foo.cc: In function 'int in_cin()':
foo.cc:6:1: warning: no return statement in function returning non-void [-Wreturn-type]
    6 | }
      | ^
foo.cc: In function 'int doing_jinxinzhong()':
foo.cc:23:1: warning: no return statement in function returning non-void [-Wreturn-type]
   23 | }
      | ^
foo.cc: In function 'int out_cout()':
foo.cc:29:1: warning: no return statement in function returning non-void [-Wreturn-type]
   29 | }
      | ^

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