Saturday, April 7, 2012

#1 famous undefined behavior C++

(See below for smart ptr, template, non-RTTI)

Deleting [1] a derived[3] object via a base[4] pointer is undefined behavior if base[6] class has non-virtual dtor.

This is well-known but it applies to a very specific situation. Many similar situations aren't described by this language rule --
[1a] This rule requires pointer delete. In contrast, automatic destruction of a non-ref "auto" variable (on stack) is unrelated.

[1b] This rule requires a heap object. Deleting a pointee on stack is a bug but it's outside this rule.

[1c] This rule is about delete-expression, not delete[]

[3] if the object's run-time type is base, then this rule is Inapplicable

[4] if the pointer is declared as pointer-to-derived, then Inapplicable

[3,4] if the object run time type is base, AND pointer is declared pointer-to-derived? Inapplicable -- compiler or runtime would have failed much earlier before reaching this point.

[6] what if derived class has non-virtual dtor? Well, that implies base non-virtual too. So Yes applicable.


*) P62 [[effC++]] points out that even in the absence of virtual functions (i.e. in a world of non-RTTI objects), you can still hit this UB by deleting a subclass instance via a base pointer.

**) The same example also shows a derived class-template is considered just like a derived class. Let me spell out the entire rule -- deleting an instance of a derived-class-template via a pointer to base-class-template is UB if the base class-template has a non-virtual dtor.

What if the pointee is deleted by a smart_ptr destructor? I think you can hit this UB.

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