What to do Dynamically Allocated Arguments

[skindig123]

Hi, Forum.

 

How do you delete new memory allocated in a function call argument? Or should you? 

 

So let's say you have a class with a constructor that has a pointer-to-object parameter (this example could apply to any function w/ a pointer parameter but for the sake of discussion, I'm using a constructor):

 

class tDailyViewPanel : public tScrolledWindow

{

    public:

        tDailyViewPanel(tDateTime* date);

        ...  // rest of class interface

    private:

         tDateTime* m_date;

 

 

A user could instantiate this two ways as my novice brain sees it.  They could create a new object dynamically with the "new" keyword such as this:

 

User Option #1

tScrolledWindow* selected = new tDailyViewPanel(new wxDateTime());

 

Or they may use a pointer to some already allocated tDateTime object, such as a member of a instantiating object.

 

User Option #2

m_selected = new tDailyViewPanel(m_member);  // m_member is a tDateTime member of some instantiating/calling object and is deleted by the calling object's deconstructor.

}

 

At first, I would think the tDailyViewPanel constructor's implementation needs to initialize the m_date member with a shallow copy of the second argument.

 

tDailyViewPanel::tDailyViewPanel(tDateTime* date) : tScrolledWindow()

{

    ...  // doing stuff

    m_date = date;

    ... // doing more stuff

}

 

I could then "delete" the memory allocated by the constructor call since "m_date" and "date" point to the same memory in User Option #1. 

 

But if I do that and the user wants does User Option #2, I could delete memory that would be better deleted by the object/function that called the tDailyViewPanel constructor. 

 

 

Alternatively, if i did a deep copy,

tDailyViewPanel::tDailyViewPanel(tDateTime* date) : tScrolledWindow()

{

    ...  // doing stuff

    m_date = new tDateTime(*date);

    ... // doing more stuff

}

 

wouldn't it create a memory leak in the case of User Option 1?

 

I must not understand the mechanics of this because it feels like I'm stuck between a memory leak or deleting memory I shouldn't.  How do you manage memory allocated in a function call argument? Are smart pointers the only way?

[ChajusSaib]

Smart pointers would be required if you must do what you have described.

You cannot check whether a pointer points to dynamically allocated memory or not. You will have to let the user of the class/function know whether to give you a pointer to dynamic memory or an object on the stack. For example in Qt(it's a GUI framework) many functions take a pointer and will take care of deleting that data so a user(such as me and you) would know what to pass to it and we would know not to free the memory as that will be handled by Qt.

Other functions in other libraries(such as tesseract) will even allocate memory that it will return and users of those functions would know that they would have to handle deleting the memory.

So for the class the above you could have it take control of the memory and free it when necessary.