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); } Regards, Jonny
