mercredi 26 avril 2017

Move constructor and vector of unique_ptr

I tried to create simple struct holding references to some values of its parent. Parent is stored within unique_ptr, inside a vector. It's instantiated before being moved there. After the movement, references of course are no longer valid. I found a way to reinstantiate them, but I hate the solution (it's shown below). I thought that move constructor is called on collection.push_back(std::move(d)), but's it's not the case for Derived. It might be for unique_ptr though, I'm not sure about that. My question is - what should be preferred way of dealing with such situations? Is there a better solution that one I presented below? Would overriding a move constructor of unique_ptr help? Is this a good idea? Or, is this even a good idea to design objects in the way presented below?

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


// Inner object of every Base instance, is used to keep reference to
// Base's inner variables
struct Ref {
    Ref(double &x, double &y)
        : x(x)
        , y(y)
    {

    }

    std::reference_wrapper<double> x;
    std::reference_wrapper<double> y;
};


struct Point {
    double x;
    double y;
};


struct Base {
    // every derived class uses this vector
    std::vector<Ref> refs;

    // some meaningless pure virtual method, ignore it
    virtual void draw() = 0; 
};


struct Derived : public Base {
    Derived() {
        std::cout << "Derived constructed" << std::endl;
    }
    // Method for adding point and relating it with
    // a reference in refs vector
    void add(double x, double y) {
        points.push_back({x, y});
        refs.push_back( {points.back().x, points.back().y} );
    }

    // some meaningless pure virtual method, ignore it
    virtual void draw() override { }

    // this vector is specific to this particular derived class
    std::vector<Point> points;
};


int main() {

    // some vector for storing objects
    std::vector<std::unique_ptr<Base>> collection;

    {
        auto d = std::unique_ptr<Derived>(new Derived());
        d->add(0.01, 0.02);
        d->add(1.111, 2.222);
        d->add(14.3333, 3.1414);
        collection.push_back(std::move(d));
    }

    // posible solution (I hate it)
    {
        auto d = std::unique_ptr<Derived>(new Derived());
        d->add(0.01, 0.02);
        d->add(1.111, 2.222);
        d->add(14.3333, 3.1414);
        collection.push_back(std::move(d));

        auto c = dynamic_cast<Derived *>(collection.back().get());
        for (int i = 0; i < c->points.size(); i++) {
            c->refs[i].x = c->points[i].x;
            c->refs[i].y = c->points[i].y;
        }
    }

    // Let's take 1st vector element and cast it to Derived
    {
        auto d = dynamic_cast<Derived *>(collection[0].get());

        std::cout << "values from points vector:" << std::endl;
        // These work correctly after moving
        std::cout << d->points[0].x << std::endl;
        std::cout << d->points[0].y << std::endl;
        std::cout << d->points[1].x << std::endl;
        std::cout << d->points[1].y << std::endl;
        std::cout << d->points[2].x << std::endl;
        std::cout << d->points[2].y << std::endl;

        std::cout << "values from refs vector:" << std::endl;
        // References of course do not work anymore
        std::cout << d->refs[0].x << std::endl;
        std::cout << d->refs[0].y << std::endl;
        std::cout << d->refs[1].x << std::endl;
        std::cout << d->refs[1].y << std::endl;
        std::cout << d->refs[2].x << std::endl;
        std::cout << d->refs[2].y << std::endl;
    }

    // Let's take 2nd vector element and cast it to Derived
    {
        auto d = dynamic_cast<Derived *>(collection[1].get());

        std::cout << "values from points vector:" << std::endl;
        // These work correctly after moving
        std::cout << d->points[0].x << std::endl;
        std::cout << d->points[0].y << std::endl;
        std::cout << d->points[1].x << std::endl;
        std::cout << d->points[1].y << std::endl;
        std::cout << d->points[2].x << std::endl;
        std::cout << d->points[2].y << std::endl;

        std::cout << "values from refs vector with ugly fix:" << std::endl;
        // References of course do not work anymore
        std::cout << d->refs[0].x << std::endl;
        std::cout << d->refs[0].y << std::endl;
        std::cout << d->refs[1].x << std::endl;
        std::cout << d->refs[1].y << std::endl;
        std::cout << d->refs[2].x << std::endl;
        std::cout << d->refs[2].y << std::endl;
    }

    return 0;
}

Output:

Derived constructed
Derived constructed
values from points vector:
0.01
0.02
1.111
2.222
14.3333
3.1414
values from refs vector:
0
0.02
4.94602e-317
4.94603e-317
14.3333
3.1414
values from points vector:
0.01
0.02
1.111
2.222
14.3333
3.1414
values from refs vector with ugly fix:
0.01
0.02
1.111
2.222
14.3333
3.1414

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