Pre-Allocate a General Class Instance

[pandy.song]

In my project, there is a class "Father",
from which lots of "CHILD" class are inherited.

For the system is real-time embedded system,
"new" operation is applied here.

I want to pre-allocate a series of general
"CHILD" instances. Every time I want a specified
"CHILD" , I get one from a link list.

In C++, virtual class "FATHER" can not be
instanced.So the method I can bring to my mind
is to instance each "CHILD" class, and every
time I want a specified "CHILD", I get a memory
From a link list, and "memcpy" it.

This "memcpy" copy not only internal data ,but
virtual funtion table.

The method I memtion above may not be clever method.

I wander if there are better methods.

Thank you for your readimg this thread.

[tom_usenet]

On Mon, 15 Dec 2003 20:03:30 +0800, "pandy.song" <pa****@263.net>
wrote:

In my project, there is a class "Father",
from which lots of "CHILD" class are inherited.

For the system is real-time embedded system,
"new" operation is applied here.

I want to pre-allocate a series of general
"CHILD" instances. Every time I want a specified
"CHILD" , I get one from a link list.

In C++, virtual class "FATHER" can not be
instanced.So the method I can bring to my mind
is to instance each "CHILD" class, and every
time I want a specified "CHILD", I get a memory
From a link list, and "memcpy" it.

This "memcpy" copy not only internal data ,but
virtual funtion table.
You can't safely memcpy objects of non-POD type. Depending on the
implementation (and presense of e.g. virtual inheritence) it may not
work. This ignores the possible problems of double deletion, etc.

The method I memtion above may not be clever method.

I wander if there are better methods.

Use placement new. E.g. to get a new CHILD1 object, do this:

void* mem = obtain_memory_from_pool(sizeof(CHILD1));
CHILD1* child1 = new (mem) CHILD1(params);

//to delete:
child1->~CHILD1(); //(or as a Father*)
return_memory_to_pool(child1);
Or, alternatively, provide new/delete in Father. e.g.

class Father
{
public:
virtual ~Father();
//etc.

void* operator new(size_t amount)
{
//return "amount" of memory from linked list
}

void* operator delete(void* ptr, size_t amount)
{
//return memory to linked list.
}
};

Finally, check out boost's pool library:
http://www.boost.org/libs/pool/doc/index.html

Tom

C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html

[pandy.song]

Thank you very much.

I tried both methods, they work great well.
"tom_usenet" <to********@hotmail.com> wrote in message
news:7d********************************@4ax.com...

On Mon, 15 Dec 2003 20:03:30 +0800, "pandy.song" <pa****@263.net>
wrote:

In my project, there is a class "Father",
from which lots of "CHILD" class are inherited.

For the system is real-time embedded system,
"new" operation is applied here.

I want to pre-allocate a series of general
"CHILD" instances. Every time I want a specified
"CHILD" , I get one from a link list.

In C++, virtual class "FATHER" can not be
instanced.So the method I can bring to my mind
is to instance each "CHILD" class, and every
time I want a specified "CHILD", I get a memory
From a link list, and "memcpy" it.

This "memcpy" copy not only internal data ,but
virtual funtion table.

You can't safely memcpy objects of non-POD type. Depending on the
implementation (and presense of e.g. virtual inheritence) it may not
work. This ignores the possible problems of double deletion, etc.

The method I memtion above may not be clever method.

I wander if there are better methods.

Use placement new. E.g. to get a new CHILD1 object, do this:

void* mem = obtain_memory_from_pool(sizeof(CHILD1));
CHILD1* child1 = new (mem) CHILD1(params);

file://to delete:
child1->~CHILD1(); file://(or as a Father*)
return_memory_to_pool(child1);
Or, alternatively, provide new/delete in Father. e.g.

class Father
{
public:
virtual ~Father();
file://etc.

void* operator new(size_t amount)
{
file://return "amount" of memory from linked list
}

void* operator delete(void* ptr, size_t amount)
{
file://return memory to linked list.
}
};

Finally, check out boost's pool library:
http://www.boost.org/libs/pool/doc/index.html

Tom

C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html