main.cpp:7:4: error: ‘String’ was not declared in this scope code example

Example 1: error: ‘cout’ was not declared in this scope

#include <iostream>
using namespace std;

adding above two lines before main will help you

NOTE:
note: suggested alternative:
In file included from class.cpp:1:0:
/usr/include/c++/7/iostream:61:18: note:   ‘std::cout’
   extern ostream cout;  /// Linked to standard output
                  ^~~~

Example 2: cout was not declared in this scope

#include<iostream>
using namespace std; //use this
int main(){
    char t = 'f';
    char *t1;
    char **t2;
    cout<<t;        
    return 0;
}