Return a NULL object if search result not found

Question

I m pretty new to C   so I tend to design with a lot of Java-isms while I m learning  Anyway  in Java  if I had class with a  search  method that would return an object T from a Collection lt  T   that matched a specific parameter  I would return that object and if the object was not found in the collection  I would return null  Then in my calling function I would just check if tResult    null           In C    I m finding out that I can t return a null value if the object doesn t exist  I just want to return an  indicator  of type T that notifies the calling function that no object has been found  I don t want to throw an exception because it s not really an exceptional circumstance   This is what my code looks like right now   class Node       Attr amp  getAttribute const string amp  attribute name  const            search collection          if found at i             return attributes i            if not found             return NULL     what should this be         private      vector lt Attr gt  attributes      How can I change it so I can give that kind of marker

User · Accepted Answer

In C    references can t be null  If you want to optionally return null if nothing is found  you need to return a pointer  not a reference   Attr  getAttribute const string amp  attribute name  const        search collection      if found at i         return  amp attributes i        if not found         return nullptr      Otherwise  if you insist on returning by reference  then you should throw an exception if the attribute isn t found    By the way  I m a little worried about your method being const and returning a non-const attribute  For philosophical reasons  I d suggest returning const Attr    If you also may want to modify this attribute  you can overload with a non-const method returning a non-const attribute as well

User · Answer

If you want a NULL return value you need to use pointers instead of references   References can t themselves be NULL     Note to the future comment posters  Yes you can have the address of a reference be NULL if you really really try to    See my answer here for a list of differences between references and pointers

User · Answer

You can easily create a static object that represents a NULL return   class Attr  extern Attr AttrNull   class Node           Attr amp  getAttribute const string amp  attribute name  const         search collection       if found at i          return attributes i         if not found          return AttrNull       bool IsNull const Attr amp  test  const       return  amp test     amp AttrNull      private      vector lt Attr gt  attributes        And somewhere in a source file   static Attr AttrNull

User · Answer

There are several possible answers here   You want to return something that might exist   Here are some options  ranging from my least preferred to most preferred    Return by reference  and signal can-not-find by exception   Attr  getAttribute const string  attribute name  const         search collection      if found at i         return attributes i        if not found         throw no such attribute error      It s likely that not finding attributes is a normal part of execution  and hence not very exceptional   The handling for this would be noisy   A null value cannot be returned because it s undefined behaviour to have null references    Return by pointer  Attr  getAttribute const string  attribute name  const         search collection      if found at i         return  attributes i        if not found         return nullptr      It s easy to forget to check whether a result from getAttribute would be a non-NULL pointer  and is an easy source of bugs    Use Boost Optional  boost  optional lt Attr   getAttribute const string  attribute name  const         search collection      if found at i         return attributes i        if not found         return boost  optional lt Attr          A boost  optional signifies exactly what is going on here  and has easy methods for inspecting whether such an attribute was found     Side note  std  optional was recently voted into C  17  so this will be a  standard  thing in the near future

User · Answer

You can try this   return  amp Type

User · Answer

As you have figured out that you cannot do it the way you have done in Java  or C    Here is another suggestion  you could pass in the reference of the object as an argument and return bool value  If the result is found in your collection  you could assign it to the reference being passed and return    true     otherwise return    false     Please consider this code   typedef std  map lt string  Operator gt  OPERATORS MAP   bool OperatorList  tryGetOperator string token  Operator amp  op        bool val   false       OPERATORS MAP  iterator it   m operators find token       if  it    m operators end                  op   it- gt second          val   true            return val      The function above has to find the Operator against the key  token   if it finds the one it returns true and assign the value to parameter Operator amp  op   The caller code for this routine looks like this  Operator opr  if  OperatorList  tryGetOperator strOperator  opr           Do something here if true is returned

User · Answer

The reason that you can t return NULL here is because you ve declared your return type as Attr amp   The trailing  amp  makes the return value a  reference   which is basically a guaranteed-not-to-be-null pointer to an existing object  If you want to be able to return null  change Attr amp  to Attr

User · Answer

You are unable to return NULL because the return type of the function is an object reference and not a pointer

[c++] Return a "NULL" object if search result not found

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