initializing unique_ptr at runtime

Question

In my implementation, I have a vector of classes. Within each class there is a unique_ptr to a linked list. Only at runtime do I know the number of nodes that should be added to each of the linked lists. Some linked lists may have zero nodes. A simplified view of my class is:

class A
{
private:
    ...
    std::unique_ptr< std::list<MyListElement> > ptrList;
    ...

public:
    ...
};

Thanks to the unique_ptr, I had to jump through the hoops of first declaring a copy-constructor and copy-assignment-operator and setting them to = delete, and then providing definitions for a default-constructor, move-constructor and move-assignment-operator. After all this, I am now ready to call the function that initializes my linked lists for each object.

void A::initListElements(unsigned int numElements)
{
    if (numElements > 0)
    {
        std::unique_ptr< std::list<MyListElement> > tmp(new std::list<MyListElement>);
        ptrList = std::move(tmp);
    }
    else
    {
        ptrList = 0;
    }
}

Is this the correct way of doing it? Is there some way I can avoid creating the temporary unique_ptr 'tmp'?

"Thanks to the `unique_ptr`, I had to jump through the hoops"... is wanting to jump through the hoops the reason the code uses a `unique_ptr` at all? Because I see no other reason. — R. Martinho Fernandes, Oct 21 '13 at 13:27
Why do you use a unique_ptr at all? `std::list` manages memory itself, no reason to allocate it on the heap. — lethal-guitar, Oct 21 '13 at 13:30
My requirement was such that when an object of class A gets destroyed, then it should destroy it's linked list as well. So I thought having a unique_ptr to the list was the easiest way to get rid of it. — Gautam, Oct 21 '13 at 13:34
A) you don't need a `unique_ptr` for this, just use an `std::list`. B) even with the unique pointer, the compiler should take care of the hoops you mention. — juanchopanza, Oct 21 '13 at 13:35
@Gautam No need to do that - they are properly destructed whenever the containing object is. — lethal-guitar, Oct 21 '13 at 13:35
You are missing the very basic concepts of C++. Get [a good book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). If an object of class A has a `std::list` member, that member gets destroyed already when A objects are destroyed. It all comes for free, there's no need to find "easier" ways. — R. Martinho Fernandes, Oct 21 '13 at 13:36

score 4 · Accepted Answer · answered Oct 21 '13 at 13:29

4

You can avoid temporaries with reset method

ptrList.reset(new std::list<MyListElement>);

And you don't need else statement, I believe.

answered Oct 21 '13 at 13:29

ixSci

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5
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79

With reset (and no else block), I was able to get it to work as expected, so I'm going to accept this answer. But it appears that I don't even need a unique_ptr for this particular problem! – Gautam Oct 21 '13 at 13:46

initializing unique_ptr at runtime

1 Answers1