++ – Why Support Pure Virtual Functions with Implementation

C++ Supports pure virtual functions with an implementation so class designers can force derived classes to override the function to add specific details , but still provide a useful default implementation that they can use as a common base.

Classic example:

class PersonBase
{
private:
    string name;
public:
    PersonBase(string nameIn) : name(nameIn) {} 

    virtual void printDetails() = 0
    {
        std::cout << "Person name " << name << endl;
    }
};

class Student : public PersonBase
{
private:
    int studentId;
public: 
    Student(string nameIn, int idIn) : PersonBase(nameIn), studentId(idIn) {  }
    virtual void printDetails()
    {
        PersonBase::printDetails(); // call base class function to prevent duplication
        std::cout << "StudentID " << studentId << endl;
    }
};

Technical Edit: The C++ standard requires the bodies of pure virtual functions to be defined outside of the class definition body. In other words a class member function cannot be both pure virtual and inline. The previous code example compiles on MSVC (Visual Studio), but not on GCC or CLANG.

Clause 10.4 paragraph 2 of C++03 tells us what an abstract class is and, as a side note, the following: [Note: a function declaration cannot provide both a pure-specifier and a definition —end note]

A pure virtual function with a body as per the C++ standard looks something like this:

#include <iostream>
using namespace std;

class PersonBase
{
private:
    string name;
public:
    PersonBase(string nameIn) : name(nameIn) {} 

    virtual void printDetails()=0;
};
void PersonBase::printDetails()
{
    std::cout << "Person name " << name << endl;
}

Best Answer

Related Solutions

C++ – Why Virtual Functions Are Needed

C++ – Why Support Pure Virtual Functions with Implementation

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