On Oct 19, 2012, at 23:27 , Andy Gibbs <andyg1...@hotmail.co.uk> wrote:
> 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...
While throwing things out there, why not just optionally allow constexpr
functions to coexist with non-constexpr functions of the same name, like inline
and non-inline? That would solve both the original problem and Andy's problem.
// Primary declaration.
int strcmp(const char *p, const char *q, size_t n);
// This declaration will be used in any constexpr contexts.
// Non-constexpr contexts will use the declaration above (which may be inline)
constexpr int strcmp(const char *p, const char *q, size_t n) {
return !n ? 0 : *p != *q ? *p - *q : !*p ? 0 : strcmp(p+1, q+1, n-1);
}
This avoids both new builtins and having to come up with a manually-mangled
name for the alternate implementation. But it is most definitely a language
change.
And for Andy's problem, you'd then get...something like this?
constexpr int foo(int n) {
return n + 1;
}
int foo(int n) = delete;
...but I have not thought about the problems in implementing this.
Jordan
