can function return 0 as reference

Question

I have this snippet of the code

Account& Company::findAccount(int id){
        for(list<Account>::const_iterator i = listOfAccounts.begin(); i != listOfAccounts.end(); ++i){
            if(i->nID == id){
                return *i;
            }
        }
        return 0;
    }

Is this right way to return 0 if I didn't find appropriate account? cause I receive an error:

no match for 'operator!' in '!((Company*)this)->Company::findAccount(id)'

I use it this way:

if(!(findAccount(id))){
            throw "hey";
          }

thanks in advance

Check http://stackoverflow.com/questions/2894891/this-code-appears-to-achieve-the-return-of-a-null-reference-in-c/2896330#2896330 for examples on how to return references — default, Jun 17 '10 at 11:12
Did you mean to write `list::const_iterator...`, because I don't think that i->nID will work with an `Account`.. — default, Jun 17 '10 at 11:18

score 4 · Answer 1 · answered Jun 17 '10 at 11:06

There is no such thing as a null reference. The standard says:

A reference shall be initialized to refer to a valid object or function. [ Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” obtained by dereferencing a null pointer, which causes undefined behavior.

score 2 · Answer 2 · answered Jun 17 '10 at 09:51

2

You cannot return a "null reference" in C++, because there is no such concept in this language.

If either an Account or no Account can be the result, one way to do it is to return an Account *.

answered Jun 17 '10 at 09:51

Daniel Daranas

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score 1 · Answer 3 · answered Jun 17 '10 at 09:52

1

No, this is not possible. It looks like the function may have been changed from returning a pointer (Account*) to returning a reference (Account&).

You cannot return 0/null as a reference, you will have to change the design of the function. Maybe reverting to Account* as return type.

If you only use it in an if-statement like that, you could change it to returning a std::bool. But the current signature suggests other usage patterns.

answered Jun 17 '10 at 09:52

H H

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why cannot return 0/null as a ref? I tried to pass a null pointer into a function as ref, it seems ok. – Alcott Oct 07 '11 at 02:43

score 1 · Answer 4 · answered Jun 17 '10 at 09:53

1

Another way (except for Account*) would be to return special Account::empty object, and/or test using account.IsEmpty().

answered Jun 17 '10 at 09:53

queen3

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Egeria · Accepted Answer · 2010-06-17T10:57:52.950

1

Why don't you use a standard algorithm like std::find_if.

Edit: Explanation of find_if.

Here is the reference for find_if algorithm. http://cplusplus.com/reference/algorithm/find_if/

bool isEqual(int id1, int id2) {
   return id1 == id2;
}

void foo() {
    std::list<Account> accountList;
    // Fill this list here.

    list<Account>::iterator it = std::find_if(accountList.begin(), accountList.end(), bind(isEqual, idYouSearch, _1));

    Account ac = *it;
}

You might need additional includes for bind and _1 placeholder. Propably there is a std Predicate for isEqual and you could use this instead.

edited Jun 17 '10 at 10:57

answered Jun 17 '10 at 10:00

Egeria

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can You explain it plese, I thought that list doesn't have such algorithm – helloWorld Jun 17 '10 at 10:03
the algorithms don't belong to the containers. All standard containers have iterators which you can use with the standard algorithms. – Egeria Jun 17 '10 at 10:59

can function return 0 as reference

5 Answers5