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Re: [PATCH v2] New mechanism for declaring const-covariant string functions.


On 11/23/2016 02:06 PM, Zack Weinberg wrote:
On Tue, Nov 22, 2016 at 4:05 AM, Florian Weimer <fweimer@redhat.com> wrote:
On 11/21/2016 06:46 PM, Zack Weinberg wrote:
diff --git a/string/bits/const-covariance.h
b/string/bits/const-covariance.h
+#define __CC_VA_NARGS(...)  __CC_VA_NARGS_(__VA_ARGS__, __CC_RSEQ)
+#define __CC_VA_NARGS_(...) __CC_VA_NARGS__(__VA_ARGS__)
+#define __CC_VA_NARGS__(_1, _2, _3, _4, _5, _6, _7, _8, N, ...) N
+#define __CC_RSEQ 8, 7, 6, 5, 4, 3, 2, 1

It's not clear to me what ensures that variadic macros are available at this
point.  <bits/const-covariance.h> seems to be included unconditionally, not
just for GNU or C++11.  I don't think you can use variadic macros for this.

This is a good point.  I had been blithely assuming that all supported
compilers provide variadic macros as an extension even in C89 mode,
but maybe I shouldn't do that.

To avoid variadic macros we would need to encode the number of
arguments in the macro name, e.g.

    __CONST_COV_PROTO_2 (strchr, char *, __s, int, __c);
    __CONST_COV_PROTO_3 (memchr, void *, __s, int, __c, size_t, __n);

Do you think we could live with that?  I won't have time to rework the
patch till the weekend.

I think the diagnostics issue is the show-stopper here. This looks increasingly like an application of Joseph's idea of generated installed headers.

Thanks,
Florian


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