Why does `int[]` decay into `int*` but not `int[][]`?

Why Does int[] Decay into int* but Not int[][]?

In C , arrays exhibit unique decay rules that differ based on their dimensionality and the presence of pointers. This phenomenon can be observed when comparing the behavior of one-dimensional arrays (int[]) and two-dimensional arrays (int[][]) when it comes to decay.

One-Dimensional Arrays: int*[] decays into int

When a one-dimensional array, such as int[], is subject to decay, it behaves like a pointer to the first element. This is evident in the following code:

<code class="cpp">std::is_same<int*, std::decay<int[]>::type>::value; // true</code>

This code returns true because the decay of int[] yields int*, indicating that it behaves like a pointer to an integer.

Two-Dimensional Arrays: int[][1] does not decay into int

In contrast to one-dimensional arrays, two-dimensional arrays (int[][]) do not decay into pointers to pointers (int**). Instead, they preserve their array nature. This is demonstrated in the following code:

<code class="cpp">std::is_same<int**, std::decay<int[][1]>::type>::value; // false</code>

This code evaluates to false, indicating that the decay of int[][1] does not produce int**.

The Role of Pointers in Array Decay

The key difference between these two cases lies in the involvement of pointers. When a one-dimensional array is declared with a pointer type, such as int[], it effectively becomes a pointer to an array. This explains why it decays into int*, which is a pointer to a pointer.

<code class="cpp">std::is_same<int**, std::decay<int*[]>::type>::value; // true</code>

Reasoning: Why Not int[][]?

The reason why two-dimensional arrays do not decay into pointers to pointers is because it would lead to difficulties in performing pointer arithmetic. In a two-dimensional array, each element is stored at a specific offset within a contiguous block of memory. To access an element, the compiler needs to know the size of both dimensions. If it were to decay into int**, it would lose this crucial information and make pointer arithmetic impossible.

The above is the detailed content of Why does `int[]` decay into `int*` but not `int[][]`?. For more information, please follow other related articles on the PHP Chinese website!