我可以在 C++ 中拥有具有值语义的多态容器吗?

2022-01-07 00:00:00 c++ stl

作为一般规则，我更喜欢在 C++ 中使用值而不是指针语义(即使用 vector 而不是 vector).通常性能上的轻微损失可以通过不必记住删除动态分配的对象来弥补.

As a general rule, I prefer using value rather than pointer semantics in C++ (ie using vector<Class> instead of vector<Class*>). Usually the slight loss in performance is more than made up for by not having to remember to delete dynamically allocated objects.

不幸的是，当您想要存储从公共基础派生的各种对象类型时，值集合不起作用.请参阅下面的示例.

Unfortunately, value collections don't work when you want to store a variety of object types that all derive from a common base. See the example below.

#include <iostream>

using namespace std;

class Parent
{
    public:
        Parent() : parent_mem(1) {}
        virtual void write() { cout << "Parent: " << parent_mem << endl; }
        int parent_mem;
};

class Child : public Parent
{
    public:
        Child() : child_mem(2) { parent_mem = 2; }
        void write() { cout << "Child: " << parent_mem << ", " << child_mem << endl; }

        int child_mem;
};

int main(int, char**)
{
    // I can have a polymorphic container with pointer semantics
    vector<Parent*> pointerVec;

    pointerVec.push_back(new Parent());
    pointerVec.push_back(new Child());

    pointerVec[0]->write(); 
    pointerVec[1]->write(); 

    // Output:
    //
    // Parent: 1
    // Child: 2, 2

    // But I can't do it with value semantics

    vector<Parent> valueVec;

    valueVec.push_back(Parent());
    valueVec.push_back(Child());    // gets turned into a Parent object :(

    valueVec[0].write();    
    valueVec[1].write();    

    // Output:
    // 
    // Parent: 1
    // Parent: 2

}

我的问题是:我可以拥有我的蛋糕(价值语义)并吃掉它(多态容器)吗?还是我必须使用指针?

My question is: Can I have have my cake (value semantics) and eat it too (polymorphic containers)? Or do I have to use pointers?

推荐答案

由于不同类的对象会有不同的大小，如果将它们存储为值，最终会遇到切片问题.

Since the objects of different classes will have different sizes, you would end up running into the slicing problem if you store them as values.

一种合理的解决方案是存储容器安全的智能指针.我通常使用 boost::shared_ptr ，它可以安全地存储在容器中.请注意 std::auto_ptr 不是.

One reasonable solution is to store container safe smart pointers. I normally use boost::shared_ptr which is safe to store in a container. Note that std::auto_ptr is not.

vector<shared_ptr<Parent>> vec;
vec.push_back(shared_ptr<Parent>(new Child()));

shared_ptr 使用引用计数，因此在删除所有引用之前它不会删除底层实例.

shared_ptr uses reference counting so it will not delete the underlying instance until all references are removed.

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