Programming Forums > Application Development > C++

Laterix · May 25th, 2006, 2:59 PM

Hi all, I'm new here.

My question is about dynamic binding and polymorphism of the C++. I'm working on my first C++ project. Before I used Java.

So, I have abstract base class that implements some of the functions, but not all. So there are also pure virtual functions in my abstract base class. Now I have function foo() which is implemented in abstract class and function goo() which is pure virtual function.

I get linker error with code like this:

(Toggle Plain Text)

Base::foo()
{
    this->goo();
}

So I'm trying to call undefined function goo, which is defnied in subclasses. Is this just not possible in C++ or am I doing something wrong here? I believe that this is possible in Java.

aznluvsmc · May 25th, 2006, 3:17 PM

It would be easier if you posted your entire code for use to see.

Laterix · May 26th, 2006, 5:16 AM

Quote:

Originally Posted by aznluvsmc

It would be easier if you posted your entire code for use to see.

Thanks for you reply. It's a lot of code and hard to cut just needed parts so that it would still make sense.

But I draw an example UML of the situation, which should be enough to illustrate my problem.

So, I have this kind of inheritance relationship. MyAdapter implements Operation1() as shown in UML. Operation2() is pure virtual function in MyAdapter and it is implmented in subclass SpesificAdapter. Now the problem is that I get error with the line this->operation2(); in MyAdaptor.

grumpy · May 26th, 2006, 6:07 AM

Yes, it is possible to implement your design in C++.

It is not necessary to post your complete program. It would help if you took your class and cut it down to a small but complete example that (most importantly) exhibits your problem. The process of creating that example may make it clear to you what the problem is (i.e. you might succeed in solving it without help). If not, by providing complete information, we have more chance of helping. It also helps if you tell us what error message you're getting from your compiler (or linker).

From the information you've given, you shouldn't need to use the "this->" prefix when calling goo() within foo(). The most likely causes of your error that I can think of (noting that you haven't provided enough information) are;

1) foo() is static (in which case there is no "this" pointer when called)

2) you have mixed up return types or argument types of the functions in some way that introduces an incompatibility.

Tegelane · May 26th, 2006, 6:49 AM

(Toggle Plain Text)

#include <iostream>

using namespace std;

class Base
{
public:
  Base() {};
  ~Base() {};
  virtual void stuff1();
  virtual void stuff2();
};

void Base::stuff1()
{
 stuff2();
}

class Derived : public Base
{
public:
  Derived() {};
  ~Derived() {};
  void stuff2();
};

void Derived::stuff2()
{
  cout<<"asdasd"<<endl;
  return;
}

int main()
{
Derived d;
d.stuff1();
return 0;
}

the code should be something like this. i think..
and compiling this gives me a linker error about undefined Base::stuff2()

/tmp/ccBAAYa4.o

.rodata._ZTV4Base[vtable for Base]+0x18): undefined reference to `Base::stuff2()'

if i implement Base::stuff2() witch does nothing, it works as needed

DaWei · May 26th, 2006, 7:04 AM

virtual void stuff2() = 0;

grumpy · May 26th, 2006, 7:33 AM

Dawei's nailed it. The reason is that you declared the function as virtual but you did not (despite what you said in the original post) declare it as pure virtual (the "=0" on the end of a declaration of a virtual function achieves that).

For reference, the error message you received is from the linker. The function as your declared it is not pure virtual, so an implementation of it is needed.

Laterix · May 26th, 2006, 2:25 PM

Thank you for your effeort everyone! I'm ashamed to say that my example was little missleading.

I actually call pure virtual function from abstract class constructor. And I think that is really a problem, because when I create a new object of SpesificAdapter it automatically calls parent class constructor before it own and there it trys to call operation2() which doesn't exists yet. Am I right?

Anyway, I found a workaround to this problem. You have been very nice. Hopefully I can help someone with Java or PHP in the future.

DaWei · May 26th, 2006, 2:33 PM

You might want to think about a couple of mundane things. What does the compiler require? Once that's satisfied, what does the linker require? Once that's satisfied, what does execution require? You'll have a hard time burping the baby if 'is mama hasn't been born yet. Which came first? The chicken or the egg or the road or the joke?

grumpy · May 26th, 2006, 4:59 PM

Quote:

Originally Posted by Laterix

I actually call pure virtual function from abstract class constructor. And I think that is really a problem, because when I create a new object of SpesificAdapter it automatically calls parent class constructor before it own and there it trys to call operation2() which doesn't exists yet. Am I rigt?

Yes, you're right. It is generally bad practice to call any virtual function from a constructor (whether pure or not). The reason is that the constructor can only call a virtual function statically (i.e. the constructor for Base can only call Base::virtual_function(), which is not normally the intent of calling a virtual function). The reason for this is construction order: a Base class constructor is invoked before the derived class constructor and various properties of the class do not come together until the derived class is constructed. This means that, when the Base constructor is called, there is no information available to say the object is really a Derived, which mean that Derived::virtual_function() cannot be invoked.

Pure virtual functions are worse: being pure virtual, they might not be defined by the base class, which means there is no Base::virtual_function() to call. That normally shows up as a linker error.

Quote:

Originally Posted by Laterix

Anyway, I found a workaround to this problem. You have been very nice. Hopefully I can help someone with Java or PHP in the future.

The workaround is to create the object, before calling the virtual function. For example (using your case);

(Toggle Plain Text)

Base *object = new Derived;
object->virtual_function();
object->nonvirtual_which_calls_virtual_function();

May 25th, 2006, 2:59 PM	#1
Laterix Newbie Join Date: May 2006 Posts: 3 Rep Power: 0	About pure virtual function calls Hi all, I'm new here. My question is about dynamic binding and polymorphism of the C++. I'm working on my first C++ project. Before I used Java. So, I have abstract base class that implements some of the functions, but not all. So there are also pure virtual functions in my abstract base class. Now I have function foo() which is implemented in abstract class and function goo() which is pure virtual function. I get linker error with code like this: (Toggle Plain Text) Base::foo() { this->goo(); } Base::foo() { this->goo(); } So I'm trying to call undefined function goo, which is defnied in subclasses. Is this just not possible in C++ or am I doing something wrong here? I believe that this is possible in Java.

May 26th, 2006, 6:49 AM	#5
Tegelane Newbie Join Date: May 2006 Location: Estonia Posts: 8 Rep Power: 0	(Toggle Plain Text) #include <iostream> using namespace std; class Base { public: Base() {}; ~Base() {}; virtual void stuff1(); virtual void stuff2(); }; void Base::stuff1() { stuff2(); } class Derived : public Base { public: Derived() {}; ~Derived() {}; void stuff2(); }; void Derived::stuff2() { cout<<"asdasd"<<endl; return; } int main() { Derived d; d.stuff1(); return 0; } #include <iostream> using namespace std; class Base { public: Base() {}; ~Base() {}; virtual void stuff1(); virtual void stuff2(); }; void Base::stuff1() { stuff2(); } class Derived : public Base { public: Derived() {}; ~Derived() {}; void stuff2(); }; void Derived::stuff2() { cout<<"asdasd"<<endl; return; } int main() { Derived d; d.stuff1(); return 0; } the code should be something like this. i think.. and compiling this gives me a linker error about undefined Base::stuff2() /tmp/ccBAAYa4.o.rodata._ZTV4Base[vtable for Base]+0x18): undefined reference to `Base::stuff2()' if i implement Base::stuff2() witch does nothing, it works as needed

May 26th, 2006, 7:04 AM	#6
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	virtual void stuff2() = 0; __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

May 26th, 2006, 2:33 PM	#9
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	You might want to think about a couple of mundane things. What does the compiler require? Once that's satisfied, what does the linker require? Once that's satisfied, what does execution require? You'll have a hard time burping the baby if 'is mama hasn't been born yet. Which came first? The chicken or the egg or the road or the joke? __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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May 25th, 2006, 3:17 PM	#2
aznluvsmc Hobbyist Programmer Join Date: Aug 2005 Posts: 137 Rep Power: 4	It would be easier if you posted your entire code for use to see.

May 26th, 2006, 6:07 AM	#4
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,221 Rep Power: 5	Yes, it is possible to implement your design in C++. It is not necessary to post your complete program. It would help if you took your class and cut it down to a small but complete example that (most importantly) exhibits your problem. The process of creating that example may make it clear to you what the problem is (i.e. you might succeed in solving it without help). If not, by providing complete information, we have more chance of helping. It also helps if you tell us what error message you're getting from your compiler (or linker). From the information you've given, you shouldn't need to use the "this->" prefix when calling goo() within foo(). The most likely causes of your error that I can think of (noting that you haven't provided enough information) are; 1) foo() is static (in which case there is no "this" pointer when called) 2) you have mixed up return types or argument types of the functions in some way that introduces an incompatibility.

May 26th, 2006, 7:33 AM	#7
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,221 Rep Power: 5	Dawei's nailed it. The reason is that you declared the function as virtual but you did not (despite what you said in the original post) declare it as pure virtual (the "=0" on the end of a declaration of a virtual function achieves that). For reference, the error message you received is from the linker. The function as your declared it is not pure virtual, so an implementation of it is needed.

May 26th, 2006, 2:25 PM	#8
Laterix Newbie Join Date: May 2006 Posts: 3 Rep Power: 0	Thank you for your effeort everyone! I'm ashamed to say that my example was little missleading. I actually call pure virtual function from abstract class constructor. And I think that is really a problem, because when I create a new object of SpesificAdapter it automatically calls parent class constructor before it own and there it trys to call operation2() which doesn't exists yet. Am I right? Anyway, I found a workaround to this problem. You have been very nice. Hopefully I can help someone with Java or PHP in the future.

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Programming Forums > Application Development > C++

About pure virtual function calls