0

我懂了

#include <vector>
using namespace std;

class A{
     protected:
         int test;
     public:
         void test(){} 
};
class B : public class A{
    public:
        void test2(){}
};

int main(){
     vector <A> new_vector;
     A a1;
     new_vector.push_back(a1);
     B b1;
     new_vector.push_back(b1);

     new_vector[0].test();
     //THE PROBLEM IS HOW DO I DO THIS:
     new_vector[1].test2();

}

逻辑上是可能的,但它给了我一个错误,我如何以正确的方式做到这一点?

在python中,我一直以一种非常简单的方式做类似的事情,在c++中可能吗?

4

2 回答 2

1

你的问题是向量只包含一个A对象的空间,没有办法在B其中放置一个。

对于多态性,您需要一个指针、引用或智能指针。为了帮助进行生命周期管理,建议使用后者。

std::vector<std::unique_ptr<A>> new_vector;
new_vector.emplace_back(new A());
new_vector.emplace_back(new B());

然后您可以使用dynamic_caston 元素来发现是否真的存在 aB并调用B特定成员。

但是,使用虚拟成员函数进行多态行为比dynamic_cast.

于 2013-08-03T04:10:19.207 回答
0

您可以使用与 python 相同的方式执行此操作,即不将对象直接存储到向量中,而是使用指向实际对象本身的指针/引用。

#include <memory>
typedef std::vector<std::unique_ptr<A>> AVector;

在这种情况下允许多态性,您可以将指针推送到从 A 派生的任何内容。

在您尝试尝试的情况下,您尝试将圆形钉子安装到方孔中。std::vector 实际上是一个简单的代码包装器,它分配了一大块内存成员 * sizeof(T)。

“unique_ptr”是一个用于指针的 C++11 容器,它知道在它消失时将其删除。如果您没有 C++11/C++0x 支持,您可以使用指针或制作自己的“自动”指针包装器。

// Old-style pointer
typedef std::vector<A*> AVector;
AVector avec;

avec.push_back(new A);
avec.push_back(new B);
avec.push_back(new A);
avec.push_back(new B);

// now it's your responsibility to 'delete' these allocations when you remove them.
void discardAvec(size_t position) {
    A* ptr = avec[position];
    delete ptr;
    avec.erase(avec.begin() + position);
}

在 ideone.com 上查看以下演练的现场演示:

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

typedef std::vector<class A*> AVector;

class A
{
protected: // so B can access it.
    int m_i;
public:
    A(int i_) : m_i(i_)
    {
        std::cout << "CTor'd A(i) " << (void*)this
                  << " with " << m_i << std::endl;
    }
    A(int i_, bool) : m_i(i_)
    {
        std::cout << "CTor'd A(i, b) " << (void*)this
                  << " with " << m_i << std::endl;

    }

    virtual ~A()
    {
        std::cout << "DTor'd A " << (void*)this << " with " << m_i << std::endl;
    }
};

class B : public A
{
    int m_j;
public:
    B(int i_, int j_) : A(i_, true), m_j(j_)
    {
        std::cout << "CTor'd B(i, j) " << (void*)this
                  << " with " << m_i << ", " << m_j << std::endl;
    }

    virtual ~B()
    {
        std::cout << "DTor'd B " << (void*)this
                  << " with " << m_i << ", " << m_j << std::endl;
    }
};

int main()
{
    AVector avec;

    std::cout << "create A(1)" << std::endl;
    avec.push_back(new A(1)); // allocated an "A" on the heap.
    std::cout << "create B(2, 1)" << std::endl;
    avec.push_back(new B(2, 1)); // allocated a "B" on the heap.
    std::cout << "create B(2, 2)" << std::endl;
    avec.push_back(new B(2, 2));
    std::cout << "create A(3) " << std::endl;
    avec.push_back(new A(3));
    std::cout << "populated avec" << std::endl;

    A* ptr = avec[2]; // take the pointer of what is actually a B
    avec.erase(avec.begin() + 2); // remove it from the vector.
    std::cout << "removed entry 2 from the vector" << std::endl;
    // 'ptr' is still valid because it's an allocation, and C++ doesn't
    // garbage collect heap allocations. We have to 'delete' it ourselves.
    // Also note that because A and B have virtual destructors,
    // you will see both of them called.
    delete ptr;

    // Now watch what DOESN'T happen as we exit.
    // everything we CTOR'd that doesn't get DTORd is a leak.
    return 0;
}
于 2013-08-03T04:16:37.357 回答