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< 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& getAttribute(const string& attribute_name) const {        //search collection        //if found at i             return attributes[i];        //if not found             return NULL; // what should this be?     }  private:     vector<Attr> attributes; } 

How can I change it so I can give that kind of marker?

Answer 1

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.)

Answer 2

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<Attr&> getAttribute(const string& attribute_name) const  {    //search collection    //if found at i         return attributes[i];    //if not found         return boost::optional<Attr&>(); }

A boost::optional signifies exactly what is going on here, and has easy methods for inspecting whether such an attribute was found.

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Side note: std::optional was recently voted into C++17, so this will be a "standard" thing in the near future.