Can I call a constructor from another constructor do constructor chaining in C

Question

As a C  developer I m used to running through constructors     class Test       public Test             DoSomething               public Test int count    this             DoSomethingWithCount count              public Test int count  string name    this count            DoSomethingWithName name             Is there a way to do this in C     I tried calling the Class name and using the  this  keyword  but both fail

User · Answer

I believe you can call a constructor from a constructor. It will compile and run. I recently saw someone do this and it ran on both Windows and Linux.

It just doesn't do what you want. The inner constructor will construct a temporary local object which gets deleted once the outer constructor returns. They would have to be different constructors as well or you would create a recursive call.

Ref: https://isocpp.org/wiki/faq/ctors#init-methods

User · Answer

If I understand your question correctly  you re asking if you can call multiple constructors in C     If that s what you re looking for  then no - that is not possible   You certainly can have multiple constructors  each with unique argument signatures  and then call the one you want when you instantiate a new object   You can even have one constructor with defaulted arguments on the end   But you may not have multiple constructors  and then call each of them separately

User · Answer

No  you can t call one constructor from another in C  03  called a delegating constructor    This changed in C  11  aka C  0x   which added support for the following syntax   example taken from Wikipedia   class SomeType     int number   public    SomeType int newNumber    number newNumber       SomeType     SomeType 42

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If you want to be evil  you can use the in-place  new  operator   class Foo         Foo        default constructor deliciousness          Foo Bar myParam          new  this  Foo             bar your param all night long                Seems to work for me   edit  As  ElvedinHamzagic points out  if Foo contained an object which allocated memory  that object might not be freed   This complicates things further   A more general example   class Foo     private    std  vector lt int gt  Stuff  public      Foo           Stuff 42                 default constructor deliciousness               Foo Bar myParam              this- gt  Foo          new  this  Foo             bar your param all night long                Looks a bit less elegant  for sure   JohnIdol s solution is much better

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Another option that has not been shown yet is to split your class into two  wrapping a lightweight interface class around your original class in order to achieve the effect you are looking for   class Test Base       public Test Base             DoSomething              class Test   public Test Base       public Test     Test Base                public Test int count    Test Base             DoSomethingWithCount count              This could get messy if you have many constructors that must call their  next level up  counterpart  but for a handful of constructors  it should be workable

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C  11  Yes  C  11 and onwards has this same feature  called delegating constructors   The syntax is slightly different from C   class Foo   public     Foo char x  int y       Foo int y    Foo  a   y         C  03  No It is worth pointing out that you can call the constructor of a parent class in your constructor e g    class A                         class B   public A               B     A                                                  But  no  you can t call another constructor of the same class upto C  03

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Would be more easy to test  than decide    Try this    include  lt iostream gt   class A   public      A  int a    m a a            std  cout  lt  lt   A  Ctor   lt  lt  std  endl                 A             std  cout  lt  lt   A  dtor   lt  lt  std  endl            public      int m a      class B   public A   public      B  int a  int b    m b b   A a     public      int m b      int main         B b 9  6       std  cout  lt  lt   Test constructor delegation a      lt  lt  b m a  lt  lt     b      lt  lt  b m b  lt  lt  std  endl          return 0      and compile it with 98 std      g   main cpp -std c  98  -o test 1   you will see   A  Ctor Test constructor delegation a   9  b   6 A  dtor   so

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No  in C   you cannot call a constructor from a constructor  What you can do  as warren pointed out  is    Overload the constructor  using different signatures Use default values on arguments  to make a  simpler  version available   Note that in the first case  you cannot reduce code duplication by calling one constructor from another  You can of course have a separate  private protected  method that does all the initialization  and let the constructor mainly deal with argument handling

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C  11  Yes   C  11 and onwards has this same feature  called delegating constructors     The syntax is slightly different from C    class Foo   public     Foo char x  int y       Foo int y    Foo  a   y          C  03  No  Unfortunately  there s no way to do this in C  03  but there are two ways of simulating this    You can combine two  or more  constructors via default parameters   class Foo   public    Foo char x  int y 0       combines two constructors  char  and  char  int               Use an init method to share common code   class Foo   public    Foo char x     Foo char x  int y            private    void init char x  int y       Foo  Foo char x      init x  int x    7               Foo  Foo char x  int y      init x  y               void Foo  init char x  int y                  See the C  FAQ entry for reference

User · Answer

If I understand your question correctly  you re asking if you can call multiple constructors in C     If that s what you re looking for  then no - that is not possible   You certainly can have multiple constructors  each with unique argument signatures  and then call the one you want when you instantiate a new object   You can even have one constructor with defaulted arguments on the end   But you may not have multiple constructors  and then call each of them separately

User · Answer

C  11  Yes   C  11 and onwards has this same feature  called delegating constructors     The syntax is slightly different from C    class Foo   public     Foo char x  int y       Foo int y    Foo  a   y          C  03  No  Unfortunately  there s no way to do this in C  03  but there are two ways of simulating this    You can combine two  or more  constructors via default parameters   class Foo   public    Foo char x  int y 0       combines two constructors  char  and  char  int               Use an init method to share common code   class Foo   public    Foo char x     Foo char x  int y            private    void init char x  int y       Foo  Foo char x      init x  int x    7               Foo  Foo char x  int y      init x  y               void Foo  init char x  int y                  See the C  FAQ entry for reference

User · Answer

No  you can t call one constructor from another in C  03  called a delegating constructor    This changed in C  11  aka C  0x   which added support for the following syntax   example taken from Wikipedia   class SomeType     int number   public    SomeType int newNumber    number newNumber       SomeType     SomeType 42

User · Answer

Another option that has not been shown yet is to split your class into two  wrapping a lightweight interface class around your original class in order to achieve the effect you are looking for   class Test Base       public Test Base             DoSomething              class Test   public Test Base       public Test     Test Base                public Test int count    Test Base             DoSomethingWithCount count              This could get messy if you have many constructors that must call their  next level up  counterpart  but for a handful of constructors  it should be workable

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In C  11  a constructor can call another constructor overload   class Foo         int d           public      Foo   int i    d i         Foo       Foo 42       New to C  11      Additionally  members can be initialized like this as well   class Foo         int d   5           public      Foo   int i    d i          This should eliminate the need to create the initialization helper method  And it is still recommended not calling any virtual functions in the constructors or destructors to avoid using any members that might not be initialized

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When calling a constructor it actually allocates memory  either from the stack or from the heap  So calling a constructor in another constructor creates a local copy  So we are modifying another object  not the one we are focusing on

User · Answer

C  11  Yes  C  11 and onwards has this same feature  called delegating constructors   The syntax is slightly different from C   class Foo   public     Foo char x  int y       Foo int y    Foo  a   y         C  03  No It is worth pointing out that you can call the constructor of a parent class in your constructor e g    class A                         class B   public A               B     A                                                  But  no  you can t call another constructor of the same class upto C  03

User · Answer

No  in C   you cannot call a constructor from a constructor  What you can do  as warren pointed out  is    Overload the constructor  using different signatures Use default values on arguments  to make a  simpler  version available   Note that in the first case  you cannot reduce code duplication by calling one constructor from another  You can of course have a separate  private protected  method that does all the initialization  and let the constructor mainly deal with argument handling

User · Answer

C  11  Yes   C  11 and onwards has this same feature  called delegating constructors     The syntax is slightly different from C    class Foo   public     Foo char x  int y       Foo int y    Foo  a   y          C  03  No  Unfortunately  there s no way to do this in C  03  but there are two ways of simulating this    You can combine two  or more  constructors via default parameters   class Foo   public    Foo char x  int y 0       combines two constructors  char  and  char  int               Use an init method to share common code   class Foo   public    Foo char x     Foo char x  int y            private    void init char x  int y       Foo  Foo char x      init x  int x    7               Foo  Foo char x  int y      init x  y               void Foo  init char x  int y                  See the C  FAQ entry for reference

User · Answer

I believe you can call a constructor from a constructor. It will compile and run. I recently saw someone do this and it ran on both Windows and Linux.

It just doesn't do what you want. The inner constructor will construct a temporary local object which gets deleted once the outer constructor returns. They would have to be different constructors as well or you would create a recursive call.

Ref: https://isocpp.org/wiki/faq/ctors#init-methods

User · Answer

This approach may work for some kinds of classes  when the assignment operator behaves  well     Foo  Foo            do what every Foo is needing            Foo  Foo char x         this   Foo            do the special things for a Foo with char

User · Answer

C  11  Yes  C  11 and onwards has this same feature  called delegating constructors   The syntax is slightly different from C   class Foo   public     Foo char x  int y       Foo int y    Foo  a   y         C  03  No It is worth pointing out that you can call the constructor of a parent class in your constructor e g    class A                         class B   public A               B     A                                                  But  no  you can t call another constructor of the same class upto C  03

User · Answer

If I understand your question correctly  you re asking if you can call multiple constructors in C     If that s what you re looking for  then no - that is not possible   You certainly can have multiple constructors  each with unique argument signatures  and then call the one you want when you instantiate a new object   You can even have one constructor with defaulted arguments on the end   But you may not have multiple constructors  and then call each of them separately

User · Answer

No  in C   you cannot call a constructor from a constructor  What you can do  as warren pointed out  is    Overload the constructor  using different signatures Use default values on arguments  to make a  simpler  version available   Note that in the first case  you cannot reduce code duplication by calling one constructor from another  You can of course have a separate  private protected  method that does all the initialization  and let the constructor mainly deal with argument handling

User · Answer

C  11  Yes   C  11 and onwards has this same feature  called delegating constructors     The syntax is slightly different from C    class Foo   public     Foo char x  int y       Foo int y    Foo  a   y          C  03  No  Unfortunately  there s no way to do this in C  03  but there are two ways of simulating this    You can combine two  or more  constructors via default parameters   class Foo   public    Foo char x  int y 0       combines two constructors  char  and  char  int               Use an init method to share common code   class Foo   public    Foo char x     Foo char x  int y            private    void init char x  int y       Foo  Foo char x      init x  int x    7               Foo  Foo char x  int y      init x  y               void Foo  init char x  int y                  See the C  FAQ entry for reference

User · Answer

In C  11  a constructor can call another constructor overload   class Foo         int d           public      Foo   int i    d i         Foo       Foo 42       New to C  11      Additionally  members can be initialized like this as well   class Foo         int d   5           public      Foo   int i    d i          This should eliminate the need to create the initialization helper method  And it is still recommended not calling any virtual functions in the constructors or destructors to avoid using any members that might not be initialized

User · Answer

C  11  Yes  C  11 and onwards has this same feature  called delegating constructors   The syntax is slightly different from C   class Foo   public     Foo char x  int y       Foo int y    Foo  a   y         C  03  No It is worth pointing out that you can call the constructor of a parent class in your constructor e g    class A                         class B   public A               B     A                                                  But  no  you can t call another constructor of the same class upto C  03

User · Answer

If I understand your question correctly  you re asking if you can call multiple constructors in C     If that s what you re looking for  then no - that is not possible   You certainly can have multiple constructors  each with unique argument signatures  and then call the one you want when you instantiate a new object   You can even have one constructor with defaulted arguments on the end   But you may not have multiple constructors  and then call each of them separately

User · Answer

Simply put  you cannot before C  11   C  11 introduces delegating constructors      Delegating constructor      If the name of the class itself appears as class-or-identifier in the   member initializer list  then the list must consist of that one member   initializer only  such constructor is known as the delegating   constructor  and the constructor selected by the only member of the   initializer list is the target constructor      In this case  the target constructor is selected by overload   resolution and executed first  then the control returns to the   delegating constructor and its body is executed       Delegating constructors cannot be recursive   class Foo   public     Foo char x  int y       Foo int y    Foo  a   y        Foo int  delegates to Foo char int        Note that a delegating constructor is an all-or-nothing proposal  if a constructor delegates to another constructor  the calling constructor isn t allowed to have any other members in its initialization list  This makes sense if you think about initializing const reference members once  and only once

User · Answer

Simply put  you cannot before C  11   C  11 introduces delegating constructors      Delegating constructor      If the name of the class itself appears as class-or-identifier in the   member initializer list  then the list must consist of that one member   initializer only  such constructor is known as the delegating   constructor  and the constructor selected by the only member of the   initializer list is the target constructor      In this case  the target constructor is selected by overload   resolution and executed first  then the control returns to the   delegating constructor and its body is executed       Delegating constructors cannot be recursive   class Foo   public     Foo char x  int y       Foo int y    Foo  a   y        Foo int  delegates to Foo char int        Note that a delegating constructor is an all-or-nothing proposal  if a constructor delegates to another constructor  the calling constructor isn t allowed to have any other members in its initialization list  This makes sense if you think about initializing const reference members once  and only once

User · Answer

No  in C   you cannot call a constructor from a constructor  What you can do  as warren pointed out  is    Overload the constructor  using different signatures Use default values on arguments  to make a  simpler  version available   Note that in the first case  you cannot reduce code duplication by calling one constructor from another  You can of course have a separate  private protected  method that does all the initialization  and let the constructor mainly deal with argument handling

User · Answer

Would be more easy to test  than decide    Try this    include  lt iostream gt   class A   public      A  int a    m a a            std  cout  lt  lt   A  Ctor   lt  lt  std  endl                 A             std  cout  lt  lt   A  dtor   lt  lt  std  endl            public      int m a      class B   public A   public      B  int a  int b    m b b   A a     public      int m b      int main         B b 9  6       std  cout  lt  lt   Test constructor delegation a      lt  lt  b m a  lt  lt     b      lt  lt  b m b  lt  lt  std  endl          return 0      and compile it with 98 std      g   main cpp -std c  98  -o test 1   you will see   A  Ctor Test constructor delegation a   9  b   6 A  dtor   so

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I would propose the use of a private friend method which implements the application logic of the constructor and is the called by the various constructors  Here is an example   Assume we have a class called StreamArrayReader with some private fields   private      istream   in           More private fields   And we want to define the two constructors   public      StreamArrayReader istream   in stream       StreamArrayReader char   filepath          More constructors      Where the second one simply makes use of the first one  and of course we don t want to duplicate the implementation of the former   Ideally  one would like to do something like   StreamArrayReader  StreamArrayReader istream   in stream          Implementation    StreamArrayReader  StreamArrayReader char   filepath        ifstream instream      instream open filepath       StreamArrayReader  amp instream       instream close        However  this is not allowed in C    For that reason  we may define a private friend method as follows which implements what the first constructor is supposed to do   private    friend void init stream array reader StreamArrayReader  o  istream   is     Now this method  because it s a friend  has access to the private fields of o  Then  the first constructor becomes   StreamArrayReader  StreamArrayReader istream   is        init stream array reader this  is       Note that this does not create multiple copies for the newly created copies  The second one becomes   StreamArrayReader  StreamArrayReader char   filepath        ifstream instream      instream open filepath       init stream array reader this   amp instream       instream close        That is  instead of having one constructor calling another  both call a private friend

User · Answer

This approach may work for some kinds of classes  when the assignment operator behaves  well     Foo  Foo            do what every Foo is needing            Foo  Foo char x         this   Foo            do the special things for a Foo with char

User · Answer

If you want to be evil  you can use the in-place  new  operator   class Foo         Foo        default constructor deliciousness          Foo Bar myParam          new  this  Foo             bar your param all night long                Seems to work for me   edit  As  ElvedinHamzagic points out  if Foo contained an object which allocated memory  that object might not be freed   This complicates things further   A more general example   class Foo     private    std  vector lt int gt  Stuff  public      Foo           Stuff 42                 default constructor deliciousness               Foo Bar myParam              this- gt  Foo          new  this  Foo             bar your param all night long                Looks a bit less elegant  for sure   JohnIdol s solution is much better

User · Answer

In Visual C   you can also use this notation inside constructor  this- Classname  Classname parameters of another constructor   See an example below   class Vertex    private    int x  y   public    Vertex int xCoo  int yCoo   x xCoo   y yCoo       Vertex          this- gt Vertex  Vertex -1  -1            I don t know whether it works somewhere else  I only tested it in Visual C   2003 and 2008  You may also call several constructors this way  I suppose  just like in Java and C    P S   Frankly  I was surprised that this was not mentioned earlier

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No  you can t call one constructor from another in C  03  called a delegating constructor    This changed in C  11  aka C  0x   which added support for the following syntax   example taken from Wikipedia   class SomeType     int number   public    SomeType int newNumber    number newNumber       SomeType     SomeType 42

User · Answer

When calling a constructor it actually allocates memory  either from the stack or from the heap  So calling a constructor in another constructor creates a local copy  So we are modifying another object  not the one we are focusing on

User · Answer

I would propose the use of a private friend method which implements the application logic of the constructor and is the called by the various constructors  Here is an example   Assume we have a class called StreamArrayReader with some private fields   private      istream   in           More private fields   And we want to define the two constructors   public      StreamArrayReader istream   in stream       StreamArrayReader char   filepath          More constructors      Where the second one simply makes use of the first one  and of course we don t want to duplicate the implementation of the former   Ideally  one would like to do something like   StreamArrayReader  StreamArrayReader istream   in stream          Implementation    StreamArrayReader  StreamArrayReader char   filepath        ifstream instream      instream open filepath       StreamArrayReader  amp instream       instream close        However  this is not allowed in C    For that reason  we may define a private friend method as follows which implements what the first constructor is supposed to do   private    friend void init stream array reader StreamArrayReader  o  istream   is     Now this method  because it s a friend  has access to the private fields of o  Then  the first constructor becomes   StreamArrayReader  StreamArrayReader istream   is        init stream array reader this  is       Note that this does not create multiple copies for the newly created copies  The second one becomes   StreamArrayReader  StreamArrayReader char   filepath        ifstream instream      instream open filepath       init stream array reader this   amp instream       instream close        That is  instead of having one constructor calling another  both call a private friend

User · Answer

No  you can t call one constructor from another in C  03  called a delegating constructor    This changed in C  11  aka C  0x   which added support for the following syntax   example taken from Wikipedia   class SomeType     int number   public    SomeType int newNumber    number newNumber       SomeType     SomeType 42

User · Answer

In Visual C   you can also use this notation inside constructor  this- Classname  Classname parameters of another constructor   See an example below   class Vertex    private    int x  y   public    Vertex int xCoo  int yCoo   x xCoo   y yCoo       Vertex          this- gt Vertex  Vertex -1  -1            I don t know whether it works somewhere else  I only tested it in Visual C   2003 and 2008  You may also call several constructors this way  I suppose  just like in Java and C    P S   Frankly  I was surprised that this was not mentioned earlier

[c++] Can I call a constructor from another constructor (do constructor chaining) in C++?

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