How to return a class object by reference in C

Question

I have a class called Object which stores some data   I would like to return it by reference using a function like this       Object amp  return Object      Then  in my code  I would call it like this       Object myObject   return Object      I have written code like this and it compiles   However  when I run the code  I consistently get a seg fault   What is the proper way to return a class object by reference

User · Answer

You can only return non-local objects by reference  The destructor may have invalidated some internal pointer  or whatever   Don t be afraid of returning values -- it s fast

User · Answer

Well  it is maybe not a really beautiful solution in the code  but it is really beautiful in the interface of your function  And it is also very efficient  It is ideal if the second is more important for you  for example  you are developing a library    The trick is this    A line A a   b make    is internally converted to a constructor of A  i e  as if you had written A a b make      Now b make   should result a new class  with a callback function  This whole thing can be fine handled only by classes  without any template    Here is my minimal example  Check only the main    as you can see it is simple  The internals aren t   From the viewpoint of the speed  the size of a Factory  Mediator class is only 2 pointers  which is more that 1 but not more  And this is the only object in the whole thing which is transferred by value    include  lt stdio h gt   class Factory     public      class Mediator       class Result         public          Result               printf   Factory  Result  Result   n                        Result Mediator fm              printf   Factory  Result  Result Mediator  n              fm call this                          typedef void   MakeMethod  Factory  factory  Result  result        class Mediator         private          Factory  factory          MakeMethod makeMethod         public          Mediator Factory  factory  MakeMethod makeMethod              printf   Factory  Mediator  Mediator Factory   MakeMethod  n              this- gt factory   factory            this- gt makeMethod   makeMethod                      void call Result  result              printf   Factory  Mediator  call Result   n                makeMethod  factory  result                         class A   class B   private Factory     private      int v     public      B int v          printf   B  B   n          this- gt v   v              int getV   const         printf   B  getV   n          return v              static void makeCb Factory  f  Factory  Result  a        Factory  Mediator make           printf   Factory  Mediator B  make   n          return Factory  Mediator static cast lt Factory  gt  this    amp B  makeCb              class A   private Factory  Result     friend class B     private      int v     public      A           printf   A  A   n          v   0              A Factory  Mediator fm    Factory  Result fm          printf   A  A Factory  Mediator  n                int getV   const         printf   A  getV   n          return v              void setV int v          printf   A  setV  i  n   v         this- gt v   v             void B  makeCb Factory  f  Factory  Result  r          printf   B  makeCb Factory   Factory  Result   n          B  b   static cast lt B  gt  f         A  a   static cast lt A  gt  r         a- gt setV b- gt getV   1           int main int argc  char   argv      B b 42     A a   b make      printf   a v    i n   a getV       return 0

User · Answer

You re probably returning an object that s on the stack  That is  return Object   probably looks like this   Object amp  return Object         Object object to return             do stuff          return object to return      If this is what you re doing  you re out of luck - object to return has gone out of scope and been destructed at the end of return Object  so myObject refers to a non-existent object  You either need to return by value  or return an Object declared in a wider scope or newed onto the heap

User · Answer

I will show you some examples   First example  do not return local scope object  for example   const string  amp dontDoThis const string  amp s        string local   s      return local      You can t return local by reference  because local is destroyed at the end of the body of dontDoThis   Second example  you can return by reference   const string  amp shorterString const string  amp s1  const string  amp s2        return  s1 size    lt  s2 size      s1   s2      Here  you can return by reference both s1 and s2 because they were defined before shorterString was called   Third example   char  amp get val string  amp str  string  size type ix        return str ix       usage code as below   string s  123456    cout  lt  lt  s  lt  lt  endl  char  amp ch   get val s  0    ch    A   cout  lt  lt  s  lt  lt  endl     A23456   get val can return elements of s by reference because s still exists after the call   Fourth example  class Student   public      string m name      int age           string  amp getName        string  amp Student  getName            you can return by reference     return m name     string amp  Test Student  amp student           we can return  m name  by reference here because  student  still exists after the call     return stu m name      usage example   Student student  student m name    jack   string name   student getName       or string name2   Test student     Fifth example   class String   private      char  str    public      String  amp operator  const String  amp str       String  amp String  operator  const String  amp str        if  this     amp str                return  this            delete    str       int length   strlen str str        str    new char length   1       strcpy str   str str        return  this      You could then use the operator  above like this   String a  String b  String c   b   a

User · Answer

You can only use       Object amp  return Object      if the object returned has a greater scope than the function  For example  you can use it if you have a class where it is encapsulated  If you create an object in your function  use pointers  If you want to modify an existing object  pass it as an argument     class  MyClass        private          Object myObj         public           Object amp  return Object                 return myObj                       Object  return created Object                 return new Object                         bool modify Object  Object amp  obj                    obj   myObj  return true  both possible             return obj modifySomething      true

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