C++17 creates a practical use of the backward array index operator

Re:C++17 creates a practical use of the backward array index... by olliej on Monday April 03, 2023 @04:19PM (#35430670)

What this is talking about is the behavior of [] in C and C++. In these

    x[y]

and

    y[x]

are the same. I was first introduced to this by a friend of mine many many years ago (because I'm an old) as

    for (i = 0; etc) 
       putc(i["hello world"])

or similar nonsense.

The C++ change that makes this matter is that apparently pre c++17 doesn't enforce sequencing such that expression1[expression2] doesn't require expression1 be evaluated before expression2. C++17 does actually fix the sequencing to be left to right, so now expression1[expression2] will always evaluate as expression1;expression2 and expression2[expression1] will always evaluated as expression2;expression1 without depending on UB.

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Re:C++17 creates a practical use of the backward array index... by jbverschoor on Monday April 03, 2023 @04:22PM (#35430721)

I’ve never seen that before. Luckily

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Re:C++17 creates a practical use of the backward array index... by olliej on Monday April 03, 2023 @04:25PM (#35430752)

He was also a tutor/TA and I think mostly used it to explain how pointers vs. indexing worked. Or possibly to traumatize students :D

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Re:C++17 creates a practical use of the backward array index... by int_19h on Monday April 03, 2023 @07:17PM (#35432724)

It is extremely rare even in ancient C code. The only reason why it exists in C at all is because rewriting a[i] as *(a+i) was inherited by C from B; the latter only has a single machine word type for everything, so there was no way to define it such that a[i] would be legal but i[a] would be illegal in B.

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Re:C++17 creates a practical use of the backward array index... by HelloNurse on Tuesday April 04, 2023 @04:47AM (#35436852)

In the competing array indexing syntax

  *(a+b)

there is the same evaluation order issue, and they should have the same behaviour: is a evaluated before b or the compiler is allowed to choose an order? Clearly it depends on the rules for

a+b

which are more important than the special case of array indexing.

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