我尝试创建简单的struct，保存对其父级的某些值的引用。父项存储在向量内unique_ptr内。它被移动到那里之前被实例化了。在运动之后，当然参考不再有效。我找到了重新实现它们的方法，但我讨厌解决方案（如下所示）。我认为在collection.push_back(std::move(d))上调用了移动构造函数，但Derived并非如此。它可能适用于unique_ptr，但我不确定。
我的问题是 - 处理这种情况的首选方法是什么？我有一个更好的解决方案吗？覆盖unique_ptr的移动构造函数会有帮助吗？这是一个好主意吗？或者，以下面给出的方式设计对象是一个好主意吗？
#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;
}
输出：
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
答案 0 :(得分：3)
根据标准，移动不应使参考无效。真正的问题是std::vector::push_back，如果容量发生变化，它会使所有内容无效。
一种解决方案是使用std::deque，因为它永远不会使push_back()的引用无效：
#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;
}
<强>输出：
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
答案 1 :(得分：1)
这一行：
refs.push_back( {points.back().x, points.back().y} );
表示refs中的新条目将引用points中最后一个条目的成员。
但下次执行points.push_back时，可能会导致向量重新分配，从而使refs中已存储的所有引用无效。
如果你真的想坚持vector<Refs>，你将不得不重新设计代码，以确保所引用对象的生命周期超过vector<Refs>的生命周期。
unique_ptr是红鲱鱼。