Why Can\'t I Initialize an Array Size with a `const int` Variable in C ?

The Elusive Dilemma of Array Size Initialization with const int

In the realm of C , the use of constants for array size initialization often poses a puzzling dilemma. While some situations seamlessly permit this practice, others provoke cryptic compilation errors.

Consider the following code snippets:

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

In this example, the compiler happily allows the declaration of an array with a size specified by the constant variable 'size'. The reason lies in the nature of the initialization expression for 'size'. It is a constant expression, meaning it can be evaluated entirely at compile time. This allows the compiler to determine the exact size of the array during compilation, facilitating memory allocation accordingly.

Contrasting the above, take a look at this snippet:

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

Here, the compiler throws a fit, bemoaning a "compile error." The crux of the problem lies in the expression used to initialize 'size'. It involves a non-constant variable ('a'). This relegates the expression to the realm of non-constant expressions. As a result, the compiler cannot ascertain the array's size at compile time.

This behavior stems from a fundamental decision by the C standards committee. The committee prioritized simplicity and explicitness over flexibility. Implementing the latter would necessitate complex flow analysis, differentiating between non-modifiable expressions like 'const int size = a;' and truly non-constant expressions like 'const int size = foo();'. Such complexity was deemed undesirable.

Thus, the rule remains: array sizes can only be initialized using constant expressions. This ensures that the compiler can determine the array's size unequivocally, ensuring predictable memory allocation and smooth compilation. Always keep this precept in mind when venturing into the world of array size initialization.

The above is the detailed content of Why Can\'t I Initialize an Array Size with a `const int` Variable in C ?. For more information, please follow other related articles on the PHP Chinese website!