cast std::vector> to std::vector>

Question

I have three class members:

public:
    std::vector<std::shared_ptr<Object> > getObjects();
    std::vector<std::shared_ptr<const Object> > getObjects() const;

private:
    std::vector<std::shared_ptr<Object> > m_objects;

I'm getting a compiler error when I return m_objects in the const version of getObjects(), because m_objects does not match the return type (std::vector<std::shared_ptr<const Object>>).

In my workaround, I first reconstruct the object vector locally by iterating and then return the local vector, but is there a more optimized way of handling this scenario?

std::vector<std::shared_ptr<const Object> > objects;
for (auto & object: m_objects)
{
    objects.push_back(object);
}
return objects;

Ok, but I suppose the content of the local variable wont be copied but moved in the case of assigning a variable to getObjects(), for example auto objects = objectManager->getObject();, where objectManager is a const pointer. — grimgrom, Jun 04 '19 at 17:23
One solution is to not return a `vector` and instead make the class itself [into a container](https://stackoverflow.com/questions/7758580/writing-your-own-stl-container). Note that this may betray the class's intent and overcomplicate things. — user4581301, Jun 04 '19 at 17:42
How about simply `return {m_objects.begin(), m_objects.end()};`? — , Jun 04 '19 at 21:35

score 1 · Answer 1 · answered Jun 04 '19 at 22:42

You cannot cast from std::vector<std::shared_ptr<T>> to std::vector<std::shared_ptr<const T>>, even reinterpret_cast would yield undefined behavior. For details, the related question cast vector<T> to vector<const T> could help.

Creating a copy is the best that you can do. But, as Frank pointed out, it can be written more concise as return {m_objects.begin(), m_objects.end()}. That also avoids some unnecessary allocations, as the size of the final container is immediately known (in contrast to the push_back approach).

cast std::vector> to std::vector>

1 Answers1