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Tip of the Day
Language: C++
Expertise: Intermediate
Oct 26, 1999

Force an Object to Destroy Itself

Sometimes, you need to force an object to destroy itself because its destructor performs an operation needed immediately. For example, when you want to release a mutex or close a file:
 
  void func(Modem& modem)
  {
      Mutex mtx(modem); // lock modem
      Dial("1234567");
      /* at this point, you want to release <br>the modem lock by invoking Mutex's dtor */  
      do_other_stuff(); //modem is still locked
  } //Mutex dtor called here
After the function Dial() has finished, the modem no longer needs to be locked. However, the Mutex object will release it only when func() exits, and in the meantime, other users will not be able to use the modem. Before you suggest to invoke the destructor explicitly, remember that the destructor will be called once again when func() exits, with undefined behavior:
 
void func(Modem& modem)
{
    Mutex mtx(modem); // lock modem
    Dial("1234567");
    mtx->~Mutex(); // very bad idea!
    do_other_stuff(); 
} //Mutex dtor called here for the second time
There is a simple and safe solution to this. You wrap the critical code in braces:
 
void func(Modem& modem)
 {
    {
      Mutex mtx(modem); // lock modem
      Dial("1234567");
    } //mtx destroyed here
     //modem is not locked anymore
    do_some_other_stuff(); 
}
Danny Kalev
 
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