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abstract classes not detected by compiler

P: n/a
I'm using interfaces in C++ by declaring classes with only pure virtual
methods. If then someone wants to implement the interface they
needs to inherit from the class. If the implementing class forgets to
implement some method I normally get a compile error(if the class is created
in some way of course). When using the Visual
Studio 6 compiler however, it does not generate a compile error in this
case:

- I have a implementing class A that inherits from an interface class. It
has not implemented all methods.
- The implementing class A is declared as an array in a container class. The
container class is created with new.

The compiler do not generate any errors, but I will of course get a runtime
error when a method with no implementation is called("pure function call" or
something). If the container class declares A as a pointer(and performs new
on it) or as a non-array variable, then the compiler generates compile
errors, but not if it is declared as an array.

So, my question is: is it a compiler fault, if it does not find out that an
abstract class is created(which is the case here since all methods are not
implemented)? Or should it be considered as normal that the compiler can not
find out cases like this(i.e more complicated cases).

/Bjorn

Jul 22 '05 #1
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10 Replies


P: n/a
Bjorn wrote:
I'm using interfaces in C++ by declaring classes with only pure virtual
methods. If then someone wants to implement the interface they
needs to inherit from the class. If the implementing class forgets to
implement some method I normally get a compile error(if the class is created
in some way of course). When using the Visual
Studio 6 compiler however, it does not generate a compile error in this
case:

- I have a implementing class A that inherits from an interface class. It
has not implemented all methods.
- The implementing class A is declared as an array in a container class. The
container class is created with new.
Posting some code is always better than describing the same.

#include <cstdlib>

class Base {
public:
virtual int donothing1(void) = 0;
virtual int donothing2(void) = 0;
};

class A: public Base {
public:
virtual int donothing1(void) {
return 1;
}

//'donothing2' not implemented
};

class Container {
private:
A obj[100];
};

int main() {
Container * contain = new Container;
delete contain;
return EXIT_SUCCESS;
}
The compiler errors out as follows -

d:\classA.cpp(20): error C2259: 'A' : cannot instantiate abstract class


The compiler do not generate any errors, but I will of course get a runtime
error when a method with no implementation is called("pure function call" or
It is not possible to call a method with no implementation, unless
there is a problem with the build process. Try cleaning the relevant
object files and building the entire thing again.

something). If the container class declares A as a pointer(and performs new
on it)
The compiler behavious is entirely determined by the type of the
object you dynamically bind to a pointer to A.

A * ptr ; // compiler is happy. you are not
// instantiating anything here.

ptr = new A ; // Assuming not all methods of A are implemented,
// compiler would error out.
or as a non-array variable, then the compiler generates compile
errors, but not if it is declared as an array.

So, my question is: is it a compiler fault,
Post some code to understand things better.
if it does not find out that an
abstract class is created(which is the case here since all methods are not
implemented)? Or should it be considered as normal that the compiler can not
find out cases like this(i.e more complicated cases).


A compiler can definitely find a scenario where not all methods of a
class are implemented.

--
Karthik. http://akktech.blogspot.com .
' Remove _nospamplz from my email to mail me. '
Jul 22 '05 #2

P: n/a
Bjorn schrieb:
I'm using interfaces in C++ by declaring classes with only pure virtual
methods. If then someone wants to implement the interface they
needs to inherit from the class. If the implementing class forgets to
implement some method I normally get a compile error(if the class is created
in some way of course). When using the Visual
Studio 6 compiler however, it does not generate a compile error in this
case:

- I have a implementing class A that inherits from an interface class. It
has not implemented all methods.
- The implementing class A is declared as an array in a container class. The
container class is created with new.

The compiler do not generate any errors, but I will of course get a runtime
error when a method with no implementation is called("pure function call" or
something). If the container class declares A as a pointer(and performs new
on it) or as a non-array variable, then the compiler generates compile
errors, but not if it is declared as an array.

So, my question is: is it a compiler fault, if it does not find out that an
abstract class is created(which is the case here since all methods are not
implemented)? Or should it be considered as normal that the compiler can not
find out cases like this(i.e more complicated cases).

/Bjorn


#include <list>

struct ABC {
virtual void x() = 0;
};

std::list<ABC *> mylist; // this is OK
std::list<ABC> anotherlist; // this should generate a compile-time error

Tom
Jul 22 '05 #3

P: n/a
Bjorn wrote:
[...]
So, my question is: is it a compiler fault, if it does not find out that an
abstract class is created
_Created_? REALLY? Where? You claim that "the container class declares
A ... as an array". Could you show how you do that?
(which is the case here since all methods are not
implemented)? Or should it be considered as normal that the compiler can not
find out cases like this(i.e more complicated cases).


------------------------------------ example 1 : no big deal
struct ABC { // abstract
virtual void foo() = 0;
};

struct CC : ABC { // concrete
void foo() {}
};

int main() {
ABC *pa; // a pointer to ABC object, uninitialised, unused
}
------------------------------------ example 2 : still no big deal
struct ABC { // abstract
virtual void foo() = 0;
};

struct CC : ABC { // concrete
void foo() {}
};

int main() {
ABC *pa[10] = {0}; // an array of pointers to ABC, all 0, unused
}
------------------------------------ example 3 : undefined behaviour
struct ABC { // abstract
virtual void foo() = 0;
};

struct CC : ABC { // concrete
void foo() {}
};

int main() {
ABC *pa; // an uninitialised pointer to ABC
pa->foo(); // an attempt to use that uninitialised pointer - BOOM!
}
------------------------------------ example 4 : undefined behaviour
struct ABC { // abstract
virtual void foo() = 0;
};

struct CC : ABC { // concrete
void foo() {}
};

int main() {
ABC *pa[10]; // an array of uninitialised pointers to ABC
pa[0]->foo(); // an attempt to use an uninitialised pointer - BOOM!
}
------------------------------------ example 5 : no big deal
struct ABC { // abstract
virtual void foo() = 0;
};

struct CC : ABC { // concrete
void foo() {}
};
#include <list>
int main() {
std::list<ABC*> lst; // a standard list of pointers to ABC - unused
}
------------------------------------
All examples should compile fine (although I didn't check, just typed
them in).

As you can see, if there is no attempt to _instantiate_ the abstract
class, it's still possible to have undefined behaviour, but not due to
the call to the pure function.

The C++ language prohibits _creation_ of _object_ of a class that is
abstract. If your program doesn't create any objects, how can it have
undefined behaviour? And if you don't create any objects in your code,
how can the compiler know what your intentions are?

Victor
Jul 22 '05 #4

P: n/a
>It is not possible to call a method with no implementation, unless
there is a problem with the build process


Not necessarily true...

struct Abstract
{
Abstract()
{
NonVirtual();
}

void NonVirtual()
{
PureVirtual();
}

virtual void PureVirtual()=0;
};

struct Concrete : public Abstract
{
void PureVirtual()
{
}
};

int main()
{
Concrete c;

return 0;
}

Using MSVC++.NET a run-time error indicates that a pure virtual function was
called.

I don't know what the language standard says regarding this. I am guessing it
is undefined behavior? Does the standard discuss attempts to invoke pure
virtual functions directly or indirectly from a base class constructor (or
destructor?)
Jul 22 '05 #5

P: n/a
da*********@aol.com (DaKoadMunky) wrote in message news:<20***************************@mb-m27.aol.com>...
It is not possible to call a method with no implementation, unless
there is a problem with the build process
Not necessarily true...

struct Abstract
{
Abstract()
{
NonVirtual();
}

void NonVirtual()
{
PureVirtual();
}

virtual void PureVirtual()=0;
};

struct Concrete : public Abstract
{
void PureVirtual()
{
}
};

int main()
{
Concrete c;

return 0;
}

Using MSVC++.NET a run-time error indicates that a pure virtual function was
called.


It's normal: "Concrete" object vtable is not yet properly constructed.
I have seen the comments on this situation, but where it was ?...

I don't know what the language standard says regarding this. I am guessing it
is undefined behavior? Does the standard discuss attempts to invoke pure
virtual functions directly or indirectly from a base class constructor (or
destructor?)

Jul 22 '05 #6

P: n/a
Oleg Polikarpotchkin wrote:
da*********@aol.com (DaKoadMunky) wrote in message news:<20***************************@mb-m27.aol.com>...
It is not possible to call a method with no implementation, unless
there is a problem with the build process


Not necessarily true...

struct Abstract
{
Abstract()
{
NonVirtual();
}

void NonVirtual()
{
PureVirtual();
}

virtual void PureVirtual()=0;
};

struct Concrete : public Abstract
{
void PureVirtual()
{
}
};

int main()
{
Concrete c;

return 0;
}

Using MSVC++.NET a run-time error indicates that a pure virtual function was
called.

It's normal: "Concrete" object vtable is not yet properly constructed.
I have seen the comments on this situation, but where it was ?...

I don't know what the language standard says regarding this. I am guessing it
is undefined behavior? Does the standard discuss attempts to invoke pure
virtual functions directly or indirectly from a base class constructor (or
destructor?)


a constructor cannot call any virtual functions. so any call to a
virtual function will be non-virtual. In consequence calling a
pure virtual function should be rejected by the compiler (IIRC)...

Jul 22 '05 #7

P: n/a
Hi again,

I provide a code example below to clarify things. The compiler version I use
is:

Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 12.00.8804 for 80x86

The following compiles without error, but of course gives a run time error
since all methods are not implemented.
#include "stdafx.h"

class Base {

public:

virtual void func1() = 0;

virtual void func2() = 0;

};

class SubClassToBase : public Base {

virtual void func1() {

int i = 7;

}

};

class Container {

public:

Base* getBase() {

return b;

}

private:

SubClassToBase b[1];

};

int main(int argc, char* argv[])

{

Container* c = new Container;

c->getBase()->func2();

// This will cause a runtime error, since

// func2() is not impemented

// The compiler does not generate error

return 0;

}

/Bjorn

"Karthik Kumar" <ka*******************@yahoo.com> wrote in message
news:417d6028$1@darkstar...
Bjorn wrote:
I'm using interfaces in C++ by declaring classes with only pure virtual
methods. If then someone wants to implement the interface they
needs to inherit from the class. If the implementing class forgets to
implement some method I normally get a compile error(if the class is created in some way of course). When using the Visual
Studio 6 compiler however, it does not generate a compile error in this
case:

- I have a implementing class A that inherits from an interface class. It has not implemented all methods.
- The implementing class A is declared as an array in a container class. The container class is created with new.
Posting some code is always better than describing the same.

#include <cstdlib>

class Base {
public:
virtual int donothing1(void) = 0;
virtual int donothing2(void) = 0;
};

class A: public Base {
public:
virtual int donothing1(void) {
return 1;
}

//'donothing2' not implemented
};

class Container {
private:
A obj[100];
};

int main() {
Container * contain = new Container;
delete contain;
return EXIT_SUCCESS;
}
The compiler errors out as follows -

d:\classA.cpp(20): error C2259: 'A' : cannot instantiate abstract class


The compiler do not generate any errors, but I will of course get a runtime error when a method with no implementation is called("pure function call" or
It is not possible to call a method with no implementation, unless
there is a problem with the build process. Try cleaning the relevant
object files and building the entire thing again.

something). If the container class declares A as a pointer(and performs

new on it)


The compiler behavious is entirely determined by the type of the
object you dynamically bind to a pointer to A.

A * ptr ; // compiler is happy. you are not
// instantiating anything here.

ptr = new A ; // Assuming not all methods of A are implemented,
// compiler would error out.
or as a non-array variable, then the compiler generates compile
errors, but not if it is declared as an array.

So, my question is: is it a compiler fault,


Post some code to understand things better.
if it does not find out that an
abstract class is created(which is the case here since all methods are not implemented)? Or should it be considered as normal that the compiler can not find out cases like this(i.e more complicated cases).


A compiler can definitely find a scenario where not all methods of a
class are implemented.

--
Karthik. http://akktech.blogspot.com .
' Remove _nospamplz from my email to mail me. '

Jul 22 '05 #8

P: n/a
Bjorn wrote:
Hi again,

I provide a code example below to clarify things. The compiler version I use
is:

Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 12.00.8804 for 80x86

The following compiles without error, but of course gives a run time error
since all methods are not implemented.
#include "stdafx.h"

class Base {

public:

virtual void func1() = 0;

virtual void func2() = 0;

};

class SubClassToBase : public Base {

virtual void func1() {

int i = 7;

}

};

class Container {

public:

Base* getBase() {

return b;

}

private:

SubClassToBase b[1];

};

int main(int argc, char* argv[])

{

Container* c = new Container;

c->getBase()->func2();

// This will cause a runtime error, since

// func2() is not impemented

// The compiler does not generate error

return 0;

}

/Bjorn


Checked with the two compilers that I have access to.

class Base {

public:

virtual void func1() = 0;
virtual void func2() = 0;

};

class SubClassToBase : public Base {

virtual void func1() {
int i = 7;
}
};

class Container { //g++ errors out here

public:

Base* getBase() {
return b;
}

private:
SubClassToBase b[1]; // VC++ .NET 2003 errors out here.
};

int main(int argc, char* argv[]) {

Container* c = new Container;
c->getBase()->func2();

// This will cause a runtime error, since
// func2() is not impemented
// The compiler does not generate error

return 0;
}
gcc 3.4.2.

C:\>g++ -ansi -pedantic pure_virtual.cpp
pure_virtual.cpp:17: error: cannot declare field `Container::b' to be of
type `SubClassToBase'
pure_virtual.cpp:17: error: because the following virtual functions
are abstract:
pure_virtual.cpp:6: error: virtual void Base::func2()
VC++ .NET is not happy either.

C:\pure_virtual.cpp(26) : error C2259: 'SubClassToBase' : cannot
instantiate abstract class
due to following members:
'void Base::func2(void)' : pure virtual function was not defined
C:\pure_virtual.cpp(6) : see declaration of 'Base::func2'
<OT>

U:\>cl
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.10.3077 for 80x86
Copyright (C) Microsoft Corporation 1984-2002. All rights reserved.

</OT>

--
Karthik. http://akktech.blogspot.com .
' Remove _nospamplz from my email to mail me. '
Jul 22 '05 #9

P: n/a
> a constructor cannot call any virtual functions. so any call to a
virtual function will be non-virtual. In consequence calling a
pure virtual function should be rejected by the compiler (IIRC)...


Which rules from the specifications show this restriction?
Would you like to adjust your wording?

How does your statement fit to the descriptions in the chapter "Item
25: Virtualizing constructors and non-member functions" from the book
"More Effective C++"?

Regards,
Markus
Jul 22 '05 #10

P: n/a
> a constructor cannot call any virtual functions. so any call to a
virtual function will be non-virtual. In consequence calling a
pure virtual function should be rejected by the compiler (IIRC)...


How do you think about this?
- What is a "virtual constructor"?
http://www.fmi.uni-konstanz.de/~kueh....html#faq-20.6

- Discussion "virtual copy constructor"
http://groups.google.de/groups?selm=...ing.google.com
Jul 22 '05 #11

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