Does C Initialization Require Lvalue-to-Rvalue Conversion?

Does Initialization Entail Lvalue-to-Rvalue Conversion?

The C 11 Standard remains ambiguous on whether initialization requires lvalue-to-rvalue conversion. However, evidence suggests that rvalues are expected as operands in built-in operators, unless specified otherwise. This approach has been extended to the context of initializers.

Conjecture: Prvalues Expected for Values

The assumed specification is that prvalues are expected wherever a value is needed, including during initialization. This implies that an lvalue-to-rvalue conversion is required when initializing an object.

Consequences of the Conjecture

If initialization expects prvalues, the following occur:

• int x = x; undergoes lvalue-to-rvalue conversion, leading to undefined behavior because x is uninitialized.
• Ill-formedness for code like int y = 0; int x = y;, where y is an lvalue.
• Inconsistent behavior, e.g. z = y being undefined behavior but z = x not being so, assuming x == y.

Further Evidence

Additional evidence supporting this conjecture includes:

• Paragraph 8.5/16 of the Standard implies the need for "standard conversions" but does not mention category conversions.
• Paragraph 4.1 mentions undefined behavior when applying lvalue-to-rvalue conversion to an uninitialized object.
• A defect report proposes rewording Paragraph 4.1 to include objects with indeterminate values, including uninitialized objects.

Conclusion

Based on the provided evidence, the most likely interpretation is that initialization expects prvalues, resulting in undefined behavior when using uninitialized lvalues as initializers. This interpretation aligns with the broader approach of expecting rvalues in built-in operators unless specified otherwise.

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