Why Does Initializing a Variable with Itself Lead to Undefined Behavior in C/C ?

Using Newly Declared Variable in Initialization (int x = x 1)

In C/C , the behavior of declaring a variable and initializing it using the same variable's value (e.g., "int x = x 1;") may seem surprising. This article delves into the mechanics and implications of such a declaration.

Variable Declaration and Initialization

Variable declaration and initialization are two distinct steps in C/C . Declaration creates a name for a variable, while initialization assigns a value to that variable. In the example provided ("int x = x 1;"), the variable x is declared with an initial value.

The Mechanics

This behavior is allowed because the variable x comes into existence at the point of initialization (the '=' sign). This means that it's accessible for use on the right-hand side of the initialization expression. It acts as an uninitialized variable until the assignment is complete.

However, it's important to note that this behavior is only allowed for variables with static storage duration (i.e., declared outside a function). For locally declared variables (e.g., within a function), using an uninitialized variable on the right-hand side of the initialization expression results in undefined behavior.

C Standard Definition

The C 03 standard states that "the point of declaration for a name is immediately after its complete declarator ... and before its initializer (if any)." This means that the second x in the example provided is initialized with its own uninitialized value, resulting in undefined behavior.

Conclusion

While it's possible to declare and initialize a variable using its own value, this practice should be avoided. It can lead to undefined behavior in certain situations and is generally considered poor coding style. Prefer to declare and initialize variables separately to ensure clarity and predictability in your code.

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