指向向量和多态性的唯一指针

2022-01-24 00:00:00 polymorphism vector c++ unique-ptr

我有问题，我想创建指向 Base 对象向量的唯一指针.我想保留 Base 的这个向量子类(SubClass)，但是初始化有问题，因为 Base 类是虚拟的.

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);

推荐答案

短版:你不想要一个指向 Base 集合的动态指针；你想要一个指向Base的动态指针集合.

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

您似乎误解了在多态集合中放置 std::unique_ptr 的位置.不是集合需要成为多态工作的指针；它是里面的对象.

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.

例如:

#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
";
    }
};

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

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();
}

输出

DerivedOne
DerivedTwo
DerivedOne
DerivedTwo
DerivedOne
DerivedTwo

您是否希望通过智能指针动态管理集合本身与此问题无关(并且有些疑问).

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

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