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c++cvisual-studio-2008language-lawyerlvalue-to-rvalue

lvalue-to-rvalue conversion of an array in ISO C

C++ ANSI ISO IEC 14882 2003 Annex C.1 (page 668):

Change: The result of a conditional expression, an assignment expression, or a comma expression may bean lvalue
Rationale: C + + is an object-oriented language, placing relatively more emphasis on lvalues. For example, functions may return lvalues.
Effect on original feature: Change to semantics of well-defined feature. Some C expressions that implicitly rely on lvalue-to-rvalue conversions will yield different results. For example, 

char arr[100];
sizeof(0, arr)

yields 100 in C + + and sizeof(char*) in C.
...

I was reading this just today and I remembered that a couple of months a go a friend of mine proposed a problem wchich was to write a function that would return 0 if it were compiled with C++ and 1 if it were compiled with C. I solved it taking advantage of the fact that in C a struct was in the outer scope. So, considering this new information, I decided that this would be another solution to the above problem, which I tried on Microsoft Visual Studio 2008, but regardless of whether it is compiled as C or C++ code sizeof(0, arr) always yields 4. So 2 questions:

1.What is ISO C? Is it the current C standard? Is it the only one (I hear C is rapidly evolving) 2. Is this a microsoft C++ bug?

TIA

Edit: Sorry got mixed up with the output and edited it:

Solution

  • Or just microsoft's C is not ISO C but some other standard C (if there exists any).

    Microsoft Visual C still supports C89 [only] whereas other compilers like gcc/clang etc support C99 too which is the current Standard.

    C99 [Section 6.5.17/2] says

    The left operand of a comma operator is evaluated as a void expression; there is a sequence point after its evaluation. Then the right operand is evaluated; the result has its type and value.95

    Thus the result of sizeof (0,arr) would be sizeof(char*)[due to the implicit lvalue to rvalue conversion /automatic decay to pointer type] not 100*sizeof(char)

    sizeof(arr) would have given 100*sizeof(char) from 6.5.3.4/3

    95) A comma operator does not yield an lvalue.

    decided that this would be another solution to the above problem, which I tried on Microsoft Visual Studio 2008, but regardless of whether it is compiled as C or C++ code sizeof(0, arr) always yields 4.

    C++03 [5.18/1] Comma Operator

    The type and value of the result are the type and value of the right operand; the result is an lvalue if its right operand is.

    So sizeof(0, arr) = sizeof (arr) and which would be equal to 100* sizeof(char) and not = sizeof(char*).

    So MSVC++ is giving incorrect result (in case of C++ code).