dcsimg
Login | Register   
RSS Feed
Download our iPhone app
TODAY'S HEADLINES  |   ARTICLE ARCHIVE  |   FORUMS  |   TIP BANK
Browse DevX
Sign up for e-mail newsletters from DevX

By submitting your information, you agree that devx.com may send you DevX offers via email, phone and text message, as well as email offers about other products and services that DevX believes may be of interest to you. DevX will process your information in accordance with the Quinstreet Privacy Policy.


Tip of the Day
Language: C++
Expertise: Intermediate
Jun 7, 1999

WEBINAR:

On-Demand

Application Security Testing: An Integral Part of DevOps


Accessing a C++ Object in C Code: Support for Virtual Member Functions

In the case of virtual functions, an additional member is inserted into the class: a pointer to the virtual table, or _vptr. The _vptr holds the address of a static table of function pointers. The exact position of the _vptr among the class' data members is implementation-dependent. Traditionally, it was placed after the user-declared data members. However, some compilers (Visual C++, for instance) have moved it to the beginning of the class for performance reasons. Theoretically, the _vptr can be located anywhere inside the class--even among user-declared members. Consider:

 
  class PolyDate
  {
  public:
    int day;
    int month;
    int year;
  
    Date();
    virtual ~Date();  //polymorphic
    bool isLeap() const;
    bool operator == (const Date& other);
  };

Defining a C struct that corresponds to the binary representation of a PolyDate object is more precarious in this case and requires intimate acquaintance with the compiler's preferred position of the _vptr as well as with its size. Such a C struct might look like this code on some implementations:

 
  struct POD_Date
  {
    int day;
    int month;
    int year;
    void * do_not_touch; /* the _vptr */
  };

However, on other implementations (Visual C++, for instance) it might look like this:

 
  struct POD_Date
  {
    void * do_not_touch; /* the _vptr */
    int day;
    int month;
    int year;
    void * do_not_touch; //
  };

Another hazard here is that the value of the _vptr is transient, which means that it might have a different value, according to the address space of the process that executes the program. Consequently, when an entire polymorphic object is stored in a file and retrieved later, the retrieved data cannot be used as a valid object. For all these reasons, accessing polymorphic objects from C code is dangerous and generally needs to be avoided.

Danny Kalev
 
Comment and Contribute

 

 

 

 

 


(Maximum characters: 1200). You have 1200 characters left.

 

 

Sitemap
Thanks for your registration, follow us on our social networks to keep up-to-date