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 After the movement, references of course are no longer valid.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 {
virtual ~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
According to the standard, references should not be invalidated by the move. The real problem is the std::vector::push_back which nvalidates everything if the capacity changes.
One solution is to use a std::deque because it never invalidates reference with push_back():
#include <iostream>
#include <vector>
#include <deque>
#include <memory>
struct Point {
double x;
double y;
};
struct Base {
// every derived class uses this vector
std::vector<Point*> refs;
// some meaningless pure virtual method, ignore it
virtual ~Base() = default;
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());
}
// some meaningless pure virtual method, ignore it
void draw() override { }
// this vector is specific to this particular derived class
std::deque<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));
// No ugly fix needed
}
// 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 still work
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
values from points vector:
0.01
0.02
1.111
2.222
14.3333
3.1414
values from refs vector:
0.01
0.02
1.111
2.222
14.3333
3.1414