Re: allocate memory of derived class

James Kanze <>
Fri, 31 Oct 2008 05:22:51 -0700 (PDT)
On Oct 30, 9:07 pm, Salt_Peter <> wrote:

On Oct 30, 1:23 pm, Steven Powers <> wrote:

 Imagine the following setup

class Parent
virtual void doStuff();
} ;

class Child : public Parent
virtual void doStuff();
} ;

and this function

bool foo(Parent *p)
p = new Parent();

I would like foo() to take a pointer and if it is null
allocate the memory for the class I pass in.

For example

Child * c = NULL;

would result in a Child() constructor and Child::doStuff()
being called. The way it is now Parent() and
Parent::doStuff() will get called.

How can this be done while keeping foo defined as foo(Parent
*p) ????

I've thought of using templates like this:
template<class T>
bool foo(Parent* p)
if(!p) p = new T();

but I am unable to get the correct type from a second
templated class that has been passed T = Child* as its
template type.

As a follow up can I get a template value T = Child* and
somehow pass foo T=Child ???

Pay attention, you will learn something today.

You said you don't want to change the function's signature

Which is, in some ways, a contradiction in terms. He wants the
function to depend on the type passed in, without passing in the
type. The obvious solution for the function to depend on the
type is to pass in the type, e.g.:

    template< typename T >
    bool foo( T* p ) ...

Of course, this doesn't solve the general problem: what to do if
he wants to call the function with "foo( NULL )".

so this would work dandy except for a few problems, i'll try
and point out those to you below:

template<class T>
void foo(Parent* p)
  if(!p) p = new T();

and you call it like so:

foo< Child >(pc);
foo< Parent >(pc);

Now, the important parts. In this language its bad news to
distribute allocation and deallocation,

Which is simply false. The rule is almost the opposite: if you
don't distribute allocation and deallocation, you shouldn't be
using dynamic allocation to begin with. The most important
single reason for using dynamic allocation is because you need
explicit deallocation, elsewhere in the program.

His case is fairly special (so special that I've never seen it
in 20 years of C++). (But I suspect that he's not described his
problem in enough detail.)

the above code is the perfect example why that rule is so
important. When you pass pointers like so:

Child* pc = 0;
foo< Child >(pc);

the pointer pc never gets modified in main, only its copy in
foo does, so once foo returns you've got a memory leak.

And to compound the issue, foo's Parent* p is no more and we
can no longer release your allocated Child. So if you were to:

    delete pc;

you are in fact deleting nothing. Hence:

template<class T>
void foo(Parent* p)
  if(!p) p = new T();
  delete p; // required

Which will wreck havoc if he calls the function with a pointer
allocated elsewhere (or pointing to a local object). What he
needs is some sort of manager class:

    template< typename T >
    class PtrManager
        PtrManager( Parent* p )
            : myPtr( p == NULL ? new T : p )
            , myIsOwned( p == NULL )

            if ( myIsOwned ) {
                delete p ;

        Parent* operator->() const
            return myPtr ;

        Parent* myPtr ;
        bool myIsOwned ;
    } ;

This will also save him if p->doStuff() throws.

Which then brings up another issue. virtual destructors.
Whenever you store derived allocations using a pointer to
base, you must declare you base d~tor virtual or you'll end up
only deallocating a portion of your objects.

No, you'll end up with undefined behavior, which is worse. It
may work, it may seem to work, but leak memory, it may crash
immediately, it may corrupt the free space arena, causing a
crash in some totally unrelated code, or it may do just about
anything else.

class Parent
  virtual ~Parent()
    std::cout << "~Parent()\n";
  virtual void doStuff()
    std::cout << "Parent::doStuff()\n";

test it, try the d~tor without 'virtual' and delete Parent* p
= new Child.

To solve the original problem [...]

We have to know what the original problem really was:-). (I
wonder, for example, if he didn't think that his allocation
actually did modify the original pointer.)

James Kanze (GABI Software)
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