Why is Initializing a Variable with Itself Syntactically Valid But Behaviorally Undefined?

Why is Initializing a Variable By Itself Valid?

Consider the following code snippet:

<code class="cpp">int a = a;</code>

This code initializes the variable a with the value of itself, but why is this syntax even valid?

Syntactic Validity

The syntax of this initialization is valid because the point of declaration of a comes before its initializer, making the name a available for use within its own initialization. This allows for less ambiguous initializations like:

<code class="cpp">void *p = &p;</code>

In this case, the name p is used to refer to itself.

Behavioral Invalidity

While syntactically valid, the behavior of initializing a variable with its own uninitialized value is undefined. This is because using the value of an uninitialized object can lead to unpredictable results. Most compilers will issue a warning for such cases, though it may not be an error requiring diagnosis.

Reasons for Validity

The reason for allowing this syntax is to provide flexibility in initialization scenarios. For instance, it allows for circular references and self-referential initialization. However, it is important to note that such initialization can result in undefined behavior if the variable is used without being properly initialized elsewhere in the code.

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