Imagine: the future

There were two competing proposals, Tom MacDonald's Variable Length Arrays (http://jclt.iecc.com/Jct13.pdf#page=67), and Ritchie's Variable-Size Arrays (http://jclt.iecc.com/Jct22.pdf#page=5). The committee eventually went with MacDonald's, apparently. Both proposals seemed to emphasize the utility for numerical computing a la Fortran, not for automatic bounds checking. Also, neither resolved the function parameter syntax problem--i.e. array parameter decay. In Ritchie's proposal, same as in MacDonald's, you were (syntactically) passed a pointer to an array, which required (syntactic) dereferencing. Ritchie's example prototype: foo(int (*a)[?][?]). MacDonald's (and VLAs since C99): foo(int n, int m, int (*a)[n][m]). In both cases sizeof (*a) worked as expected, but resolving at runtime.

Notably, Ritchie's proposal lacked variable size automatic storage arrays. With the VLA proposal, you can declare arrays on the stack: int a[n][m];. In Ritchie's proposal you were required to use malloc (or alloca?): int (*a)[?][?] = (int (*)[n][m])
malloc(n * m * sizeof (int)));. I don't think this was an intrinsic limitation; I think Ritchie just objected to the ambiguity of how and when sizeof evaluated the integral expression(s) used in the type definition (i.e. n and m above). MacDonald's paper mentions that the compiler would have to cache the evaluated value so it reflected the value of the type at it's declaration; subsequently modifying n or m wouldn't change the result of sizeof. Except when using VLAs in parameters, or with variable length structures (part of the original proposal), this wasn't quite true (or true but irrelevant), something fat pointers avoids.

It would have been better if we ended up with Ritchie's proposal. But I surmise (based on those papers alone) that MacDonald's won the day because 1) it was easier to implement--no ABI changes nor introduction of fat pointers into the compiler architecture; 2) GCC seemed to already have most of the implementation already, albeit with a slightly different syntax; 3) Ritchie gave short shrift to declaring automatic storage variable size arrays; 4) MacDonald was working at Cray so presumably was deemed to speak for pressing industry demands. #3 and #4 seem especially pivotal given everybody's preoccupation with numerical computing rather than bounds safety per se.