Access violation error when creating pointer to pointer List

Question

i,m trying to create pointer to pointer List

list<ObjectClass*> *lst_testList;

and trying to use it this way

void FunctioningClass::functioningMethod()
{
    ObjectClass *object = new ObjectClass();
    object->i_testing = 234;
    lst_testList->push_back(object);
    object = lst_testList->front();
    cout<<object->i_testing;
    std::getchar();
}

I can build the program. But when i run it,it gives me this error.

Unhandled exception at 0x012885DA in ConsoleApplication7.exe: 0xC0000005: Access violation reading location 0x00000004.

notice that when i create list like this

list<ObjectClass*> lst_testList;

and use it like this,

lst_testList.push_back(object);

it didn,t give me any error.

http://stackoverflow.com/questions/6500313/why-should-c-programmers-minimize-use-of-new — Marian Spanik, Aug 24 '16 at 06:37

score 3 · Answer 1 · edited Feb 09 '17 at 21:26

3

list<ObjectClass*> *lst_testList is a pointer to a list of ObjectClass pointers. In order to use it you either need to allocate it first or (as you already found out) uselist<ObjectClass*> lst_testList (which is a list to ObjectClass pointers) .

edited Feb 09 '17 at 21:26

Yasitha Bandara

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answered Aug 24 '16 at 06:28

Ton Plooij

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score 1 · Accepted Answer · answered Aug 24 '16 at 06:53

1

Such list<ObjectClass*> lst_testList; variable is default-initialized.

For pointer list<ObjectClass*>* lst_testList; default initialization is not performed. Proper initialization for it is:

list<ObjectClass*>* lst_testList = new list<ObjectClass*>();

or

list<ObjectClass*>* lst_testList = NULL;

It depends on your goals.

answered Aug 24 '16 at 06:53

Nikita

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Are there any advantages & disadvantages between two methods? – Yasitha Bandara Aug 24 '16 at 07:34
@YasithaBandara It's hard to say. It depends on architecture of your application (e.g. on `lst_testList` visibility). If it's global variable you can create single instance `new list()` and use it over the application. If it's a field of `FunctioningClass` you can initialize `lst_testList` to `NULL` and create an instance `new list()` later when it will be needed. – Nikita Aug 24 '16 at 07:46
@YasithaBandara General approach is to always initialize pointers to some value. `NULL` will guarantee predictable behavior. It's easy to understand why pointer is `NULL`, but very hard to debug unpredictable behavior of [dangling pointer](https://en.wikipedia.org/wiki/Dangling_pointer). – Nikita Aug 24 '16 at 07:48

Access violation error when creating pointer to pointer List

2 Answers2