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I have two classes; Salary that is intended to hold information and calculations regarding the salary of an employee and Employee that has an object of type class Salary and some members like name and address of the employee...

• 
  

  What I want to do is to prevent class Salary from being instantiated and only class Employee can instantiate it. So I declared the constructors of Salary private and made Employee friend of Salary. But I get errors:

  
  
  

  class Employee;
  
  class Salary 
   public:
  
   private:
   Salary() : revenue_, cost_ 
   Salary(int x, int y) : revenue_ x ,
   cost_ y 
  
   
   int revenue_, cost_;
   friend class Employee;
  ;
  
  class Employee 
   public:
   std::string name_;
   Salary sal;
  ;
  
  int main()
  
   Employee emp; // "Salary::Salary()" is inaccessible
  
  

  
  


• 
  

  The problem raised for me if I forward declare main:

  
  
  

  int main(int, char*[]);
  

  
  
  

  And make main friend of class Salary so in Salary:

  
  
  

  class Salary 
   //...
   friend int main(int argc, char* argv[]);
  ;
  

  
  

Now the program compiles correctly!

*** Another thing in main if I declare an object this way:

Employee emp; // ok
Employee emp; // error?

Because you don't provide a constructor for Employee the braces in your initialization Employee emp; will perform an aggregate initialization, which essentially means that each member is initialized one-by-one using the default rules, in the context of main(). Since main() doesn't have access to the Salary constructor, it fails.

As others have pointed out, adding an Employee default constructor will resolve your problem:

class Employee 
 public:
 Employee() = default;
 std::string name_;
 Salary sal;
;

You have to explicitly declare the default constructor of class Employee thus you can initialize an abject via uniform initialization:

class Employee 
 public:
 Employee() // add it
 std::string name_;
 Salary sal;
;

int main()
 Employee emp; // now this should compile

You need Employee's ctor to call the ctor of Salary. The ctor of Salary is not accessible from main.

eg:

class Employee 
public:
 Employee() : sal() 
 public:
 std::string name_;
 Salary sal;
;

If you erase the "" after "Employee emp" in your main() function it compiles just fine (gcc 7.3.1 on Fedora 27).