存储子类指针的方法

Question

I have superclass A and subclasses B and C ,

I have to satisfy sort of an interface in a sense that I want to do something like this:

x.Add(B());

or

x.Add(C());

or any other subclass of an A. where B() and C() are default construtors my Add method looks like this:

Add(A & data){
vector<A*> vec;
vec.push_back(new A())
}`

its giving me an error invalid initialization of non-const reference of type 'A&' from an rvalue of type 'A'

What is the problem here?

Answer 1

You are constructing temporary objects, which cannot be assigned to a non-const reference. You would have to make the parameter be a const reference instead:

class MyClass
{
public:
    void Add(const A &data);

private:
    std::vector<A*> vec;
};

void MyClass::Add(const A &data)
{
    vec.push_back(&data);
}

MyClass x;
x.Add(B());
x.Add(C());

But it does not really make sense to store pointers to temporary objects, since they will be destroyed, leaving the vector holding invalid pointers.

So, one option would be to push caller-allocated pointers instead:

class MyClass
{
public:
    void Add(A *data);

private:
    std::vector<A*> vec;
};

void MyClass::Add(A *data)
{
    vec.push_back(data);
}

MyClass x;
x.Add(new B);
x.Add(new C);

But that can cause memory leaks if vec.push_back() fails.

Another option is to let Add() itself perform the allocation internally, and free it if something goes wrong:

class MyClass
{
public:
    template <typename T>
    void Add()
    {
        std::auto_ptr<T> data(new T);
        vec.push_back(data.get());
        data.release();
    }

private:
    std::vector<A*> vec;
};

MyClass x;
x.Add<B>();
x.Add<C>();