On Thu, 9 Feb 2023 at 16:30, Xi Ruoyao wrote: > > On Thu, 2023-02-09 at 14:56 +0000, Jonathan Wakely via Gcc-help wrote: > > > Note, my code isn't like this, it is just an example to suggest > > > adding the nullptr attribute, as its clearly already rejected at > > > runtime. > > > > I assume you mean the nonnull attribute. That was added in 2020 and > > then reverted because it broke some things: > > I remember I'd once made the same mistake when I suggested to add > nonnull for ostream::operator<<(const string &) and I was lectured: > nonnull is not only a diagnostic attribute, it also allows the compiler > to assume the parameter is never null and rendering std::string(nullptr) > an undefined behavior. Yes, I think that's what might have happened with the std::string change. > Then the example may just silently continue to run, instead of throwing > an exception. It would be an ironic example: an attempt to improve > diagnostic finally made diagnostic more difficult. Indeed. Maybe we can add __attribute__((access(read, 1))) instead, which says that we will read from the pointer, which also implies it must be non-null. N.B. in C++23 string(nullptr) produces an error, although string((const char*)nullptr) doesn't, so in practice it only prevents the dumbest calls with a literal 'nullptr' token, and not the more realistic problems where you have a pointer that happens to be null.
