Late Binding Concept
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It maybe but Are you sure? What compiler are you using? What code? What else? :-) Anyway
CBaseClass * pBase = new CBaseClass();
CChildClass * pChild = (CChildClass *) pBase;is a gross mistake.
Veni, vidi, vici.
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Hello. I am little confused in Late Binding Concept (although I understand it theoretically) as I could not understand it programatically. I am defining two classes here and then casting and using their pointers. CBaseClass
void CBaseClass::Function() // this is virtual function { cout<<"\nBase Class."; }CChildClassvoid CChildClass::Function() // overriden virtual function { cout<<"\nChild Class."; }MainClassvoid main() { CBaseClass* pBase = new cBaseClass(); CChildClass* pChild = (CChildClass*)pBase; pBase->Function(); pChild->Function(); }What should the two function calls produce inMainClass. I think it should produce BaseClass. ChildClass. But it actually produces BaseClass. BaseClass. What is wrong with my understanding. ThanksThis world is going to explode due to international politics, SOON.
The bug in your code is obvious. CPallini already gave you the correct code but I feel this is something that needs further clarification. The bug is the following, and its a major bug in its current context:
CChildClass* pChild = (CChildClass*)pBase;
There are 2 kinds of static casts: downcast and upcast. If you are drawing the uml diagram of your class hierarchy then it looks like a tree with its root node (base class) at the top of the diagram. Down and upcast are named based on the direction of the cast in this tree so an upcast means that you cast a derived class to one of its base classes in the hierarchy, this type of cast is safe, you don't have to mark it explicitly in your code because the compiler does it automatically for you:
Derived * d = new Derived;
Base* b = d;However downcasts are not only dangerous but they usually mean that you committed a design mistake. A base class can have a lot of derived classes and if you are casting a base class to one of its derived classes than its a bug if the actual object isn't an instance of the downcast type or its descendants. People often say that dowcasts defeat the whole purpose of polymorphism/late binding. Example:
class Base;
class A : public Base;
class B : public Base;
Base* b = new B;
A* p = (A*)b; // huge mistake because b points to a B instance so the code that uses p can crash anytime if you are not lucky!!!!So why doesn't your code crash??? Because you were lucky and currently in your simple example the binary representation of instances created by the compiler for your base and derived classes are probably identical, they just differ in their vtable pointer but in any more complex situations it could easily crash. OK, so when you have a complex hierarchy and you call a virtual function on a base class pointer then which method is executed??? It depends on which object have you actually created! (with new). Since in your example you created just a Base instance it would be magic to see executing the method of the derived class, and its just luck that using the pointer that you filled with your invalid downcast haven't crashed your program. So based on what we have learned up until now you should only use upcasts that are done automatically for you by the compiler, for example:
CBaseClass* p = new CChildClass; // the compiler automatically upcasts the CChildClass* to CBaseClass*
p->Function(); // calls the Function -
The bug in your code is obvious. CPallini already gave you the correct code but I feel this is something that needs further clarification. The bug is the following, and its a major bug in its current context:
CChildClass* pChild = (CChildClass*)pBase;
There are 2 kinds of static casts: downcast and upcast. If you are drawing the uml diagram of your class hierarchy then it looks like a tree with its root node (base class) at the top of the diagram. Down and upcast are named based on the direction of the cast in this tree so an upcast means that you cast a derived class to one of its base classes in the hierarchy, this type of cast is safe, you don't have to mark it explicitly in your code because the compiler does it automatically for you:
Derived * d = new Derived;
Base* b = d;However downcasts are not only dangerous but they usually mean that you committed a design mistake. A base class can have a lot of derived classes and if you are casting a base class to one of its derived classes than its a bug if the actual object isn't an instance of the downcast type or its descendants. People often say that dowcasts defeat the whole purpose of polymorphism/late binding. Example:
class Base;
class A : public Base;
class B : public Base;
Base* b = new B;
A* p = (A*)b; // huge mistake because b points to a B instance so the code that uses p can crash anytime if you are not lucky!!!!So why doesn't your code crash??? Because you were lucky and currently in your simple example the binary representation of instances created by the compiler for your base and derived classes are probably identical, they just differ in their vtable pointer but in any more complex situations it could easily crash. OK, so when you have a complex hierarchy and you call a virtual function on a base class pointer then which method is executed??? It depends on which object have you actually created! (with new). Since in your example you created just a Base instance it would be magic to see executing the method of the derived class, and its just luck that using the pointer that you filled with your invalid downcast haven't crashed your program. So based on what we have learned up until now you should only use upcasts that are done automatically for you by the compiler, for example:
CBaseClass* p = new CChildClass; // the compiler automatically upcasts the CChildClass* to CBaseClass*
p->Function(); // calls the Function -
Thanks!
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The bug in your code is obvious. CPallini already gave you the correct code but I feel this is something that needs further clarification. The bug is the following, and its a major bug in its current context:
CChildClass* pChild = (CChildClass*)pBase;
There are 2 kinds of static casts: downcast and upcast. If you are drawing the uml diagram of your class hierarchy then it looks like a tree with its root node (base class) at the top of the diagram. Down and upcast are named based on the direction of the cast in this tree so an upcast means that you cast a derived class to one of its base classes in the hierarchy, this type of cast is safe, you don't have to mark it explicitly in your code because the compiler does it automatically for you:
Derived * d = new Derived;
Base* b = d;However downcasts are not only dangerous but they usually mean that you committed a design mistake. A base class can have a lot of derived classes and if you are casting a base class to one of its derived classes than its a bug if the actual object isn't an instance of the downcast type or its descendants. People often say that dowcasts defeat the whole purpose of polymorphism/late binding. Example:
class Base;
class A : public Base;
class B : public Base;
Base* b = new B;
A* p = (A*)b; // huge mistake because b points to a B instance so the code that uses p can crash anytime if you are not lucky!!!!So why doesn't your code crash??? Because you were lucky and currently in your simple example the binary representation of instances created by the compiler for your base and derived classes are probably identical, they just differ in their vtable pointer but in any more complex situations it could easily crash. OK, so when you have a complex hierarchy and you call a virtual function on a base class pointer then which method is executed??? It depends on which object have you actually created! (with new). Since in your example you created just a Base instance it would be magic to see executing the method of the derived class, and its just luck that using the pointer that you filled with your invalid downcast haven't crashed your program. So based on what we have learned up until now you should only use upcasts that are done automatically for you by the compiler, for example:
CBaseClass* p = new CChildClass; // the compiler automatically upcasts the CChildClass* to CBaseClass*
p->Function(); // calls the Function:thumbsup: Good explanation about the perils of upcasting and downcasting and how in this simple example the downcasting worked, but that in a real world case, it would likely crash. :)
Chris Meech I am Canadian. [heard in a local bar] In theory there is no difference between theory and practice. In practice there is. [Yogi Berra] posting about Crystal Reports here is like discussing gay marriage on a catholic church’s website.[Nishant Sivakumar]
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:thumbsup: Good explanation about the perils of upcasting and downcasting and how in this simple example the downcasting worked, but that in a real world case, it would likely crash. :)
Chris Meech I am Canadian. [heard in a local bar] In theory there is no difference between theory and practice. In practice there is. [Yogi Berra] posting about Crystal Reports here is like discussing gay marriage on a catholic church’s website.[Nishant Sivakumar]
Thank you!
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The bug in your code is obvious. CPallini already gave you the correct code but I feel this is something that needs further clarification. The bug is the following, and its a major bug in its current context:
CChildClass* pChild = (CChildClass*)pBase;
There are 2 kinds of static casts: downcast and upcast. If you are drawing the uml diagram of your class hierarchy then it looks like a tree with its root node (base class) at the top of the diagram. Down and upcast are named based on the direction of the cast in this tree so an upcast means that you cast a derived class to one of its base classes in the hierarchy, this type of cast is safe, you don't have to mark it explicitly in your code because the compiler does it automatically for you:
Derived * d = new Derived;
Base* b = d;However downcasts are not only dangerous but they usually mean that you committed a design mistake. A base class can have a lot of derived classes and if you are casting a base class to one of its derived classes than its a bug if the actual object isn't an instance of the downcast type or its descendants. People often say that dowcasts defeat the whole purpose of polymorphism/late binding. Example:
class Base;
class A : public Base;
class B : public Base;
Base* b = new B;
A* p = (A*)b; // huge mistake because b points to a B instance so the code that uses p can crash anytime if you are not lucky!!!!So why doesn't your code crash??? Because you were lucky and currently in your simple example the binary representation of instances created by the compiler for your base and derived classes are probably identical, they just differ in their vtable pointer but in any more complex situations it could easily crash. OK, so when you have a complex hierarchy and you call a virtual function on a base class pointer then which method is executed??? It depends on which object have you actually created! (with new). Since in your example you created just a Base instance it would be magic to see executing the method of the derived class, and its just luck that using the pointer that you filled with your invalid downcast haven't crashed your program. So based on what we have learned up until now you should only use upcasts that are done automatically for you by the compiler, for example:
CBaseClass* p = new CChildClass; // the compiler automatically upcasts the CChildClass* to CBaseClass*
p->Function(); // calls the Function+5. Great explanation. For me too, I've been reversing upcast and downcast terms, didn't think of the diagram. Saw the obvious error in his example though.
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+5. Great explanation. For me too, I've been reversing upcast and downcast terms, didn't think of the diagram. Saw the obvious error in his example though.
Thank you!
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+5. Great explanation. For me too, I've been reversing upcast and downcast terms, didn't think of the diagram. Saw the obvious error in his example though.
Regarding the upcast/downcast naming: A lot of coders exchange these names so when someone starts using them I always ask him to clarify how to interpret "his upcast" right at the start of the conversation. I also used them in reverse order for a long time until reading some uml docs that clarified it for me. For some reason thinking that upcast is casting up to a higher level smarter derived class seemed to be more logical for me. :-)
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Regarding the upcast/downcast naming: A lot of coders exchange these names so when someone starts using them I always ask him to clarify how to interpret "his upcast" right at the start of the conversation. I also used them in reverse order for a long time until reading some uml docs that clarified it for me. For some reason thinking that upcast is casting up to a higher level smarter derived class seemed to be more logical for me. :-)
Which is how I use "upcast", to a "bigger" container, the derived class. Of course, that's just a convenience notation. The real idea is
Cast Derived to Base - OK
Cast Base to Derived - Bad
