Why Does GCC (g ) Generate Multiple Destructors for the Same Class?

GNU GCC (g ): Why Does It Generate Multiple Destructors?

When inspecting the symbol table of a compiled C program, you may observe multiple destructors (dtors) generated for the same class. Understanding this phenomenon is crucial for achieving comprehensive code coverage in unit tests.

Reasons for Multiple Destructors:

The GCC compiler generates multiple destructors for the following reasons:

• Virtual Inheritance: Virtual base classes require the generation of an additional dtor to destruct them correctly.
• Optimization: The compiler may create aliases for dtors with the same implementation to improve performance.

Types of Destructors:

GCC generates three types of dtors:

• Base Object Destructor (D2): Destroys the object, data members, and non-virtual base classes.
• Complete Object Destructor (D1): Additionally destroys virtual base classes.
• Deleting Object Destructor (D0): Calls the operator delete function to free the memory.

How Destructors Are Used:

Normally, the operator delete function will call the correct dtor based on the type of object being deleted. However, you can explicitly invoke specific dtors during testing to ensure complete coverage.

Example:

The following code demonstrates the multiple dtors generated for the BaseClass and DerivedClass:

<code class="cpp">class BaseClass { public: ~BaseClass(); void someMethod(); };
class DerivedClass : public BaseClass { public: virtual ~DerivedClass(); virtual void someMethod(); };</code>

Upon compiling with GCC (g ), you will notice three dtors for DerivedClass in addition to two for BaseClass, as shown by the nm command.

Conclusion:

The generation of multiple dtors is a feature of GCC to handle different scenarios, including virtual inheritance and optimization. Understanding the purpose and usage of these dtors is essential for achieving 100% function coverage in your unit tests.

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