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Null Pointer? will this code work
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Join Date: Aug 2005
Posts: 11
Reputation: 
Solved Threads: 0
Newbie Poster
As null pointer points to nothing. So, How does the following code works
#include <stdio.h>
class cb
{
public:
void HelloB()
{
printf("Calling cb\n");
};
};
int main(void)
{
cb* pb = 0;
pb->HelloB(); // should it crash here ?
return 0;
}
#include <stdio.h>
class cb
{
public:
void HelloB()
{
printf("Calling cb\n");
};
};
int main(void)
{
cb* pb = 0;
pb->HelloB(); // should it crash here ?
return 0;
}
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Originally Posted by ankitrastogi82
As null pointer points to nothing. So, How does the following code works
#include <stdio.h>
class cb
{
public:
void HelloB()
{
printf("Calling cb\n");
};//This semicolon should not be here...
};
int main(void)
{
cb* pb = 0;
pb->HelloB(); // should it crash here ?
return 0;
}
Whereas crashing of program is concern(it won't..no reason)...it doesn't matter if u initailise it will 0.....member function will be called depending on the type of the pointer
You can even write
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Join Date: Aug 2005
Posts: 11
Reputation:
Solved Threads: 0
Newbie Poster
Sunny,
but it is null ptr and null ptr cant be dereferenced . Also, they cant be used for calling functions. A null pointer is known not to point to any object or function.
It is not the initialization, it is saying that ptr is null.
Now look into the problem again.
but it is null ptr and null ptr cant be dereferenced . Also, they cant be used for calling functions. A null pointer is known not to point to any object or function.
It is not the initialization, it is saying that ptr is null.
Now look into the problem again.
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Originally Posted by ankitrastogi82
but it is null ptr and null ptr cant be dereferenced .
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Originally Posted by ankitrastogi82
Also, they cant be used for calling functions.
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Originally Posted by ankitrastogi82
A null pointer is known not to point to any object or function.It is not the initialization, it is saying that ptr is null.
Example
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Join Date: Aug 2005
Posts: 15,407
Reputation: 
Solved Threads: 1468
it will compile, but dereferencing a null pointer like that will often (but maybe not always) crash the program -- it's undefined behavior. In the case of c++ classes, the function will be called with an invalid "this" pointer and any attempt to reference or use class objects will also result in undefined behavior. So "it works" is all relative to this undefined behavior, and won't "work" very long!
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Originally Posted by sunnypalsingh
Neither in your program nor the example which i gave you derefrenced null pointer.
PT* ptr = 0;
ptr->whatever();
is the same as
PT* ptr = 0;
(*ptr).whatever();
Sorry about that...yes it does derefrence the null pointer.....as far as working of code is concerned...what you are doing is wrong
Almost always when a program does something wrong like this, the C++ standard does NOT says that the program must crash, its does NOT say that the program must produce an error message, what it says is that the program has UNDEFINED BEHAVIOUR.
UNDEFINED BEHAIOUR means exactly what it says, anything could happen,
including the program working. If you ran this program on a different
computer, or with a different compiler or even on a different day of the
week you might get different behaviour.
Most compilers will not crash untill you access a member variable
within the function.
In other Words,
Since the member function in this case does not use its "this"
pointer for anything.....therefore it will work most of the time
Almost always when a program does something wrong like this, the C++ standard does NOT says that the program must crash, its does NOT say that the program must produce an error message, what it says is that the program has UNDEFINED BEHAVIOUR.
UNDEFINED BEHAIOUR means exactly what it says, anything could happen,
including the program working. If you ran this program on a different
computer, or with a different compiler or even on a different day of the
week you might get different behaviour.
Most compilers will not crash untill you access a member variable
within the function.
In other Words,
Since the member function in this case does not use its "this"
pointer for anything.....therefore it will work most of the time
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