How Does `delete[]` Know the Size of the Array It's Deallocating?

How Memory Allocators Know the Array Size for Delete[]

In C , the delete[] operator is used to deallocate an array of objects from memory. However, unlike the new[] operator, which explicitly requires the array size, delete[] seemingly operates without this information. How does it determine the size of the array?

When you allocate memory on the heap using new[], the memory allocator allocates a contiguous block of memory for the array elements and stores additional information about the allocation, including its size. This information is typically stored in a header immediately preceding the allocated memory.

Standardization

The method of storing the array size with the allocated memory is not standardized in C itself. Different memory allocators implement various techniques:

• Heap Metadata: Some allocators use an in-memory data structure to track allocation details, including array sizes. This data structure is managed by the allocator and is not directly visible to the program.
• Memory Header: As mentioned earlier, many allocators store the array size and other information in a header immediately before the allocated memory. This header is usually prefixed with a unique tag or flag to distinguish it from the actual data.
• Special Pointer Value: Some allocators may use a special value in the array pointer to indicate the end of the array. For example, the pointer might be set to a null pointer when the array is fully deallocated.

Retrieval of Array Size

During deletion, delete[] retrieves the array size from the associated header or memory allocator. By knowing the size, delete[] can efficiently deallocate the entire array of objects.

Conclusion

In summary, the memory allocated by new[] carries information about its size, either through heap metadata, memory headers, or pointer values. This information is used by the delete[] operator to determine the array size during deallocation, enabling efficient memory management without the need to explicitly provide the size.

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