foo.cc: In function 'int main()':
foo.cc:5:9: error: 'cin' was not declared in this scope
    5 |         cin>>a>>b;
      |         ^~~
foo.cc:11:9: error: 'cout' was not declared in this scope
   11 |         cout<<"最大公因数="<<b;
      |         ^~~~
foo.cc:4:17: warning: unused variable 'c' [-Wunused-variable]
    4 | {       int a,b,c;
      |                 ^

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