Why Can\'t Array Sizes Be Initialized with `const int` Variables in C ?

Why is Array Size Limitation with Const Int Sometimes Enforced?

In C , array sizes can be specified using constant integers. However, this flexibility has limitations, as demonstrated by the following examples:

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

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

Why is it that the first example compiles successfully while the second fails?

Explanation

The C standard dictates these array size limitations based on the nature of the expression used to initialize the size.

Constant Expression

In the first example, const int size = 2; is a constant expression because the value of size is known at compilation time. Since the compiler knows the array size, it can allocate the necessary memory during compilation.

Non-Constant Expression

In the second example, const int size = a; is not a constant expression because the value of a is not determined until runtime. This prevents the compiler from knowing the array size during compilation, making it impossible to allocate memory at compile time.

Immutability is Irrelevant

Note that the second example still has an effectively constant value for size, but this is not considered by the compiler. The rules focus on the type of expression used, and int a = 2; uses mutable variables, making it a non-constant expression.

Flow Analysis Complexity

Allowing runtime initialization for compile-time allocation would require flow analysis. The compiler would need to differentiate between expressions like:

<code class="cpp">int a = 2;
const int size = a;</code>

and

<code class="cpp">int a = foo();
const int size = a;</code>

where the size expression is identical, but the actual value depends on runtime behavior. This complexity is deemed unnecessary by the C committee.

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