On Sun, 12 Mar 2023 at 22:10, Jonny Grant wrote: > > > > On 09/02/2023 17:52, Jonathan Wakely wrote: > > 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. > > How about adding a method that is called by these operators that has the __attribute__ ((nonnull)) ? > > example: > https://godbolt.org/z/bqW86PP34 > > >> 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. > > There is a way to generate a build error for even string((const char*)nullptr) > > I made another example that detects nullptr being passed around (should such stupid code occur) at build time providing optimizer is on. With -O0 it just gives the error always; so I put in an __OPTIMIZE__ check. This example doesn't need -fanalyzer. > > https://godbolt.org/z/TdGnno4K5 > > #if __OPTIMIZE__ > void nullptr_compile_abort() __attribute__((error("nullptr compile error"))); > #endif > > static void f2(const char * str) > { > #if __OPTIMIZE__ > if (str == nullptr) nullptr_compile_abort(); > #endif > } > > int main() > { > f2((const char *)nullptr); > } This causes compilation to fail for code which is never executed at run-time, which is not permitted by the standard. You can use __attribute__((warning(""))) instead, but that is broken^W inconvenient for inline functions. You need a non-inline definition of the function, which means exporting a new function from the shared library just for this diagnostic. All these techniques you're rediscovering have been tried before :-)
