Why Were Designated Initializer Lists Excluded from C 11?

Designated Initializer List Exclusion in C 11

C 11 lacks support for designated initializer lists, a feature present in C99. This omission has puzzled some developers, who wonder why such a convenient syntax was not included in the newer standard.

Rationale

The C standard committee has consistently rejected proposals to incorporate designated initializer lists into the language. Their rationale stems from several inherent challenges:

Evaluation Order Indeterminacy:

In C99, the order of evaluation for designated initializer subexpressions is indeterminately sequenced. This can lead to unexpected behavior, especially when the initialization involves interacting functions or side effects.

Compatibility Breach:

If C were to implement designated initializer lists with indeterminate evaluation order, it would break compatibility with existing C99 implementations, which typically provide a consistent evaluation order.

Feasibility Concerns:

The standard committee has repeatedly expressed doubts about the feasibility of implementing designated initializer lists in a way that meets the strict initializer-list requirements of C (11.6.4[dcl.init.list]4).

Designator Limitations in C 20

While designated initializer lists remain excluded from C 11 and C 17, the C 20 standard introduces partial support with limitations to ensure determinacy and compatibility. These limitations include:

• Only data members declared in the order of their initialization can be designated.
• Array and nested initialization is not supported.
• All or none of the data members must be initialized by designators.

Conclusion

The exclusion of designated initializer lists from C 11 was a deliberate decision by the standard committee, motivated by concerns over evaluation order indeterminacy, compatibility issues, and implementation challenges. While limited support was eventually added in C 20, the C standard continues to focus on enforcing strict initializer order and avoiding unpredictable behavior that could compromise program correctness or portability.

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