‘cout’ was not declared in this scope

From Cppsyntax

You can get the error

 ‘cout’ was not declared in this scope

if you forget to include iostream or to use the std namespace.

WRONG

 #include <iostream>  // no using namespace std;
 int main(void) 
 {
   cout << "A";
   return 0;
 }

ALSO WRONG

 using namespace std;   // no #include iostream
 int main(void) 
 {
   cout << "A";
   return 0;
 }

RIGHT

 #include <iostream>   // one of two
 using namespace std;    // two of two, yay!
 int main(void) 
 {
   cout << "A";
   return 0;
 }