Unique pointer to vector and polymorphism

Question

I have problem, I want to create unique pointer to vector of Base objects. I want keep in this vector subclass of Base (SubClass), but i have problem with initialization, because Base class is virtual.

std::unique_ptr<std::vector<Base>> baseVector = std::make_unique<std::vector<Base>>();
SubClass newObject();
baseVector->push_back(newObject);

You don't need a pointer to the vector, but a pointer to the base class. Please try to find a good beginners book as it will all be explained there. — Some programmer dude
– Some programmer dude, Commented Jun 8, 2017 at 11:41
You should use pointer to Base for storing the objects. See this stackoverflow.com/questions/2394581/… — 0x0001
– 0x0001, Commented Jun 8, 2017 at 11:41
You probably want std::vector<std::unique_ptr<Base>> instead (or std::vector<Base*> if vector doesn't have ownership). — Jarod42
– Jarod42, Commented Jun 8, 2017 at 11:41
You'll need a vector of pointers to your base class. Also, why do you need a pointer to a vector? What does it give you that aplain vector instance does not? — Rook
– Rook, Commented Jun 8, 2017 at 11:42

WhozCraig · Accepted Answer · 2017-06-08 12:09:39Z

6

Short Version: You don't want a dynamic pointer to a collection of Base; you want a collection of dynamic pointer-to-Base.

You seem to be misunderstanding where to place std::unique_ptr in your polymorphic collection. It isn't the collection that needs to be pointers for polymorphism to work; it's the object held within.

For example:

#include <iostream>
#include <vector>
#include <memory>

struct Base
{
    virtual ~Base() {}

    virtual void foo() const = 0;
};

class DerivedOne : public Base
{
public:
    virtual void foo() const
    {
        std::cout << "DerivedOne\n";
    }
};

class DerivedTwo : public Base
{
public:
    virtual void foo() const
    {
        std::cout << "DerivedTwo\n";
    }
};

int main()
{
    std::vector< std::unique_ptr<Base> > objs;

    objs.emplace_back(std::make_unique<DerivedOne>());
    objs.emplace_back(std::make_unique<DerivedTwo>());

    // via operator[]
    objs[0]->foo();
    objs[1]->foo();

    // via range-for
    for (auto const& p : objs)
        p->foo();

    // via iterators
    for (auto it = objs.begin(); it !=objs.end(); ++it)
        (*it)->foo();
}

Output

DerivedOne
DerivedTwo
DerivedOne
DerivedTwo
DerivedOne
DerivedTwo

Whether you want the collection itself to be managed dynamically via a smart pointer is unrelated (and somewhat questionable) to this issue.

answered Jun 8, 2017 at 12:09

WhozCraig

66.5k11 gold badges82 silver badges148 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Radosław Gadowski Over a year ago

Thank you for your solution :)

CharlesW Over a year ago

Yes, this worked for me and I made a complicated example out of it since my needs vary a tad from this. But it works, thanks.

mattideluxe · Accepted Answer · 2017-06-08 12:00:41Z

2

A vector of Base doesn't support polymorphism. For this you need references or pointers or even better smart pointers:

auto baseVector = std::make_unique<std::vector<Base*>>();
auto baseVector = std::make_unique<std::vector<std::unique_ptr<Base>>>();

add elements via

baseVector.push_back(std::make_unique<SubClass>());

answered Jun 8, 2017 at 12:00

mattideluxe

4682 silver badges13 bronze badges

Collectives™ on Stack Overflow

Unique pointer to vector and polymorphism

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related