Vector Iterators Casting

问题

Hey, In C++, I have a vector of type:

vector<BaseClass*> myVector;

In which, I insert (push_back) pointers of derived classes into it.

Now, I want to pop back its elements so I do this:

vector<ADlgcDev*>::iterator iter;

for (iter = myVector.rbegin(); iter != myVector.rend(); iter++)
{
 // but before I pop it, I need to shutdown it down
 // so I cast this
 // but this way, I'm unable to call the function
 (DerivedClass*(*iter))->Shutdown();

 myVector.pop_back();
}

but as mention in the comments before I pop it, I need to call its Shutdown() method and the cast is not working properly too. Any resolutions? or is impossible?

回答1:

while (!myVector.empty())
{
  ((DerivedClass*)(myVector.back()))->Shutdown();
  myVector.pop_back();
}

Notes:

You should probably use dynamic_cast instead of the hard cast. (If it's sure that there are only DerivedClass objects in the vector, why isn't it std::vector<DerivedClass>?)
You should probably not have to cast at all, since Shutdown() should be declared in the base class.
You should probably delete the objects, too, before you pop them off the vector. (But that might not be so.)
You should probably use a smart pointer which calls Shutdown() (and delete, probably).

Edit: Using std::vector<T>::clear(), as shown by markh44 is probably better than the pop_back().

回答2:

Could you make Shutdown a virtual function in BaseClass? Then you wouldn't need a cast.

Also you'll probably have trouble removing items from a vector while iterating. I'd do it like this:

vector<BaseClass*>::iterator iter;

for (iter = myVector.rbegin(); iter != myVector.rend(); iter++)
{
    (*iter)->Shutdown();
}
myVector.clear();

Edit: and another thing, ++iter is generally preferred over iter++.

回答3:

The constructor casting doesn't work for pointers. Use static_cast if you're sure or dynamic_cast and check.

回答4:

If Shutdown() is a virtual method of the Base class i.e. BaseClass::ShutDown() you should directly call iter->ShutDown();

Otherwise if the method isn't virtual you should use dynamic_cast.

vector<ADlgcDev*>::iterator iter;

for (iter = myVector.rbegin(); iter != myVector.end(); iter++)
{
 DerivedClassA* a = dynamic_cast<DerivedClassA*>( *iter ) ;
 if ( a ) a->ShutDownA();
 else
 {
 DerivedClassB* b = dynamic_cast<DerivedClassB*>(*iter);
 if ( b ) b->ShutDownB();
 // ... repeat for every class in hierarchy that might be in the vector.
 }
 myVector.pop_back();
}

Anyway you're probably leaking memory, unless ShutDown() deletes the object from itself (which is generally a bad idea ) or you're keeping duplicated pointers and deleting them elsewhere, which is another risky idea.

来源：https://stackoverflow.com/questions/1372288/vector-iterators-casting

标签

c++

vector

pointers

casting