How Do Multiple Destructors in GNU GCC (g ) Differ and Why Are They Generated?

Multiple Destructors in GNU GCC (g )

Question:

While investigating code coverage in GNU GCC (g ), multiple destructors were observed for both BaseClass and DerivedClass. Why are these multiple destructors generated, and how do they differ?

Answer:

In the Itanium C ABI, three types of destructors are defined:

• D2 (base object destructor): Destroys the object itself, non-virtual base classes, and data members.
• D1 (complete object destructor): Destroys the object, including virtual base classes.
• D0 (deleting object destructor): Destroys the object, calls operator delete to free memory.

Multiple Destructors in the Given Code:

The given code generates multiple destructors because it includes virtual base classes. In this case:

• DerivedClass::~DerivedClass() is the D1 destructor, which destroys the DerivedClass object and its virtual base class (BaseClass).
• DerivedClass::~DerivedClass() is the D2 destructor, which destroys only the DerivedClass object, excluding the virtual base class.
• BaseClass::~BaseClass() is the D1 destructor for the BaseClass subobject within the DerivedClass.

Selective Use of Destructors:

The Itanium C ABI specifies which destructor is executed:

• Deleting a pointer to a base class (e.g., delete b_ptr) invokes D0 (default) or D1 (if the class has a virtual destructor).
• Deleting a pointer to a complete object pointer (e.g., DerivedClass* dptr) invokes D1.

Coverage Implications:

To achieve 100% function coverage, it's important to understand these destructors and invoke them accordingly in unit tests. For example, b_ptr should be deleted as BaseClass* b_ptr to execute D0 and cover both destructors.

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