[Boost-users] [corrected]How to delay the initialisation of a member in a C++ base class until the ctor of derived class is executed?

Subject: [Boost-users] [corrected]How to delay the initialisation of a member in a C++ base class until the ctor of derived class is executed?
From: tom tan (ttan_at_[hidden])
Date: 2009-03-25 05:02:08

Here's the scenario: I want define a base class that allocate some
buffer once for all derived classes, while the size of the buffer varies
among different derived classes. I could achieve it in this way:

class base
{
public:
     base():size(), p_array(0){}
private:
     size_t size;
     boost::shared_array<unsigned char> p_array;

};

in the derived class:

class derived
{
public:
     derived(size_t array_size):size(array_size)
     {
          p_array.reset(new unsigned char[size]);
     }    
};

However, in order to simplify the design of the derived class, I really
want to put this line:

p_array.reset(new unsigned char[size]);

to somewhere in the base class, thus writing it only once.

In other words,I wished to achieve this: 
1) the buffer is allocated inside the base class while 
2) the size is designated inside the derived class.

Is there any C++ design pattern could achieve it? Thanks