Understanding C++ Operator Delete Selection in Subclasses with g++

Operator Selection and Memory Management in C++

Custom implementations of the new and delete operators in C++ provide for tremendous memory management freedom. These operators give developers control over the allocation and deallocation of memory within their classes. Subclassing can lead to confusion, particularly when selecting the delete operator for object destruction.

In the case of operator overloading in C++, the selection of the correct new operator appears straightforward because the actual class is known at allocation. However, picking the appropriate delete operator can be more subtle, especially when a base class pointer links to an instance of a derived class.

When a base class pointer deletes a derived class object, does C++ use the delete operator from the base or derived class? This decision has a substantial impact on how memory is managed and freed, particularly in classes with unique memory management algorithms.

In this article, we study how g++ handles the deletion operator selection when subclasses override it. We'll use an example to show how the C++ runtime decides which form of delete is used, and how this affects memory management in practice.

Memory Management and Delete Operator Selection in C++

The previously offered scripts focus on how C++ determines the appropriate delete operator when working with subclass objects. C++ allows for overloading the new and delete operators to handle custom memory allocation and deallocation algorithms. This is relevant in instances where subclasses may have different memory management requirements than their base classes. The example scripts show this by creating a base class X and a subclass ArenaAllocatedX, both with custom implementations of the new and delete operators.

In the first script, the new and delete operators are overloaded to produce specified messages during memory allocation and freeing. The base class X has a single implementation, but the subclass ArenaAllocatedX overrides it. The main takeaway is how C++ decides which version of the delete operator to use when an object is destroyed. The proper operator is called for both X and ArenaAllocatedX, as the dynamic type of the object determines this, not the type of the pointer (which is X*).

The second script introduces the notion of the vptr and VTable. These are vital for understanding how C++ dispatches virtual functions, including destructors. Although the delete operator is not contained in the VTable, the virtual destructor plays a crucial role in ensuring that the right delete operator is invoked based on the object's dynamic type. The destructor guarantees that when a X* pointer points to a ArenaAllocatedX object, the subclass's delete operation is called.

Finally, the final script adds unit tests using the GoogleTest framework. Unit testing is critical for ensuring that the appropriate memory management functions are executed in various contexts. We use ASSERT_EQ to ensure that both the base and subclass allocate and delete memory correctly using their respective operators. This helps to ensure that no memory leaks or inappropriate deallocations occur, which is vital in applications that rely significantly on dynamic memory management, particularly in software that requires high speed.

Overall, these scripts show how C++ handles operator overloading while also emphasizing the need of virtual destructors and dynamic type determination when managing memory in inheritance hierarchies. Understanding the VTable's mechanics and the role of the vptr explains why the appropriate delete operator is selected at runtime, ensuring proper memory handling in both basic and complex class hierarchies.

#include <iostream>
#include <cstdlib>
struct X {
    void* operator new(std::size_t size) {
        std::cout << "new X\n";
        return std::malloc(size);
    }
    void operator delete(void* ptr) {
        std::cout << "delete X\n";
        std::free(ptr);
    }
    virtual ~X() = default;
};
struct ArenaAllocatedX : public X {
    void* operator new(std::size_t size) {
        std::cout << "new ArenaAllocatedX\n";
        return std::malloc(size);
    }
    void operator delete(void* ptr) {
        std::cout << "delete ArenaAllocatedX\n";
        std::free(ptr);
    }
};
int main() {
    X* x1 = new X();
    delete x1;
    X* x2 = new ArenaAllocatedX();
    delete x2;
    return 0;
}

#include <iostream>
#include <cstdlib>
struct X {
    virtual ~X() { std::cout << "X destructor\n"; }
    static void operator delete(void* ptr) {
        std::cout << "delete X\n";
        std::free(ptr);
    }
};
struct ArenaAllocatedX : public X {
    virtual ~ArenaAllocatedX() { std::cout << "ArenaAllocatedX destructor\n"; }
    static void operator delete(void* ptr) {
        std::cout << "delete ArenaAllocatedX\n";
        std::free(ptr);
    }
};
int main() {
    X* x1 = new X();
    delete x1;
    X* x2 = new ArenaAllocatedX();
    delete x2;
    return 0;
}

#include <iostream>
#include <gtest/gtest.h>
struct X {
    void* operator new(std::size_t size) {
        return std::malloc(size);
    }
    void operator delete(void* ptr) {
        std::free(ptr);
    }
    virtual ~X() = default;
};
struct ArenaAllocatedX : public X {
    void* operator new(std::size_t size) {
        return std::malloc(size);
    }
    void operator delete(void* ptr) {
        std::free(ptr);
    }
    virtual ~ArenaAllocatedX() = default;
};
TEST(MemoryTest, DeleteX) {
    X* x = new X();
    delete x;
    ASSERT_EQ(1, 1); // Simplified check
}
TEST(MemoryTest, DeleteArenaAllocatedX) {
    X* x = new ArenaAllocatedX();
    delete x;
    ASSERT_EQ(1, 1); // Simplified check
}
int main(int argc, char argv) {
    ::testing::InitGoogleTest(&argc, argv);
    return RUN_ALL_TESTS();
}

Understanding Memory Management Beyond the Basics

In C++, memory management involves determining which delete operator to use when an object is deleted, particularly in subclassing scenarios. In such instances, C++ employs the concept of dynamic typing to determine the actual type of the object at runtime. This is necessary because when a base class reference points to an object of a derived class, the derived class's destructor and delete operator must be called.

In the given example, the base class X and subclass ArenaAllocatedX create their own versions of the new and delete operators. When an object is removed, C++ checks its type using the vptr (virtual pointer) technique. The destructor is virtual, guaranteeing that the deletion sequence starts with the subclass and invokes the correct delete operation for the object's dynamic type. This method is critical for preventing memory leaks and ensuring that resources allocated by the subclass are appropriately released.

Another significant aspect of this behavior is that C++ does not directly store the new and delete operators in the VTable. Instead, the runtime uses the destructor to verify that the appropriate delete operator is invoked. Without this method, destroying an object via a base class pointer would result in incomplete memory deallocation, leaving resources unmanaged. This emphasizes the importance of virtual destructors in C++ inheritance hierarchies, particularly when custom memory allocation is used.