Pointers

[hassan2]

int *p;
p=NULL;
cout<<"p :"<<p<<endl;

char *q;
q=0;
cout<<"q: "<<q<<endl;

string *t;
t=NULL;
cout<<"t: "<<t<<endl;

OUTPUT
p: 0
q: Press any key to continue...



Why the output is this? Can anyone tell me?
I want to ask that why the statement cout<<"t: "<<t<<endl; is not executed?

[vlad from moscow]

Remove

char *q;
 q=0;
 cout<<"q: "<<q<<endl;

and you will get two zeroes as a result of outputing. In fact your program has undefined behavior becuase it tries to access data at zero adsress in statement

cout<<"q: "<<q<<endl;

When you use a pointer of type char * in the operator << for stream std::cout it outputs data at the address stored in the pointer. So it doe not ouutput the value of the address itself but outputs data at this address.

[zb0S216C]

q is a pointer to a char. Typically, a pointer to a char points to a string of characters. std::ostream overloads operator << to support char*, which effectively interprets the memory the pointer points to as a sequence of characters until the null-character ('\0') is reached. Since the null-character is equivalent to zero, nothing is printed.

Because of q, std::ostream fails to print the address of *t. As to why this happens, I'm not too sure.

_Wazzak

[Akshit]

q=0 means setting address of pointer to 0?

and why output of p is 0?It should be address like 0x8fb50000

[Moschops]

q=0 means setting address of pointer to 0?

No, it does not. q=0 means give q the value zero.

You need to learn what a pointer is:
http://www.cplusplus.com/articles/EN3hAqkS/

and why output of p is 0?It should be address like 0x8fb50000
No, it should not. This cout<<"p :"<<p<<endl; outputs the value of p, NOT the location in memory where p is. What is the value of p? NULL. Which is zero. You're trying somethign too advanced for your level of understanding. You need to learn what a pointer is: http://www.cplusplus.com/articles/EN3hAqkS/

[Akshit]

sorry but my compiler shows this output only and I have learnt this only in class.

we use *p or *q for values and p and q for address(in general term).

[Moschops]

sorry but my compiler shows this output only

Your compiler is doing the right thing.

[vlad from moscow]

@Akshit

q=0 means setting address of pointer to 0?

The address of the pointer itself is not changed until it is alive. You assign a value to the pointer that is an address of some other memory or variable.

[Lowest0ne]

char *q;
q=0;
cout<<"q: "<<q<<endl;

The cout crashes my program. Something to do with:
streamsize _Count = (streamsize)_Traits::length(_Val); // may overflow

[Moschops]

The cout crashes my program.

Well let's take a look at the code.

char *q;
So, q is a char pointer. What value does it have? You didn't set it yet, so it could be any value at all, thus it could be pointing anywhere in memory at all.

q=0;

Now you set the value of the pointer to zero. The pointer is pointing at memory address zero. This is commonly known as a null pointer; it is a special memory address used to indicate that the pointer is deliberately not pointing at anything. In C++11, we now have the nullptr keyword for this.

cout<<"q: "<<q<<endl;
Now, we pass the pointer to cout, using the << operator, which is cleverly overloaded to dela with char pointers. In this case, the result is: go to the address is points to, output that as a char, and output all the following addresses as a char until you get to a zero value, and then stop. You have thus passed a pointer that deliberately doesn't point at anything to cout. What happens when cout tries to read that memory address to output its contents as a char? Well, you can see for yourself in this case - a crash. I'd normally expect a segFault if you tried to dereference a null pointer yourself. Exactly what happens when you feed it to the << operator is not something I'd like to guess.

However, I would have expected it to deal with it elegantly. Using g++ 4.7.0 and the version of clang++ I have on my machine, it is dealt with elegantly without crashing - what compiler are you using?

[Lowest0ne]

what compiler are you using?

Good Question. I guess the VS 2010 default.

Maybe this is an answer:

/Zi /nologo /W3 /WX- /O2 /Oi /Oy- /GL /D "WIN32" /D "NDEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /Gm- /EHsc /GS /Gy /fp:precise /Zc:wchar_t /Zc:forScope /Fp"Release\Fun.pch" /Fa"Release\" /Fo"Release\" /Fd"Release\vc100.pdb" /Gd /analyze- /errorReport:queue

I see a few options there that are listed here:
http://msdn.microsoft.com/en-us/library/fwkeyyhe