foo.c: In function 'main':
foo.c:5:3: error: unknown type name 'using'
   using namespace std;
   ^~~~~
foo.c:5:19: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
   using namespace std;
                   ^~~
foo.c:6:5: warning: 'main' is normally a non-static function [-Wmain]
 int main(){
     ^~~~
foo.c: In function 'main':
foo.c:7:16: error: 'cin' undeclared (first use in this function); did you mean 'main'?
     int h,m,s; cin>>h>>m>>s;
                ^~~
                main
foo.c:7:16: note: each undeclared identifier is reported only once for each function it appears in
foo.c:23:5: error: 'cout' undeclared (first use in this function)
     cout<<h<<" "<<m<<" "<<s;
     ^~~~
foo.c: In function 'main':
foo.c:25:1: error: expected declaration or statement at end of input
 }
 ^
At top level:
foo.c:6:5: warning: 'main' defined but not used [-Wunused-function]
 int main(){
     ^~~~

[Hydro](https://hydro.ac)提供评测服务