https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105995
Bug ID: 105995
Summary: QoI: constexpr basic_string variable must use all of
its SSO buffer
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: de34 at live dot cn
Target Milestone: ---
The current implementation in libstdc++ only supports constexpr
std::basic_string variable of static storage duration which uses all of its SSO
buffer:
#include <string>
constexpr std::string str15{"0123456789abcde"};
// constexpr std::string str14{"0123456789abcd"}; // error: should we support
this?
// constexpr std::string str16{"0123456789abcdef"}; // error: unsupportable
constexpr std::u16string u16str7{u"0123456"};
constexpr std::u32string u32str3{U"012"};
int main()
{
// constexpr std::string strdyn{"0123456789abcde"}; // error: unsupportable
}
Compiler Explorer links:
https://gcc.godbolt.org/z/cvbY1hK3G
https://gcc.godbolt.org/z/634888Y96
I think constexpr string variable that requires heap allocation is currently
unimplementable, and constexpr string variable of dynamic storage duration
can't be supported by libstdc++-v3 without ABI breakage now.
However, should we allow shorter strings to be stored into constexpr
basic_string variables? We can do this by always fully initializing the SSO
buffer (if it's used) during constant evluation.
