Does `std::array` Have the Same Size and Memory Layout as a Built-in Array?

Is the Size and Memory Layout of std::array Identical to an Array?

Despite the standard defining std::array with contiguous storage and performance comparable to a regular array, it leaves some room for ambiguity regarding its exact size and memory layout.

Standard Requirements

Section 23.3.2.1/2 of the standard states that an array is an aggregate initialized with an initializer list containing up to N elements convertible to T. This suggests that std::array must store the values themselves, excluding any auxiliary data.

Potential Variations

However, the standard does not explicitly guarantee that std::array will have the same size and memory layout as a built-in array. Theoretically, a compiler could store additional auxiliary data after the specified data or apply different padding or alignment rules.

Super-Alignment Considerations

Built-in arrays cannot support super-alignment requirements, such as data for Intel's SSE instructions. While std::array's specification might allow for super-alignment, it is not explicitly guaranteed.

Implementation-Specific Implications

Conclusion: The standard's requirements do not definitively impose that std::array must have the same size and memory layout as a built-in array. While documentation suggests that it should be equally memory efficient, it remains implementation-specific. Therefore, one cannot rely on the assumption of identical sizeof and memory layout.

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