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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?

c++ constructor friend-class

share|improve this question

edited Apr 24 at 19:14

Boann

37.7k1291123

asked Apr 23 at 21:56

Syfu_HSyfu_H

429112

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  

  
  
  
  

add a comment | 

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?

c++ constructor friend-class

share|improve this question

edited Apr 24 at 19:14

Boann

37.7k1291123

asked Apr 23 at 21:56

Syfu_HSyfu_H

429112

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  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
  

  
  
  
  

add a comment | 

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

edited Apr 24 at 19:14

Boann

37.7k1291123

asked Apr 23 at 21:56

Syfu_HSyfu_H

429112

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

share|improve this question

edited Apr 24 at 19:14

Boann

37.7k1291123

asked Apr 23 at 21:56

Syfu_HSyfu_H

429112

share|improve this question

edited Apr 24 at 19:14

Boann

37.7k1291123

asked Apr 23 at 21:56

Syfu_HSyfu_H

429112

edited Apr 24 at 19:14

Boann

37.7k1291123

edited Apr 24 at 19:14

Boann

37.7k1291123

asked Apr 23 at 21:56

Syfu_HSyfu_H

429112

asked Apr 23 at 21:56

Syfu_HSyfu_H

429112

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

edited Apr 23 at 22:31

answered Apr 23 at 22:15

zdanzdan

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
  Apr 25 at 7:50
  

  
  
  
  

add a comment | 

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

answered Apr 23 at 22:14

Raindrop7Raindrop7

3,85031224

add a comment | 

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

edited Apr 23 at 22:24

Pavan Manjunath

20.1k1181108

answered Apr 23 at 22:04

schuessschuess

536416

add a comment | 

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

answered Apr 23 at 22:03

Eric SokolowskyEric Sokolowsky

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
  

  
  
  
  

add a comment | 

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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

edited Apr 23 at 22:31

answered Apr 23 at 22:15

zdanzdan

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
  Apr 25 at 7:50
  

  
  
  
  

add a comment | 

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

edited Apr 23 at 22:31

answered Apr 23 at 22:15

zdanzdan

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
  Apr 25 at 7:50
  

  
  
  
  

add a comment | 

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

edited Apr 23 at 22:31

answered Apr 23 at 22:15

zdanzdan

22.3k34865

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

share|improve this answer

edited Apr 23 at 22:31

answered Apr 23 at 22:15

zdanzdan

22.3k34865

answered Apr 23 at 22:15

zdanzdan

22.3k34865

answered Apr 23 at 22:15

zdanzdan

22.3k34865

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

answered Apr 23 at 22:14

Raindrop7Raindrop7

3,85031224

add a comment | 

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

answered Apr 23 at 22:14

Raindrop7Raindrop7

3,85031224

add a comment | 

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

answered Apr 23 at 22:14

Raindrop7Raindrop7

3,85031224

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

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

share|improve this answer

answered Apr 23 at 22:14

Raindrop7Raindrop7

3,85031224

answered Apr 23 at 22:14

Raindrop7Raindrop7

3,85031224

answered Apr 23 at 22:14

Raindrop7Raindrop7

3,85031224

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

edited Apr 23 at 22:24

Pavan Manjunath

20.1k1181108

answered Apr 23 at 22:04

schuessschuess

536416

add a comment | 

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

edited Apr 23 at 22:24

Pavan Manjunath

20.1k1181108

answered Apr 23 at 22:04

schuessschuess

536416

add a comment | 

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

edited Apr 23 at 22:24

Pavan Manjunath

20.1k1181108

answered Apr 23 at 22:04

schuessschuess

536416

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

share|improve this answer

edited Apr 23 at 22:24

Pavan Manjunath

20.1k1181108

answered Apr 23 at 22:04

schuessschuess

536416

edited Apr 23 at 22:24

Pavan Manjunath

20.1k1181108

edited Apr 23 at 22:24

Pavan Manjunath

20.1k1181108

answered Apr 23 at 22:04

schuessschuess

536416

answered Apr 23 at 22:04

schuessschuess

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

answered Apr 23 at 22:03

Eric SokolowskyEric Sokolowsky

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
  

  
  
  
  

add a comment | 

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

answered Apr 23 at 22:03

Eric SokolowskyEric Sokolowsky

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
  

  
  
  
  

add a comment | 

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

answered Apr 23 at 22:03

Eric SokolowskyEric Sokolowsky

594

share|improve this answer

answered Apr 23 at 22:03

Eric SokolowskyEric Sokolowsky

594

answered Apr 23 at 22:03

Eric SokolowskyEric Sokolowsky

594

answered Apr 23 at 22:03

Eric SokolowskyEric Sokolowsky

594