foo.cc: In function 'int main()':
foo.cc:20:30: error: cannot convert 'const char*' to 'FILE*'
   20 |         freopen("in.txt","r","stdin");
      |                              ^~~~~~~
      |                              |
      |                              const char*
In file included from foo.cc:1:
/nix/store/y8wfrgk7br5rfz4221lfb9v8w3n0cnyd-glibc-2.37-8-dev/include/stdio.h:267:40: note:   initializing argument 3 of 'FILE* freopen(const char*, const char*, FILE*)'
  267 |                       FILE *__restrict __stream) __wur;
      |                       ~~~~~~~~~~~~~~~~~^~~~~~~~

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