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The memory required by an object of some base class is usually less than the memory required for an object of a class you derive from that base class (because the derived classes usually add more fields). Therefore, you cannot allow the base class constructor to allocate the storage when you call it from inside the derived class's constructor. You can easily solve this problem by checking ESI within the derived class constructor and allocating any neces- sary storage for the object before calling the base class constructor. The second problem is the initialization of the VMT pointer. When you call the base class's constructor, it will initialize the VMT pointer with the address of the base class's virtual method table. A derived class object's_PVMT_ field, however, must point at the virtual method table for the derived class. Calling the base class constructor will always initialize the _pVMT_ field with the wrong pointer. To properly initialize the _pVMT_ field with the appropriate value, the derived class constructor must store the address of the derived class's virtual method table into the PVMT_ field after the call to the base class constructor (so that it overwrites the value written by the base class constructor).

The memory required by an object of some base class is usually less than the memory required for an object of a class you derive from that base class (because the derived classes usually add more fields). Therefore, you cannot allow the base class constructor to allocate the storage when you call it from inside the derived class's constructor. You can easily solve this problem by checking ESI within the derived class constructor and allocating any neces- sary storage for the object before calling the base class constructor. The second problem is the initialization of the VMT pointer. When you call the base class's constructor, it will initialize the VMT pointer with the address of the base class's virtual method table. A derived class object's_PVMT_ field, however, must point at the virtual method table for the derived class. Calling the base class constructor will always initialize the _pVMT_ field with the wrong pointer. To properly initialize the _pVMT_ field with the appropriate value, the derived class constructor must store the address of the derived class's virtual method table into the PVMT_ field after the call to the base class constructor (so that it overwrites the value written by the base class constructor).

Computer Networking: A Top-Down Approach (7th Edition)
Computer Networking: A Top-Down Approach (7th Edition)
7th Edition
ISBN:9780133594140
Author:James Kurose, Keith Ross
Publisher:James Kurose, Keith Ross
Chapter1: Computer Networks And The Internet
Section: Chapter Questions
Problem R1RQ: What is the difference between a host and an end system? List several different types of end...
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The memory required by an object of some base class is usually less than the memory required for an object of a class you derive from that base class (because the derived classes usually add more fields). Therefore, you cannot allow the base class constructor to allocate the storage when you call it from inside the derived class's constructor. You can easily solve this problem by checking ESI within the derived class constructor and allocating any neces- sary storage for the object before calling the base class constructor. The second problem is the initialization of the MT pointer. When you call the base class's constructor, it will initialize the VMT pointer with the address of the base class's virtual method table. A derived class object's_PVMT_ field, however, must point at the virtual method table for the derived class. Calling the base class constructor will always initialize the _PVMT_ field with the wrong pointer. To properly initialize the _PVMT_ field with the appropriate value, the derived class constructor must store the address of the derived class's virtual method table into the pVMT_field after the call to the base class constructor (so that it overwrites the value written by the base class constructor).
Transcribed Image Text:The memory required by an object of some base class is usually less than the memory required for an object of a class you derive from that base class (because the derived classes usually add more fields). Therefore, you cannot allow the base class constructor to allocate the storage when you call it from inside the derived class's constructor. You can easily solve this problem by checking ESI within the derived class constructor and allocating any neces- sary storage for the object before calling the base class constructor. The second problem is the initialization of the MT pointer. When you call the base class's constructor, it will initialize the VMT pointer with the address of the base class's virtual method table. A derived class object's_PVMT_ field, however, must point at the virtual method table for the derived class. Calling the base class constructor will always initialize the _PVMT_ field with the wrong pointer. To properly initialize the _PVMT_ field with the appropriate value, the derived class constructor must store the address of the derived class's virtual method table into the pVMT_field after the call to the base class constructor (so that it overwrites the value written by the base class constructor).
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