error: new types may not be defined in a return type

Hello Peeps, 

I just moved to United Kingdom last week, and I continuously want this learning rhythm to go. As I'm also learning OOPs concepts, this quarantine period helped me with little bit of programming. Click here to know the concepts I learnt so far in C++. 

#include <iostream>

class army                         // Class declaration
{
public:                            // Access Specifiers
    int number_suffix;             //Member Variable
    string key;
    
    void enrollment(string name, int age)
    {key = name; number_suffix = age;}
    
    void retirement()             // Member Functions
    {cout << key << " is retiring";}
}

int main()
{
    army a1;                     // Object creation and Memory allocation

    a1.enrollment("Shirley", 31);
    a1.retirement();
    return 0; 
}

The declaration of a class doesn't have a ; at the end, so it threw an error as 'new types may not be defined in a return type'

#include <iostream>

class army                         // Class declaration
{
public:                            // Access Specifiers
    int number_suffix;             //Member Variable
    string key;
    
    void enrollment(string name, int age)
    {key = name; number_suffix = age;}
    
    void retirement()             // Member Functions
    {cout << key << " is retiring";}
};

int main()
{
    army a1;                     // Object creation and Memory allocation

    a1.enrollment("Shirley", 31);
    a1.retirement();
    return 0; 
}

In the above program, I haven't used 'using namespace std;', but still it worked. I'm not sure of the use of 'namespace std'.

That's all guys!! Bye Bye...