Return a "Null" Object If Search Result Not Found

Return a NULL object if search result not found

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& attribute_name) const {
   //search collection
   //if found at i
        return &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.)

don't return null -- what to return for search function

Throwing an exception is an expensive operation, since there is a context switch and a lot of debug information has to be gathered, so you want to avoid throwing them as a way to control process flow (especially if you can handle the situation without throwing exceptions). Returning a null can be perfectly acceptable in order to avoid catching exceptions.

An example of this in action would be a couple of LINQ functions in C#. These methods can return a null:

SingleOrDefault(); // returns a single instance of an object, or null if not found

FirstOrDefault(); // returns the first matching object, or null if not found

This allows you to check for null without trying to figure out control flow using exception handling.

One exception (pardon the pun) that I can think of is using exceptions to communicate across program boundries. If you had, for example, a data access layer in a separate DLL, and you needed to communicate a database failure back to the parent program, sometimes the best way to do that is through exception handling.

Return null default value if no result found

You can use below aggregation

Mongodb doesn't produce the result if there is not $matched data found with the query.

But you can use $facet aggregation which processes multiple aggregation pipeline within single stage.

So first use $facet to get the $matched documents and use $projection if no ($ifNull) data found.

let searchTerm = "form2"

db.myCollec.aggregate([
  { "$facet": {
    "data": [
      { "$match": { "name": searchTerm  }},
      { "$project": { "name": 1, "type": 1, "_id": 0 }}
    ]
  }},
  { "$project": {
    "name": {
      "$ifNull": [{ "$arrayElemAt": ["$data.name", 0] }, searchTerm ]
    },
    "type": {
      "$ifNull": [{ "$arrayElemAt": ["$data.type", 0] }, null]
    }
  }}
])

How to return NULL object in C++

I think you need something like

Normal* Sphere::hit(Ray ray) {
   //stuff is done here
   if(something happens) {
       return NULL;
   }
   //other stuff
   return new Normal(something, somethingElse);
}

to be able to return NULL;

throw an exception or return NULL object in C++?

You can either return a pointer to an object, or a reference. In C++, unlike C#, you return the object by value, therefore, the object gets copied. To return it by reference, write Step& Config::getStep(string stepName).

To throw an exception, just write throw MissingStepException(); and then handle it like this:

try {
    Step s = c.getStep("step");
}
catch (MissingStepException& ex) {
    // handle
}

You need to define MissingStepException class first, of course.

Choosing what to do in general: return NULL or throw exception depends on your logic: if missing step is a logic error or an unlikely condition, it's better to use exception, otherwise returning a NULL pointer would do.

Should a retrieval method return 'null' or throw an exception when it can't produce the return value?

If you are always expecting to find a value then throw the exception if it is missing. The exception would mean that there was a problem.

If the value can be missing or present and both are valid for the application logic then return a null.

More important: What do you do other places in the code? Consistency is important.

Is there a standard function to check for null, undefined, or blank variables in JavaScript?

You can just check if the variable has a truthy value or not. That means

if( value ) {
}

will evaluate to true if value is not:

• null
• undefined
• NaN
• empty string ("")
• 0
• false

The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section.

Furthermore, if you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. For instance

if( typeof foo !== 'undefined' ) {
    // foo could get resolved and it's defined
}

If you can be sure that a variable is declared at least, you should directly check if it has a truthy value like shown above.

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