Should a std::vector of objects use pointers, references, or nothing?

Question

Suppose we have some class, say class Apple, and wanted to store a std::vector containing Apple.

Should the vector contain pointers, references, or plain objects?

std::vector<Apple*> vector; // Definitely won't copy the Apple on lookup, 
    // but I would prefer not to use pointers if I don't have to
std::vector<Apple&> vector; // Seems circuitous?
std::vector<Apple> vector; // I don't know if this copies the Apple on each lookup

My goal is to make it so when I call

Apple& a = vector[4];

the Apple won't get copied.

I've been using C++ for over a year and I've always worked around this and never understood it. Is there a simple explanation of why one of these three approaches is the best practice?

I'll ask a question back: who shall own the objects you can access by index? The vector? Or will the vector point or reference to another actual owner? — Zeta
– Zeta, Commented Dec 24, 2018 at 18:07
HolyBlackCat: that's a good question, Apple is not polymorphic. But if it was, we'd definitely want to use a pointer, right? — ATOMP
– ATOMP, Commented Dec 24, 2018 at 18:09
@HolyBlackCat: I think so. They sell hardware and software. <g> — Rudy Velthuis
– Rudy Velthuis, Commented Dec 24, 2018 at 18:10
If it was polymorphic, I'd suggest figure out how to make a copyable unique_ptr wrapper (using type erasure) and use that. If it sounds too hard, use pointers. — HolyBlackCat
– HolyBlackCat, Commented Dec 24, 2018 at 18:11

Zeta · Accepted Answer · 2018-12-24 18:11:52Z

22

Use the type T. Remember that operator[] returns a (const) reference, so your Apple& works fine there:

      T& vector<T>::operator[](size_t);
const T& vector<T>::operator[](size_t) const;

That's why you can use vec[3].make_older() to begin with if return_type Apple::make_older(void) exists.

However, keep in mind that there are many methods that invalidate references, so

Apple& reference = vec[3];
vec.push_back(other_apple);

reference.get_eaten();

might result in undefined behaviour if push_back reallocated the referenced apple.

answered Dec 24, 2018 at 18:11

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David Schwartz · Accepted Answer · 2018-12-24 18:11:38Z

11

Use:

std::vector<Apple> vector; // I don't know if this copies the Apple on each lookup

While all of them would achieve your goal (a lookup never copies), this is the one that allows the vector to own the Apples in it so that the lifetimes are sensibly managed.

answered Dec 24, 2018 at 18:11

David Schwartz

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3 Comments

Rakete1111 Over a year ago

Note that not all of them work. A vector of references is not allowed.

Vassilis Over a year ago

@Rakete1111, A vector of references is not allowed but a vector of reference_wrapper does work.

Rakete1111 Over a year ago

@Vassilis Jup, but a reference_wrapper is not a reference :)

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