Compilare: main.cpp: In function 'int main()': main.cpp:22:24: warning: passing NULL to non-pointer argument 3 of 'constexpr std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::insert(size_type, size_type, _CharT) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; size_type = long unsigned int]' [-Wconversion-null] 22 | S.insert(0, 1, NULL); | ^~~~ In file included from /usr/include/c++/12.2.1/string:53, from /usr/include/c++/12.2.1/bits/locale_classes.h:40, from /usr/include/c++/12.2.1/bits/ios_base.h:41, from /usr/include/c++/12.2.1/ios:42, from /usr/include/c++/12.2.1/istream:38, from /usr/include/c++/12.2.1/fstream:38, from main.cpp:1: /usr/include/c++/12.2.1/bits/basic_string.h:1950:53: note: declared here 1950 | insert(size_type __pos, size_type __n, _CharT __c) | ~~~~~~~^~~