Can not connvert int to AcDbObjectIdArray error when NULL is assign to AcDbObjectIdArray variable in ObjectARX 2023.

[nkokate052]

Please resolve this issue.

If I assign NULL to AcDbObjectIdArray variable in ObjectARX 2023 in visual studio 2019 it is giving error can not convert from int to AcDbObjectIdArray. Screen shot is attached with this.

Example : 

AcDbObjectIdArray arrObj = NULL;

this giving error can not convert from int to AcDbObjectIdArray.

[tbrammer]

Just write  AcDbObjectIdArray arrObj;   to get an empty array.

[nkokate052]

but i have to set default value to AcDbObjectIdArray variable in function declaration.

What can I do to set default value?

[tbrammer]

AcDbObjectIdArray has a very nice default constructor that creates an empty array of AcDbObjectIds. You just don't need a default value. If you insist in assigning a default value (i.e. for syntactical reasons) you can do something like this:

AcDbObjectIdArray emptyIdArray; // global variable

void fkt() {
    AcDbObjectIdArray ids1 = emptyIdArray;
    AcDbObjectIdArray ids2 = AcDbObjectIdArray();
}

AcDbObjectIdArray is a typedef: 

    typedef AcArray<AcDbObjectId> AcDbObjectIdArray;

The assignment operator of AcArray<T,R> only accepts another AcArray:

    AcArray<T,R>&         operator =  (const AcArray<T,R>&);
    AcArray<T,R>&         operator =  (AcArray<T,R>&&);

[autodaug · Accepted solution]

---

@nkokate052 wrote:

but i have to set default value to AcDbObjectIdArray variable in function declaration.

What can I do to set default value?

---

It sounds like you might be passing the array by value, as in:  myfunc(AcDbObjectIdArray idArr)

That's not usually the best practice, because it can cause unnecessary copying of the array during func calls.

If the array is not being modified, you can pass a const ref, as in:  myfunc(const AcDbObjectIdArray & idArr)

And if it *is* being modified, then pass it as a non-const ref: myfunc(AcDbObjectIdArray & idArr)

Or, if the array arg is optional, you can make it a ptr :  myfunc(const AcDbObjectIdArray *pArr = nullptr);

[daniel_cadext]

looks like you're trying to do an optional argument, you can pass by pointer.

    static void Foo()
    {
        AcDbObjectIdArray ids;
        func(10, &ids);
        acutPrintf(_T("\nfoo %ld"), ids.length());
    }

    static void Far()
    {
        AcDbObjectIdArray ids;
        func(10);
        acutPrintf(_T("\nfar = %ld"), ids.length());
    }

    static void foobar()
    {
        std::unique_ptr<AcDbObjectIdArray> pids; //is null
        func(10, pids.get());
        if (pids)
            acutPrintf(_T("\nfoobar = %ld"), pids->length());
        else
            acutPrintf(_T("\nfoobar = %ld"), 0);
    }

    static void furbar()
    {
        std::unique_ptr<AcDbObjectIdArray> pids(new AcDbObjectIdArray());
        func(10, pids.get());
        if (pids)
            acutPrintf(_T("\nfurbar = %ld"), pids->length());
        else
            acutPrintf(_T("\nfurbar = %ld"), 0);

    }

    static void func(size_t cnt, AcDbObjectIdArray* ids = nullptr)
    {
        if (ids != nullptr)
        {
            for (size_t idx = 0; idx < cnt; idx++)
                ids->append(AcDbObjectId::kNull);
        }
    }

[tbrammer · Accepted solution]

As I suggested before you can pass a default value to a function like this: 

int defArr(AcDbObjectIdArray arr = AcDbObjectIdArray());

 or like this:

//Header file:
extern AcDbObjectIdArray defaultIdArray; // global variable
int defArr2(AcDbObjectIdArray arr=defaultIdArray);


//CPP file:
AcDbObjectIdArray defaultIdArray;

int defArr2(AcDbObjectIdArray arr) {
    return arr.length(); //i.e.
}

@autodaug's approach is a little more efficient.

But if you don't want to change function signatures you can do it like this.

[autodaug · Accepted solution]

I agree, if you want to pass the array by value, and have a default parameter, then it should be = AcDbObjectIdArray()

The second approach with the global defaultIdArray object is more complicated and less flexible, imo.

[daniel_cadext]

I assumed it was an [in][out] parameter, with a global or a new there's no way to know since there's no null.