我怎样才能拥有一个基本 class 对象的列表并访问派生的 class 字段？

Question

Let's say I have a base class and two derived classes:

class a {};
class b : a { int a; };
class c : a { int b; };

And I have a list of the base class, and I insert the derived class into the list:

std::list<a> list;

list.emplace_back(b());
list.emplace_back(c());

Now, I want to access int a like this:

for(auto i : list)
{
    i.a = 5;
}

I already have checks in place to see what class it is, but the compiler is still not letting me access it.

How can I access any field in the derived class from the base class?

This question has been asked many times, but none of the methods have worked for me so far.

Answer 1

First, you need a list of base class pointers , otherwise you will slice the objects when you insert them into the list.

Then, you need to type-cast the pointers to access derived classes as needed.

If all of the list elements are pointing at the same derived type, you can use static_cast :

struct A {};
struct B : A { int a; };
struct C : A { int b; };

std::list<std::unique_ptr<A>> lst;

lst.push_back(std::make_unique<B>());
lst.push_back(std::make_unique<B>());

for(auto &ptr : lst)
{
    static_cast<B*>(ptr.get())->a = 5;
}

Otherwise, use dynamic_cast instead to test the derived class type before accessing its fields:

struct A {};
struct B : A { int a; };
struct C : A { int b; };

std::list<std::unique_ptr<A>> lst;

lst.push_back(std::make_unique<B>());
lst.push_back(std::make_unique<C>());

for(auto &ptr : lst)
{
    B *b = dynamic_cast<B*>(ptr.get());
    if (b)
        b->a = 5;
}