Why can I not instantiate a class whose constructor is private in a friend class?Can an abstract class have a constructor?Can I call a constructor from another constructor (do constructor chaining) in C++?Why can templates only be implemented in the header file?Why do this() and super() have to be the first statement in a constructor?Why can't I change a private member of a class from a friend class in a different namespace?Passing a class as argument which has a private constructor that takes no parametersFriend function is not accessing private members of another friend classDeclaring constructors as private shows errors. Is at least one public constructor mandatory?Cannot access private member declared in class, even declared friend classPrivate Data member is inaccessible in Friend Function

How to reduce LED flash rate (frequency)

How exactly does Hawking radiation decrease the mass of black holes?

Stop and Take a Breath!

How would one muzzle a full grown polar bear in the 13th century?

What are the potential pitfalls when using metals as a currency?

Pass By Reference VS Pass by Value

Was there a Viking Exchange as well as a Columbian one?

Will tsunami waves travel forever if there was no land?

Why isn't the definition of absolute value applied when squaring a radical containing a variable?

What makes accurate emulation of old systems a difficult task?

Is there a way to get a compiler for the original B programming language?

Error message with tabularx

What does KSP mean?

Shrinkwrap tetris shapes without scaling or diagonal shapes

Do I have to worry about players making “bad” choices on level up?

Is there really no use for MD5 anymore?

Using a Lyapunov function to classify stability and sketching a phase portrait

How to stop co-workers from teasing me because I know Russian?

Reducing vertical space in stackrel

What is the difference between `command a[bc]d` and `command `ab,cd`

What happened to Captain America in Endgame?

What's the polite way to say "I need to urinate"?

Unexpected email from Yorkshire Bank

Does the sign matter for proportionality?



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


Can an abstract class have a constructor?Can I call a constructor from another constructor (do constructor chaining) in C++?Why can templates only be implemented in the header file?Why do this() and super() have to be the first statement in a constructor?Why can't I change a private member of a class from a friend class in a different namespace?Passing a class as argument which has a private constructor that takes no parametersFriend function is not accessing private members of another friend classDeclaring constructors as private shows errors. Is at least one public constructor mandatory?Cannot access private member declared in class, even declared friend classPrivate Data member is inaccessible in Friend Function






.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty height:90px;width:728px;box-sizing:border-box;








16















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 except by class Employee. So I declared the constructors of Salary private and made Employee a 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 goes away if I forward declare main:



    int main(int, char*[]);


    And make main a friend of class Salary like 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
    Apr 23 at 22:40

















16















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 except by class Employee. So I declared the constructors of Salary private and made Employee a 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 goes away if I forward declare main:



    int main(int, char*[]);


    And make main a friend of class Salary like 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
    Apr 23 at 22:40













16












16








16


8






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 except by class Employee. So I declared the constructors of Salary private and made Employee a 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 goes away if I forward declare main:



    int main(int, char*[]);


    And make main a friend of class Salary like 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 except by class Employee. So I declared the constructors of Salary private and made Employee a 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 goes away if I forward declare main:



    int main(int, char*[]);


    And make main a friend of class Salary like 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 Apr 24 at 19:14









Boann

37.7k1291123




37.7k1291123










asked Apr 23 at 21:56









Syfu_HSyfu_H

417111




417111












  • 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
    Apr 23 at 22:40

















  • 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
    Apr 23 at 22:40
















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
Apr 23 at 22:40





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
Apr 23 at 22:40












4 Answers
4






active

oldest

votes


















17














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
    Apr 23 at 22:32







  • 1





    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
    Apr 23 at 22:43












  • Thanks. it saves the day!

    – Syfu_H
    2 days 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


















      • 4





        I recommend explaining why.

        – user4581301
        Apr 23 at 22:06











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

        – Syfu_H
        Apr 23 at 22:07






      • 1





        @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
        Apr 23 at 22:15











      Your Answer






      StackExchange.ifUsing("editor", function ()
      StackExchange.using("externalEditor", function ()
      StackExchange.using("snippets", function ()
      StackExchange.snippets.init();
      );
      );
      , "code-snippets");

      StackExchange.ready(function()
      var channelOptions =
      tags: "".split(" "),
      id: "1"
      ;
      initTagRenderer("".split(" "), "".split(" "), channelOptions);

      StackExchange.using("externalEditor", function()
      // Have to fire editor after snippets, if snippets enabled
      if (StackExchange.settings.snippets.snippetsEnabled)
      StackExchange.using("snippets", function()
      createEditor();
      );

      else
      createEditor();

      );

      function createEditor()
      StackExchange.prepareEditor(
      heartbeatType: 'answer',
      autoActivateHeartbeat: false,
      convertImagesToLinks: true,
      noModals: true,
      showLowRepImageUploadWarning: true,
      reputationToPostImages: 10,
      bindNavPrevention: true,
      postfix: "",
      imageUploader:
      brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
      contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
      allowUrls: true
      ,
      onDemand: true,
      discardSelector: ".discard-answer"
      ,immediatelyShowMarkdownHelp:true
      );



      );













      draft saved

      draft discarded


















      StackExchange.ready(
      function ()
      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55819962%2fwhy-can-i-not-instantiate-a-class-whose-constructor-is-private-in-a-friend-class%23new-answer', 'question_page');

      );

      Post as a guest















      Required, but never shown

























      4 Answers
      4






      active

      oldest

      votes








      4 Answers
      4






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      17














      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
        Apr 23 at 22:32







      • 1





        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
        Apr 23 at 22:43












      • Thanks. it saves the day!

        – Syfu_H
        2 days ago















      17














      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
        Apr 23 at 22:32







      • 1





        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
        Apr 23 at 22:43












      • Thanks. it saves the day!

        – Syfu_H
        2 days ago













      17












      17








      17







      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 Apr 23 at 22:31

























      answered Apr 23 at 22:15









      zdanzdan

      22.3k34865




      22.3k34865







      • 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
        Apr 23 at 22:32







      • 1





        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
        Apr 23 at 22:43












      • Thanks. it saves the day!

        – Syfu_H
        2 days 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
        Apr 23 at 22:32







      • 1





        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
        Apr 23 at 22:43












      • Thanks. it saves the day!

        – Syfu_H
        2 days 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
      Apr 23 at 22:32






      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
      Apr 23 at 22:32





      1




      1





      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
      Apr 23 at 22:43






      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
      Apr 23 at 22:43














      Thanks. it saves the day!

      – Syfu_H
      2 days ago





      Thanks. it saves the day!

      – Syfu_H
      2 days 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 Apr 23 at 22:14









          Raindrop7Raindrop7

          3,81531224




          3,81531224





















              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 Apr 23 at 22:24









                  Pavan Manjunath

                  20.2k1181108




                  20.2k1181108










                  answered Apr 23 at 22:04









                  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


















                      • 4





                        I recommend explaining why.

                        – user4581301
                        Apr 23 at 22:06











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

                        – Syfu_H
                        Apr 23 at 22:07






                      • 1





                        @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
                        Apr 23 at 22:15















                      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


















                      • 4





                        I recommend explaining why.

                        – user4581301
                        Apr 23 at 22:06











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

                        – Syfu_H
                        Apr 23 at 22:07






                      • 1





                        @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
                        Apr 23 at 22:15













                      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 Apr 23 at 22:03









                      Eric SokolowskyEric Sokolowsky

                      594




                      594







                      • 4





                        I recommend explaining why.

                        – user4581301
                        Apr 23 at 22:06











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

                        – Syfu_H
                        Apr 23 at 22:07






                      • 1





                        @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
                        Apr 23 at 22:15












                      • 4





                        I recommend explaining why.

                        – user4581301
                        Apr 23 at 22:06











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

                        – Syfu_H
                        Apr 23 at 22:07






                      • 1





                        @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
                        Apr 23 at 22:15







                      4




                      4





                      I recommend explaining why.

                      – user4581301
                      Apr 23 at 22:06





                      I recommend explaining why.

                      – user4581301
                      Apr 23 at 22:06













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

                      – Syfu_H
                      Apr 23 at 22:07





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

                      – Syfu_H
                      Apr 23 at 22:07




                      1




                      1





                      @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
                      Apr 23 at 22:15





                      @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
                      Apr 23 at 22:15

















                      draft saved

                      draft discarded
















































                      Thanks for contributing an answer to Stack Overflow!


                      • Please be sure to answer the question. Provide details and share your research!

                      But avoid


                      • Asking for help, clarification, or responding to other answers.

                      • Making statements based on opinion; back them up with references or personal experience.

                      To learn more, see our tips on writing great answers.




                      draft saved


                      draft discarded














                      StackExchange.ready(
                      function ()
                      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55819962%2fwhy-can-i-not-instantiate-a-class-whose-constructor-is-private-in-a-friend-class%23new-answer', 'question_page');

                      );

                      Post as a guest















                      Required, but never shown





















































                      Required, but never shown














                      Required, but never shown












                      Required, but never shown







                      Required, but never shown

































                      Required, but never shown














                      Required, but never shown












                      Required, but never shown







                      Required, but never shown







                      Popular posts from this blog

                      Sum ergo cogito? 1 nng

                      三茅街道4182Guuntc Dn precexpngmageondP