The behavior is undefined because the destructor is invoked twice for the same object:
- Once when you invoke it explicitly
- Once when the scope ends and the automatic variable is destroyed
Invoking the destructor on an object whose lifetime has ended results in undefined behavior per C++03 §12.4/6:
the behavior is undefined if the destructor is invoked for an object whose lifetime has ended
An object's lifetime ends when its destructor is called per §3.8/1:
The lifetime of an object of type T
ends when:
— if T
is a class type with a non-trivial destructor (12.4), the destructor call starts, or
— the storage which the object occupies is reused or released.
Note that this means if your class has a trivial destructor, the behavior is well-defined because the lifetime of an object of such a type does not end until its storage is released, which for automatic variables does not happen until the end of the function. Of course, I don't know why you would explicitly invoke the destructor if it is trivial.
What is a trivial destructor? §12.4/3 says:
A destructor is trivial if it is an implicitly-declared destructor and if:
— all of the direct base classes of its class have trivial destructors and
— for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial destructor.
As others have mentioned, one possible result of undefined behavior is your program appearing to continue running correctly; another possible result is your program crashing. Anything can happen and there are no guarantees whatsoever.