The only storage type available on Linux is "generic Unix". It is meant to interoperate with Posix-style APIs that deal with char *.
sys_string logically stores its content as a sequence of chars in UTF-8 encoding. Conversions from const char * always incur copying and sometimes memory allocation (sys_string does small string optimization similar to std::string).
Conversion to const char * are 0-cost.
const char * cstr_in = "abc";
sys_string str(cstr_in);
const char * cstr_out = str.c_str();
assert(strcmp(cstr_out, cstr_in) == 0);
assert(cstr_out != cstr_in); //in and out are NOT the same!
//nullptr
assert(sys_string().c_str() != nullptr); //NO null preservation
assert(strcmp(sys_string().c_str(), "") == 0);
assert(sys_string((const char *)nullptr).c_str() != nullptr); //NO null preservation
assert(strcmp(sys_string((const char *)nullptr).c_str(), "") == 0);Note that unlike other storage types there is no null preservation here. c_str() returns an empty C string for default constructed sys_string or one constructed from nullptr. This is deliberate to align with std::string behavior that never produces nullptr from its c_str().