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Why I cannot instantiate a class whose constructor is private in a friend class?



Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30 pm US/Eastern)
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7















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?









share|improve this question
























  • Why are you making Salary's constructor private? It seems like there are contexts when you'd want to use Salary outside of Employee

    – J. Antonio Perez
    1 hour ago

















7















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?









share|improve this question
























  • Why are you making Salary's constructor private? It seems like there are contexts when you'd want to use Salary outside of Employee

    – J. Antonio Perez
    1 hour ago













7












7








7


6






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?









share|improve this question
















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?






c++ constructor friend-class






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 1 hour ago







Syfu_H

















asked 2 hours ago









Syfu_HSyfu_H

35518




35518












  • Why are you making Salary's constructor private? It seems like there are contexts when you'd want to use Salary outside of Employee

    – J. Antonio Perez
    1 hour ago

















  • Why are you making Salary's constructor private? It seems like there are contexts when you'd want to use Salary outside of Employee

    – J. Antonio Perez
    1 hour ago
















Why are you making Salary's constructor private? It seems like there are contexts when you'd want to use Salary outside of Employee

– J. Antonio Perez
1 hour ago





Why are you making Salary's constructor private? It seems like there are contexts when you'd want to use Salary outside of Employee

– J. Antonio Perez
1 hour ago












4 Answers
4






active

oldest

votes


















8














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;
;





share|improve this answer




















  • 1





    I'm trying on MSVS and only Employee() ; allows Employee emp; to compile. Clang seems to accept Employee() = default;, but then again, Clang seems to accept having no default constructor here.

    – wally
    1 hour ago












  • GCC does the same as Clang, and doesn't need a default constructor to compile in this case. Have you tried this answer on any specific compiler?

    – wally
    1 hour ago



















2














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







share|improve this answer






























    2














    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;
    ;





    share|improve this answer
































      0














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






      share|improve this answer


















      • 1





        I recommend explaining why.

        – user4581301
        1 hour ago











      • Yes. Not onyl GCC but also MSVC14 also compiles Employee emp; but why?

        – Syfu_H
        1 hour ago











      • @Syfu_H Value Initialization. And I could be mistaken here (been caught on this in the past), but the Value Initialization is being replaced by Aggregate Initialization

        – user4581301
        1 hour ago











      Your Answer






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      4 Answers
      4






      active

      oldest

      votes








      4 Answers
      4






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      8














      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;
      ;





      share|improve this answer




















      • 1





        I'm trying on MSVS and only Employee() ; allows Employee emp; to compile. Clang seems to accept Employee() = default;, but then again, Clang seems to accept having no default constructor here.

        – wally
        1 hour ago












      • GCC does the same as Clang, and doesn't need a default constructor to compile in this case. Have you tried this answer on any specific compiler?

        – wally
        1 hour ago
















      8














      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;
      ;





      share|improve this answer




















      • 1





        I'm trying on MSVS and only Employee() ; allows Employee emp; to compile. Clang seems to accept Employee() = default;, but then again, Clang seems to accept having no default constructor here.

        – wally
        1 hour ago












      • GCC does the same as Clang, and doesn't need a default constructor to compile in this case. Have you tried this answer on any specific compiler?

        – wally
        1 hour ago














      8












      8








      8







      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;
      ;





      share|improve this answer















      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;
      ;






      share|improve this answer














      share|improve this answer



      share|improve this answer








      edited 1 hour ago

























      answered 1 hour ago









      zdanzdan

      22.2k34864




      22.2k34864







      • 1





        I'm trying on MSVS and only Employee() ; allows Employee emp; to compile. Clang seems to accept Employee() = default;, but then again, Clang seems to accept having no default constructor here.

        – wally
        1 hour ago












      • GCC does the same as Clang, and doesn't need a default constructor to compile in this case. Have you tried this answer on any specific compiler?

        – wally
        1 hour ago













      • 1





        I'm trying on MSVS and only Employee() ; allows Employee emp; to compile. Clang seems to accept Employee() = default;, but then again, Clang seems to accept having no default constructor here.

        – wally
        1 hour ago












      • GCC does the same as Clang, and doesn't need a default constructor to compile in this case. Have you tried this answer on any specific compiler?

        – wally
        1 hour ago








      1




      1





      I'm trying on MSVS and only Employee() ; allows Employee emp; to compile. Clang seems to accept Employee() = default;, but then again, Clang seems to accept having no default constructor here.

      – wally
      1 hour ago






      I'm trying on MSVS and only Employee() ; allows Employee emp; to compile. Clang seems to accept Employee() = default;, but then again, Clang seems to accept having no default constructor here.

      – wally
      1 hour ago














      GCC does the same as Clang, and doesn't need a default constructor to compile in this case. Have you tried this answer on any specific compiler?

      – wally
      1 hour ago






      GCC does the same as Clang, and doesn't need a default constructor to compile in this case. Have you tried this answer on any specific compiler?

      – wally
      1 hour ago














      2














      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







      share|improve this answer



























        2














        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







        share|improve this answer

























          2












          2








          2







          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







          share|improve this answer













          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








          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 1 hour ago









          Raindrop7Raindrop7

          3,74531224




          3,74531224





















              2














              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;
              ;





              share|improve this answer





























                2














                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;
                ;





                share|improve this answer



























                  2












                  2








                  2







                  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;
                  ;





                  share|improve this answer















                  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;
                  ;






                  share|improve this answer














                  share|improve this answer



                  share|improve this answer








                  edited 1 hour ago









                  Pavan Manjunath

                  20.1k1181108




                  20.1k1181108










                  answered 1 hour ago









                  schuessschuess

                  536416




                  536416





















                      0














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






                      share|improve this answer


















                      • 1





                        I recommend explaining why.

                        – user4581301
                        1 hour ago











                      • Yes. Not onyl GCC but also MSVC14 also compiles Employee emp; but why?

                        – Syfu_H
                        1 hour ago











                      • @Syfu_H Value Initialization. And I could be mistaken here (been caught on this in the past), but the Value Initialization is being replaced by Aggregate Initialization

                        – user4581301
                        1 hour ago















                      0














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






                      share|improve this answer


















                      • 1





                        I recommend explaining why.

                        – user4581301
                        1 hour ago











                      • Yes. Not onyl GCC but also MSVC14 also compiles Employee emp; but why?

                        – Syfu_H
                        1 hour ago











                      • @Syfu_H Value Initialization. And I could be mistaken here (been caught on this in the past), but the Value Initialization is being replaced by Aggregate Initialization

                        – user4581301
                        1 hour ago













                      0












                      0








                      0







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






                      share|improve this answer













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







                      share|improve this answer












                      share|improve this answer



                      share|improve this answer










                      answered 1 hour ago









                      Eric SokolowskyEric Sokolowsky

                      514




                      514







                      • 1





                        I recommend explaining why.

                        – user4581301
                        1 hour ago











                      • Yes. Not onyl GCC but also MSVC14 also compiles Employee emp; but why?

                        – Syfu_H
                        1 hour ago











                      • @Syfu_H Value Initialization. And I could be mistaken here (been caught on this in the past), but the Value Initialization is being replaced by Aggregate Initialization

                        – user4581301
                        1 hour ago












                      • 1





                        I recommend explaining why.

                        – user4581301
                        1 hour ago











                      • Yes. Not onyl GCC but also MSVC14 also compiles Employee emp; but why?

                        – Syfu_H
                        1 hour ago











                      • @Syfu_H Value Initialization. And I could be mistaken here (been caught on this in the past), but the Value Initialization is being replaced by Aggregate Initialization

                        – user4581301
                        1 hour ago







                      1




                      1





                      I recommend explaining why.

                      – user4581301
                      1 hour ago





                      I recommend explaining why.

                      – user4581301
                      1 hour ago













                      Yes. Not onyl GCC but also MSVC14 also compiles Employee emp; but why?

                      – Syfu_H
                      1 hour ago





                      Yes. Not onyl GCC but also MSVC14 also compiles Employee emp; but why?

                      – Syfu_H
                      1 hour ago













                      @Syfu_H Value Initialization. And I could be mistaken here (been caught on this in the past), but the Value Initialization is being replaced by Aggregate Initialization

                      – user4581301
                      1 hour ago





                      @Syfu_H Value Initialization. And I could be mistaken here (been caught on this in the past), but the Value Initialization is being replaced by Aggregate Initialization

                      – user4581301
                      1 hour ago

















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