如何正确实现一个对象列表的创建（和分配）只有在C ++的抽象基础知道？ [英] How to properly implement creation (and allocation) of a list of objects known only by abstract base in C++?

问题描述

我应该将堆栈分配优先于堆分配。 最好通过值（，尤其是如果您要创建新的对象，但同时如果返回基类，则对象将被切片），或至少通过引用而不是传递指针 （虽然您无法创建引用向量） 。

I should prefer stack allocation to heap allocation. It's better to pass by value (especially if you're creating new objects — but at the same time, if you return by base class, your object will be sliced), or at least by reference than passing pointers (although you can't create a vector of references).

我仔细阅读所有这些，现在我觉得我知道的不多，我以前知道。我不知道如何写一个代码，IMO，应该是微不足道，同时尊重所有这些精心编写和思考的答案中提到的最佳做法。

I read all of this carefully, and now I feel that I know less that I knew before. I don't have a slightest idea about how to write a code that, IMO, should be trivial, while respecting the best practices mentioned in all of these well-written and thought-through answers.

这里是我想要实现的。 （我不假装它是正确的C ++，但我只是想传达的想法）。

Here's what I want to implement. (I don't pretend that it's correct C++, but I just want to convey the idea).

// This thing is purely virtual!
class BaseStuff { }

// Has important behaviour and data members that shouldn't be sliced
class SomeStuff : public BaseStuff { }

// Don't slice me plz
class OtherStuff : public BaseStuff { }

BaseStuff CreateStuff()
{
    // falls a set of rules to create SomeStuff or OtherStuff instance based on phase of the moon
}

std::vector<BaseStuff> CreateListOfStuff()
{
    // calls CreateStuff a lot
}

public static void main()
{
    List<BaseStuff> allTheStuff = CreateListOfStuff();
    // to things with stuff
}

推荐答案

你将存储多态指针到一个实际指向派生类型的基类（注意我在这里使用 std :: unique_ptr ）： p>

You're going to store polymorphic pointers to a base class that actually point to a derived type (note I used std::unique_ptrs here):

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

class Base
{
public:
  virtual void shout() = 0;
};
class Child : public Base
{
public:
  void shout() { std::cout << "Child!\n"; }
};
class Orphan : public Base
{
public:
  void shout() { std::cout << "Orphan!\n"; }
};

int main()
{
  std::vector<std::unique_ptr<Base>> the_list;
  the_list.reserve(3);
  the_list.emplace_back(std::make_unique<Child>());
  the_list.emplace_back(std::make_unique<Orphan>());
  the_list.emplace_back(std::make_unique<Child>());

  for(const auto& item : the_list)
  {
    item->shout(); // remember, items are (smart) pointers!
  }
}

实例。
只能在标准C ++容器中存储一种类型，因此必须在此处存储一个（智能）指针到Base。

Live example. You can only store one type in a Standard C++ container, so you must store a (smart) pointer to Base here.

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