Does the C Standard Guarantee the Same Size and Memory Layout for `std::array` as for Built-in Arrays?

Does C Standard Define the Size of std::array?

In C 11, std::array offers contiguous storage with performance comparable to regular arrays. However, it's unclear if std::array's size and memory layout mirror that of normal arrays.

Standard Requirements

The C standard (§23.3.2.1/2) defines an array as an aggregate initialized via:

array<T, N> a = { initializer-list };

As an aggregate, std::array cannot use constructors to convert data in the initializer-list. This implies that it primarily stores the actual data values.

Potential Implementation-Specific Behavior

Technically, it's possible for an std::array to include auxiliary data or non-standard alignment, which would deviate from a normal array's behavior.

For instance, a compiler could add a sentinel value at the end of an std::array to detect out-of-bounds write attempts. Alternatively, super-alignment (e.g., Intel SSE instructions) could be supported by an std::array but not by a built-in array.

Expected Behavior

In general, you can expect the following code to behave as intended:

std::vector< std::array<int, N> > x(M);
typedef int (*ArrayPointer)[N];
ArrayPointer y = (ArrayPointer) &amp;x[0][0];
// Use y as a multidimensional array

Most compilers (e.g., GNU and Intel) adhere to this behavior. However, it's important to note that the standard does not explicitly guarantee identical memory layout for std::array and normal arrays.

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