Why Can\'t I Use a Variable to Initialize an Array Size in C ?

Variable-Initialized Array Sizes: A Paradox of C Constant Expressions

In C , using a const int to declare the size of an array can be both permissible and forbidden. Consider the following examples:

<code class="cpp">const int size = 2;
int array[size] = {0}; // Valid</code>

<code class="cpp">int a = 2;
const int size = a;
int array[size] = {0}; // Compile error</code>

The crucial difference lies in the initialization of size. The first example uses a constant expression (2) to set size at compile time. This allows the compiler to reserve memory for the array during compilation.

In contrast, the second example uses a variable (a) to initialize size. This is not a constant expression, so the compiler cannot determine the array size until runtime. As a result, memory allocation cannot be performed at compile time, leading to a compilation error.

This restriction is not purely technical. The C committee intentionally forbade variable-initialized array sizes to ensure code stability and efficiency. It eliminates the possibility of dynamically changing the array size, which could lead to undefined behavior and memory corruption.

By limiting array sizes to constant expressions, C developers are forced to determine the size of their arrays during compilation. This eliminates uncertainty and enhances code reliability. While this restriction may seem overly strict, it ultimately contributes to the safety and predictability of C programs.

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