foo.cc:4:1: error: 'string' does not name a type; did you mean 'stdin'?
 string decode(string a)
 ^~~~~~
 stdin
foo.cc: In function 'int main()':
foo.cc:25:4: error: 'decode' was not declared in this scope
  a=decode(a);
    ^~~~~~
foo.cc:25:4: note: suggested alternative: 'remove'
  a=decode(a);
    ^~~~~~
    remove