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Why Do Multidimensional Arrays Decay to Pointers Differently Than Single-Dimensional Arrays?
Why Do Multidimensional Arrays Decay to Pointers Differently Than Single-Dimensional Arrays?
Oct 26, 2024 am 08:24 AM
Why Arrays Decay into Pointers Differently Depending on Dimensionality
Introduction
When working with arrays and pointers, it's important to understand how type-decay occurs. While you may expect two-dimensional arrays to decay into double pointers, this isn't always the case. Let's delve into why this happens and explore the difference in behavior.
Decay for One-Dimensional Arrays
As the test case demonstrates, one-dimensional arrays indeed decay into single pointers:
<code class="cpp">std::is_same<int*, std::decay<int[]>::type>::value; // true</code>
This is because pointer arithmetic can be performed with a single pointer.
Decay for Multi-Dimensional Arrays
However, two-dimensional arrays don't decay into double pointers:
<code class="cpp">std::is_same<int**, std::decay<int[][1]>::type>::value; // false</code>
The reason is that double pointers require additional information about the dimensions of the array. For instance, in the case of int[5][4], the compiler knows that each "inner" array has a length of 4. Casting to int (*)[4] retains this information, making pointer arithmetic possible.
However, casting to int ** loses this dimension information. It becomes simply a pointer to a pointer, which isn't enough to perform meaningful pointer arithmetic.
Understanding the Difference
Consider the following:
<code class="cpp">char *tmp = (char *)p // Work in units of bytes (char)
+ i * sizeof(int[4]) // Offset for outer dimension (int[4] is a type)
+ j * sizeof(int); // Offset for inner dimension
int a = *(int *)tmp; // Back to the contained type, and dereference</code>
This code manually performs array access, demonstrating that the compiler relies on dimension information. int** doesn't provide this information, making it unsuitable for pointer arithmetic.
Conclusion
While one-dimensional arrays decay into single pointers, multi-dimensional arrays do not decay into double pointers because they lack the necessary dimension information. This behavior ensures that meaningful pointer arithmetic remains possible with single-dimension pointers.
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