How can I initialize C object member variables in the constructor

Question

I ve got a class that has a couple of objects as member variables  I don t want the constructors for these members to be called when declared  so I m trying to hang onto a pointer to the object explicitly  I have no idea what I m doing  I thought maybe I could do the following  where the constructor is called immediately when initializing the object member variable  class MyClass       public          MyClass int n       private          AnotherClass another 100      Construct AnotherClass right away      But I want the MyClass constructor to call the AnotherClass constructor  Here s what my code looks like  FIle BigMommaClass h  include  quot ThingOne h quot   include  quot ThingTwo h quot   class BigMommaClass            public                  BigMommaClass int numba1  int numba2            private                  ThingOne  ThingOne                  ThingTwo  ThingTwo      FIle BigMommaClass cpp  include  quot BigMommaClass h quot   BigMommaClass  BigMommaClass int numba1  int numba2            this- gt ThingOne   ThingOne 100           this- gt ThingTwo   ThingTwo numba1  numba2      Here s the error I m getting when I try to compile  g   -Wall -c -Iclasses -o objects BigMommaClass o classes BigMommaClass cpp In file included from classes BigMommaClass cpp 1 0  classes BigMommaClass h 12 8  error  declaration of   ThingTwo  BigMommaClass  ThingTwo   classes ThingTwo h 1 11  error  changes meaning of   ThingTwo   from   class ThingTwo   classes BigMommaClass cpp  In constructor   BigMommaClass  BigMommaClass int  int     classes BigMommaClass cpp 4 30  error  cannot convert   ThingOne   to   ThingOne    in assignment classes BigMommaClass cpp 5 37  error      BigMommaClass  this - gt BigMommaClass  ThingTwo   cannot be used as a function make       BigMommaClass o  Error 1  Am I using the right approach  but the wrong syntax  Or should I be coming at this from a different direction

User · Accepted Answer

You can specify how to initialize members in the member initializer list   BigMommaClass       BigMommaClass int  int    private      ThingOne thingOne      ThingTwo thingTwo      BigMommaClass  BigMommaClass int numba1  int numba2        thingOne numba1   numba2   thingTwo numba1  numba2

User · Answer

I know this is 5 years later  but the replies above don t address what was wrong with your software   Well  Yuushi s does  but I didn t realise until I had typed this - doh    They answer the question in the title How can I initialize C   object member variables in the constructor  This is about the other questions  Am I using the right approach but the wrong syntax  Or should I be coming at this from a different direction  Programming style is largely a matter of opinion  but an alternative view to doing as much as possible in a constructor is to keep constructors down to a bare minimum  often having a separate initialization function  There is no need to try to cram all initialization into a constructor  never mind trying to force things at times into the constructors initialization list  So  to the point  what was wrong with your software  private      ThingOne  ThingOne      ThingTwo  ThingTwo   Note that after these lines  ThingOne  and ThingTwo  now have two meanings  depending on context  Outside of BigMommaClass  ThingOne is the class you created with  include  quot ThingOne h quot  Inside BigMommaClass  ThingOne is a pointer  That is assuming the compiler can even make sense of the lines and doesn t get stuck in a loop thinking that ThingOne is a pointer to something which is itself a pointer to something which is a pointer to     Later  when you write this- gt ThingOne   ThingOne 100   this- gt ThingTwo   ThingTwo numba1  numba2    bear in mind that inside of BigMommaClass your ThingOne is a pointer  If you change the declarations of the pointers to include a prefix  p  private      ThingOne  pThingOne      ThingTwo  pThingTwo   Then ThingOne will always refer to the class and pThingOne to the pointer  It is then possible to rewrite this- gt ThingOne   ThingOne 100   this- gt ThingTwo   ThingTwo numba1  numba2    as pThingOne   new ThingOne 100   pThingTwo   new ThingTwo numba1  numba2    which corrects two problems  the double meaning problem  and the missing new   You can leave this- gt  if you like   With that in place  I can add the following lines to a C   program of mine and it compiles nicely  class ThingOne public ThingOne int n       class ThingTwo public ThingTwo int x  int y        class BigMommaClass        public              BigMommaClass int numba1  int numba2        private              ThingOne  pThingOne              ThingTwo  pThingTwo      BigMommaClass  BigMommaClass int numba1  int numba2        pThingOne   new ThingOne numba1   numba2       pThingTwo   new ThingTwo numba1  numba2       When you wrote this- gt ThingOne   ThingOne 100   this- gt ThingTwo   ThingTwo numba1  numba2    the use of this- gt  tells the compiler that the left hand side ThingOne is intended to mean the pointer  However we are inside BigMommaClass at the time and it s not necessary  The problem is with the right hand side of the equals where ThingOne is intended to mean the class  So another way to rectify your problems would have been to write this- gt ThingOne   new   ThingOne 100   this- gt ThingTwo   new   ThingTwo numba1  numba2    or simply ThingOne   new   ThingOne 100   ThingTwo   new   ThingTwo numba1  numba2    using    to change the compiler s interpretation of the identifier

User · Answer

You re trying to create a ThingOne by using operator  which isn t going to work  incorrect syntax   Also  you re using a class name as a variable name  that is  ThingOne  ThingOne  Firstly  let s fix the variable names   private      ThingOne  t1      ThingTwo  t2    Since these are pointers  they must point to something  If the object hasn t been constructed yet  you ll need to do so explicitly with new in your BigMommaClass constructor   BigMommaClass  BigMommaClass int n1  int n2        t1   new ThingOne 100       t2   new ThingTwo n1  n2       Generally initializer lists are preferred for construction however  so it will look like   BigMommaClass  BigMommaClass int n1  int n2        t1 new ThingOne 100    t2 new ThingTwo n1  n2

User · Answer

This question is a bit old  but here s another way in C  11 of  quot doing more work quot  in the constructor before initialising your member variables  BigMommaClass  BigMommaClass int numba1  int numba2        thingOne    int n1  int n2  return n1 n2   numba1 numba2          thingTwo numba1  numba2      The lambda function above will be invoked and the result passed to thingOnes constructor  You can of course make the lambda as complex as you like

User · Answer

Regarding the first  and great  answer from chris who proposed a solution to the situation where the class members are held as a  quot true composite quot  members  i e - not as pointers nor references   The note is a bit large  so I will demonstrate it here with some sample code  When you chose to hold the members as I mentioned  you have to keep in mind also these two things   For every  quot composed object quot  that does not have a default constructor - you must initialize it in the initialization list of all the constructor s of the  quot father quot  class  i e - BigMommaClass or MyClass in the original examples and MyClass in the code below   in case there are several  see InnerClass1 in the example below   Meaning  you can  quot comment out quot  the m innerClass1 a  and m innerClass1 15  only if you enable the InnerClass1 default constructor   For every  quot composed object quot  that does have a default constructor - you may initialize it within the initialization list  but it will work also if you chose not to  see InnerClass2 in the example below     See sample code  compiled under Ubuntu  18 04  Bionic Beaver  with g   version 7 3 0    include  lt iostream gt   using namespace std   class InnerClass1       public          InnerClass1 int a    m a a                        cout  lt  lt   quot InnerClass1  InnerClass1 - set m a  quot   lt  lt  m a  lt  lt  endl                        No default constructor         InnerClass1     m a 15                        cout  lt  lt   quot InnerClass1  InnerClass1   - set m a  quot   lt  lt  m a  lt  lt  endl                                 InnerClass1                         cout  lt  lt   quot InnerClass1   InnerClass1 quot   lt  lt  endl                 private          int m a      class InnerClass2       public          InnerClass2 int a    m a a                        cout  lt  lt   quot InnerClass2  InnerClass2 - set m a  quot   lt  lt  m a  lt  lt  endl                     InnerClass2     m a 15                        cout  lt  lt   quot InnerClass2  InnerClass2   - set m a  quot   lt  lt  m a  lt  lt  endl                      InnerClass2                         cout  lt  lt   quot InnerClass2   InnerClass2 quot   lt  lt  endl                 private          int m a      class MyClass       public          MyClass int a  int b    m innerClass1 a      m innerClass2 a     m b b                        cout  lt  lt   quot MyClass  MyClass int b  - set m b to  quot   lt  lt  m b  lt  lt  endl                      MyClass     m innerClass1 15     m innerClass2 15     m b 17                        cout  lt  lt   quot MyClass  MyClass   - m b  quot   lt  lt  m b  lt  lt  endl                      MyClass                         cout  lt  lt   quot MyClass   MyClass quot   lt  lt  endl                 private          InnerClass1 m innerClass1          InnerClass2 m innerClass2          int m b      int main int argc  char   argv        cout  lt  lt   quot main - start quot   lt  lt  endl       MyClass obj       cout  lt  lt   quot main - end quot   lt  lt  endl      return 0

[c++] How can I initialize C++ object member variables in the constructor?

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