Dynamic_cast：在这种情况下应替换

Question

There is a base class A, which is virtual

class A
{
  ~virtual A() = 0;
};

and more derived classes B, C, D, E...

class B : public A
{
};

class C: public A
{
};

and analogously for other classed D, E... We have a list of A pointers

std::list <A*> a_list;

We remove any element which type is unknown, for example

A *a = a_list.front();

Base on the type of a pointed object we decide, what to do... There more possibilities how to do that:

A) dynamic_cast case

Recasting of a to derived types.

if (dynamic_cast <B*> (a))
{
   //do something (but nothing with a)
}

else if (dynamic_cast <C*> (a))
{
    //do other (but nothing with a)
}

But the dynamic_cast usage indicates a bad design.

B) Additional attribute

Some aditional attribute, for example an object ID is impelemented;

class A
{
  virtual ~A() = 0;
  virtual short getID() = 0;
};

class B : public A
{
  virtual short getID() {return 1;}
};

class C: public A
{
  virtual short getID() {return 2;}
};

So the modified condition

switch ( a->getID())
{
   case 1: // do something (but nothing with a)
   case 2: // do other (but nothing with a)
}

A note:

We do not perform any action directly with the object, but on the basis of its type we do some different computations.

Questions:

1) Is it the case, when we should avoid dynamic_cast?

2) is there any prefereable solution (may be different to presented)?

Thanks for your help.

Answer 1

According to Item 90 in C++ Coding Standards ( Amazon ): Avoid type-switching (regardless whether you do it with an if-else ladder and dynamic_cast , or a switch statement with getID() function). Prefer instead to rely on polymorphism via virtual functions

class A
{
public:
  ~virtual A() = 0;

  void fun()  // non-virtual
  { 
     // main algorithm here

     // post-processing step
     post_fun();
  }

  virtual void post_fun() = 0;
};

class B : public A
{
public:
   virtual void post_fun() { /* bla */ }
};

class C: public A
{
public:
   virtual void post_fun() { /* meow */ }
};

A* a = a_list.front();
a->fun(); // will be resolved at run-time to whatever type a points to

The reason is that having an explicit type-switch is hard to maintain and update. If you get a new derived class from A , you need to update every place where you loop over types. Instead, the compiler will do that automatically for you if you rely on virtual functions.

Answer 2

In most cases, when you need to use anything B -specific (keeping your naming), you should store B * (or shared_ptr<B> , rather), instead of A * . In all other cases, hide everything behind polymorphism.

Consider following hierarchy:

class Animal 
{
public:
    Animal() {}
    virtual ~Animal() = 0;
    virtual void Breathe();
};

class Bird : public Animal
{
public:
    Bird() {}
    virtual ~Bird() {}
    virtual void Breathe() {...}
    virtual void Fly() {...}
};

and imagine you're storing Animal * s - you shouldn't call Fly() now. If you need to call it, then store Bird * from the beginning. But all animals must breathe - that's why the function is inherited from base class.

---

To sum it up: if you need to do something Child -specific, store pointer to Child , not to Base .

Answer 3

Usually, dynamic cast are perform to get specific type of object to work with them.

struct base
{
    virtual void a() = 0;
};

struct foo : base
{
    virtual void a() { ... }
    void specificFooMethod();
};

struct bar : base
{
    virtual void a() { ... }
    void specificBarMethod();
};

base * pBase = ...;
pBase->a(); // no casting required here
if ( foo * p = dynamic_cast<foo*>(pBase) )
{
    p->specificFooMethod();
}
else if ( bar * p = dynamic_cast<bar*>(pBase) )
{
    p->specificBarMethod();
}

specific* methods are not virtual and can't be acccessed by base interface without casting. So dynamic_cast should be used when you really need specific type of object which have additional functionality/properties that can't be move to abstract layer in base class.