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Hello.
I made a class:
//the problem is, that the return value of a_function gets destroyed BEFORE the assignment is done, thus the "cout ... " will print unexpected results.Code:class Test
{
public:
int a;
}; //there is no special usage for this class. I just want to show my problem
Test a_function()
{
Test temp;
temp.a = 4;
return temp;
}
int main()
{
Test b = a_function();
cout << b.a << endl;
}
How can I make a copy of the return value before it's destroyed.
Return a pointer to new'ed memory. I think if you make the variable (temp) static it might work because it never gets deleted until the end. You'd have to try though, I'm not sure at all if this is correct.
but what about the functions that return 'int'-s as their return values. those return values are used to specify the success or failure of the function - like int main(). This should mean, that actually the variable is not destroyed right away, but e.g.: after the expression it occurrs in.
I am not sure about the behaviour. Does anyone else have an idea?
You are returning a copy of a local variable (temp). There is nothing wrong with that.
Can it be you are trying instead to return a local reference?
I shortened the code. It looks something like this:
Code:class List
{
public:
List();
List cut(Iterator&);
};
List List::cut(Iterator& it)
{
List new_list; //List is a class defined by me, not the std::list<>
pos = pos->next;
for (;pos != NULL;pos = pos->next)
{
new_list.push_back(pos->value);
new_list.view();
}
return new_list;
}
int main()
{
List li = an_existing_List.cut(SomeIterator);
}
And what is the error you get?
there are 2 elements in the class List:
Elem start;
Elem end;
they are respectively the start- and endpoint of the list; //their position is constant - they don't move
They are initialized in the List() constructor.
There is a member function void List::View() which prints the linked list (all the elementsbetween start and end (inclusive)). Elements are added between the start and end with "new".
Before the new_list is returned in the List::cut function, the view shows all the elements correctly, but:
List new_li = an_existing_list.cut(SomeIterator);
new_li.View(); // prints correctly the first element and all the elements between start and end, but then the last element will contain some random numbers(which didn't appear before the "return" statement in the cut function).
Because the list is now shortened and start and end remained the same?
Ok, here's the whole class and the 'Elem' stuct:Quote:
Originally Posted by Mario F.
Code:class List
{
int ID_j2rg;
Elem start, end;
public:
List();
void push_front(const int&);
void push_back(const int&);
void add_after(const Iterator&, const int value);
void add_after(const int, const int);
void view(bool mode = 0); //kui mode = 0, siis edaspidi, kui 1 siis tagurpidi
List bond(List&);
List cut(Iterator&);
Elem* get_begin() { return &start; }
Elem* get_end() { return &end; }
};
struct Elem
{
struct Elem* prev, *next;
int value;
int ID;
Elem() { value = 0; ID = -1; prev = next = NULL; }
Elem(const int& a, const int new_id) { value = a; ID = new_id; prev = next = NULL; }
};
I shortened the list for other's to read the important parts like the constructors. List::push_front doesn't influence the problem imho, so I left it out from the shortened version.Quote:
Originally Posted by Mario F.
I suppose you are allocating memory in your push_xxx functions
For such classes you have to provide copy constructors
If you don't - the compiler creates one for you - just using member by member assignments...
In this case all pointers point to the same memory as the original class instance,
And after the original class is destroyed - your copy has dangling pointer here and there...
With your function you invoke copy-constructor in the return statement from the the local variable to the temp-variable being returned...
After that the operator = is invoked to copy values from the temp variable to the newly created var in the calling function. It means this operator also should be provided
I created another member in the class that returns a List, in there, the copying works.
Here's how I test it:
I add a deconstructor into the List class that prints "Destroying"
then I write the following command:
(a_list.cut(Someiterator)).view();
now, after the cut function returns the value, I see "Destroying" and then all the elements of the partly destroyed list are printed on the screen(the last element has been deteriorated).
But when I write:
(a_list.returner()).view();
I see the elements printed on the screen and after the printing finishes, I see "Destroying", which is logical, as the command ends and the variables in the previous scope(List::returner()'s scope) are destroyed.Code:List List::returner()
{
List a;
return a;
}
Yes, the push_xxx functions use the dynamic memory allocation - "new".
No... I meant your list was cut. You are returning a shortened version of your list. If you are getting problems with it by having seemingly random results at the end of the list, then it's probably because your Elem members are not being correctly updated.
It was a guess. I can't see the problem with the code provided.
How can it be then, that the View() functions correctly display all the elements BEFORE the return statement, but AFTER the returning is done, the elements are no longer displayed correctly.Quote:
Originally Posted by Mario F.
I think that's the problem. So I'll have to find another way around it.Quote:
Originally Posted by vart
Spot on, I was just trying to put that into words myself :)Quote:
Originally Posted by vart
You can't declare something on the stack then expect the code to just know that you've extended it onto the heap when you go and pass it by value :P
Personally I'd declare the list on the heap and pass a pointer. You're just creating overhead by copying everything constantly anyway... I suppose that depends on the purpose behind your program to some extent though.
Create a copy constructor that handles the heap-based allocations you made with new() or keep it all on the heap and pass a pointer around.Quote:
Originally Posted by hardi
Note that for classes using dynamic memory allocation directly you should always provide copyConstructor and operator =Quote:
Originally Posted by hardi
If you think you don't need them and don't want to implement - a good practice is to create empty functions for them and make them private.
In this case if you still use one of the above by mistake - compiler will note you