Determining Array Size After Dynamic Allocation in C++

In C++, arrays allocated dynamically using the new operator do not inherently expose their size programmatically. This question arises from the observation that delete[] must be aware of the allocated array's size to deallocate memory effectively.

Why No Built-in Function to Obtain Array Size?

Unlike arrays declared on the stack, whose size can be determined using sizeof(), dynamically allocated arrays' size is unknown during compilation. This is because the runtime or operating system's memory manager manages the allocated memory, not the compiler. sizeof() operates as a compile-time constant and cannot dynamically evaluate sizes of dynamically allocated arrays.

Alternative Strategies

Even though C++ does not provide a built-in function to obtain the array's size, there are alternative approaches:

• Pointer Semantics: Arrays in C++ decay to pointers. By manipulating the pointer, one can infer the array's size through subtraction, as demonstrated in the following example:

int *arr = new int[256];
int *p = &arr[100];
int size = p - arr; // Subtracting pointers yields the size

• Custom Memory Management: If control over memory management is desired, one can implement a custom memory management scheme that tracks allocated memory and its sizes. This approach necessitates careful handling and introduces its own complexities.

Implications

The lack of a standardized way to determine the size of dynamically allocated arrays in C++ stems from the language's focus on memory management flexibility and platform independence. While it presents limitations in certain scenarios, it also allows for diverse implementation strategies and optimization opportunities.