On Saturday, October 20, 2012 7:50 AM, Chandler Carruth wrote:
[...snip...] Let me hypothesize a different interface:
This stays the same...
constexpr int constexpr_strncmp(const char *p, const char *q, size_t n) {
return !n ? 0 : *p != *q ? *p - *q : !*p ? 0 : constexpr_strncmp(p+1,
q+1, n-1);
}
But here we do something different on the actual declaration:
[[constexpr_alias(constexpr_strncmp)]]
int strncmp(const char *p, const char *q, size_t n);
When parsing the *declaration* of this function, we lookup the function
name passed to constexpr_alias. We must find a constexpr function with an
identical signature. Then, at function invocation substitution of strncmp,
we instead substitute the body of constexpr_strncmp.
This seems more direct (no redirection in the code), and it also provides
a specific advantage of allowing this to be easily added to an existing
declaration in a declaration-only header file without impacting or
changing the name of the runtime executed body or definition.
I'd be very happy with this solution. I come across precisely the problem
raised by Richard on a very regular basis and have different workarounds
for both gcc and clang. I'd love to see something "standard" emerging!
For my side, I'd still like some way of declaring a function to be used
only in a constexpr environment, meaning that the compiler gives an error
up front when a function is then used in a non-constexpr environment. The
above proposal will provide a link-time error if the non-constexpr function
is not defined, which is half-way there. Perhaps using the "unavailable"
attribute in conjunction with "constexpr_alias" would be the compile-time
solution...
Cheers
Andy
