CODEWITHSUNDEEP
c++pointersnull
masoud khanlo
masoud khanlo

Reputation: 307

null pointer doesn't address to 0x0 location in cpp

I want to evaluate a pointer is null or not by an if condition like below :

Model * model; 
if(model == nullptr){
    //do something...
}

But this does not work, Strangly the model pointer instead of pointing to 0x0 location it points to 0xcdcdcdcdcdcdcdcd memory location then model==nullptr doesn't work for that , i also did if(model) , if(model== NULL) these are also not working!

any idea about this problem ?

Upvotes: 1

Views: 748

Answers (4)

nav
nav

Reputation: 1665

The pointer Model * model; will take any random garbage value depending on the memory location at which the pointer gets allocated. C/C++ standards do not mandate that declared but uninitialized pointers be automatically initialized to NULL (or any default value).

Now, I'm not really clear why the pointer is compared to NULL immediately after declaration, but that comparison makes sense after the pointer is assigned a value returned by a function or some expression:

Model * model = some_func(/* .. */);  // This would be 
if (model == NULL) {                  // a valid usage
}

Upvotes: 0

haccks
haccks

Reputation: 106012

An uninitialized pointer need not point to NULL. It can point anywhere.

Upvotes: 7

gsamaras
gsamaras

Reputation: 73366

This code invokes Undefined Behavior, since your pointer is uninitialized!

You need to initialize your pointer to nullptr, and then check for it, otherwise, its value is garbage (it can be anything).

PS: Null pointers must compare equal to the expression 0x0.

Upvotes: 1

Adnan
Adnan

Reputation: 195

which IDE you are using if you are using Visual Studio it use NULL like this

but here is the solution 

Model * model=NULL;
if(model == NULL){
    //do something...
}

Upvotes: -3

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