Is `int x = x;` Undefined Behavior in C Due to Lvalue-to-Rvalue Conversion?

Initialization and lvalue-to-rvalue conversion

The provided code snippet raises a commonly debated issue in C . Initializing a variable with itself, as in int x = x;, involves an lvalue-to-rvalue conversion. The question arises whether this conversion leads to undefined behavior (UB) considering that the right-hand 'x' is an uninitialized value, and lvalue-to-rvalue conversion on uninitialized values is generally prohibited.

Evidence of Expected lvalue-to-rvalue Conversion

While the C 11 Standard lacks an explicit specification regarding value categories expected by language constructs, circumstantial evidence suggests that rvalues are intended to be the default expectation.

• Built-in operators consistently require a prvalue operand and explicitly mention lvalue-to-rvalue conversion when necessary.
• The initialization of references implies that lvalue-to-rvalue conversion is expected for object initialization.

Application to Initializer Conversion

By analogy to the behavior of built-in operators, it is reasonable to assume that copy-initialization also expects a prvalue as an initializer. This assumption is further supported by the following:

• The Standard emphasizes using standard conversions to convert the initializer expression to the destination type.
• Performing lvalue-to-rvalue conversion enables the correct initialization of variables with lvalue initializers.
• The lack of user-defined conversions suggests that value category conversions are intended to be handled by the Standard.

Implications for the Code Snippet

Under the assumption that copy-initialization expects a prvalue, the code snippet int x = x; would indeed lead to UB. This is because the right-hand 'x' is an uninitialized lvalue, and its lvalue-to-rvalue conversion would result in an indeterminate value.

Further Evidence

A proposed defect report highlights the need to clarify that lvalue-to-rvalue conversion on objects with an indeterminate value causes UB. This proposed change further supports the notion that copy-initialization should not allow such conversions.

Conclusion

Based on the available evidence, it is most likely that int x = x; constitutes UB in C , as the lvalue-to-rvalue conversion on the uninitialized 'x' is prohibited. However, it is important to note that the Standard lacks a definitive specification on expected value categories, leaving some room for ongoing debate.

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