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C++ and virtual methods (for C# or Java programmers)

Last friday I stumbled over a seroius shortcomming of C++ (compared to C# or Java) I’d like to share here with you. It’s about virtual methods called from a class constructor.

The C# Example

Let me start with an example. Here’s some C# code that simply calls a virtual method (named test()) from the class’ constructor:

class TestBaseClass {
  public TestBaseClass() {
    Console.Write("From base class: ");
    test();
  }

  protected virtual void test() {
    Console.WriteLine("in base class");
  }
}

Creating an instance of this class results in nothing special:

From base class: in base class

Now lets create a sub class of TestBaseClass and override the virtual method:

class TestSubClass : TestBaseClass {
  public TestSubClass() {
    Console.Write("From sub class: ");
    test();
  }

  protected override void test() {
    Console.WriteLine("in sub class");
  }
}

Now, creating an instance of TestSubClass will print this:

From base class: in sub class
From sub class: in sub class

This means that the sub class’ implementation of test() was executed (and not TestBaseClass‘ implementation) – just as expected.

Note: In Java all methods are automatically virtual. In contrast to C# or C++ you can’t create “non-virtual” methods in Java.

The C++ Problem

And exactly here is the problem in C++. Let’s create a C++ version of the two classes above (compiled with Visual C++).

Header file (TestClass.h):

#pragma once

class TestBaseClass {
public:
  TestBaseClass();

protected:
  virtual void test();
};


class TestSubClass : public TestBaseClass {
public:
  TestSubClass();

protected:
  virtual void test();
};

Source file (TestClass.cpp):

#include "TestClass.h"
#include <stdio.h>

TestBaseClass::TestBaseClass() {
  printf("From base class: ");
  test();
}

void TestBaseClass::test() {
  printf("in base class\n");
}


TestSubClass::TestSubClass() : TestBaseClass() {
  printf("From sub class: ");
  test();
}

void TestSubClass::test() {
  printf("in sub class\n");
}

Now, creating an instance of TestSubClass results in the following output:

From base class: in base class
From sub class: in sub class

Note how the base class’ implementation of test() is used in the base class constructor while the sub class’ implementation of test() is used in the sub class constructor.

The problem here (in constrast to C# or Java) is that the sub class constructor hasn’t been executed yet and therefore the “redirection” from TestBaseClass::test() to TestSubClass::test() hasn’t been established yet.

Rule: There is no way to call a sub class’ implementation of a virtual function in the base class constructor!

The problem becomes even more severe with pure virtual (which is abstract in C# and Java) methods. These methods don’t even have an implementation in the base class and therefore can’t be executed at all.

For your interest: A C++/CLI class will behave like a C# class (and not like a C++ class).

Example Visual Studio Solution

I’ve created a solution (for Visual Studio 2010) containing the source code above. In addition to a C# and a C++ project, I’ve also added a C++/CLI project. You can download it here:

VirtualMethodTest.zip

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