How to get alternative value from function that gives wanted data via non-const output parameter for assigning reference-to-const variable to it?

问题

The commented code works, but it is not a reference, so it has more computational cost.

void CClass::Function(const CArray<CItem*>* ItemsInput) const
{
    /*
    CArray<CItem*> Items;

    if (ItemsInput != nullptr)
        Items.Copy(*ItemsInput);
    else
        GetContainer().GetInnerItems(Items, NULL, true);
    */


    const CArray<CItem*>& Items=
        (ItemsInput!= nullptr)?
            *ItemsInput
        :
        [this] () -> const CArray<CItem*> 
        {
            CArray<CItem*> InnerItems;
            GetContainer().GetInnerItems(InnerItems, NULL, true);
            return const_cast <const CArray<CItem*>& > (InnerItems);
        }
    ;

    //...
 }

The uncommented code is the maximal extent where I got trying to use an approach of ternary operator and lambda expression, but until now it was unsuccessful, giving the error:

1>Class.cpp(line of the last semicolon): error C2446: ':' : no conversion from '`anonymous-namespace'::<lambda2>' to 'const CArray<TYPE>'
1>          with
1>          [
1>              TYPE=CItem *
1>          ]
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

Important things to notice:

I pass this on the lambda capture, just to have access to the GetInnerItems method
The GetInnerItems method gives the inner items via its first parameter of type CArray<CItem*>&, which is non-const and could never be const.

Therefore, it seems to me the problem is the way the GetInnerItems works and has nothing to do with ternary operator. Assume I can not change its signature to return a CArray, because CArrayis derived from CObject which declares:

class AFX_NOVTABLE CObject
{
//...
private:
    CObject(const CObject& objectSrc);              // no implementation
    void operator=(const CObject& objectSrc);       // no implementation
//...
}

So, questions are:

How can I assign the alternative value to the reference-to-const Items variable keeping const-correctness?
If you successfully answered the first question using a const_cast, can you now give a solution without it?

回答1:

I declared the lambda-function, but I forget to call it. So, parentheses are needed. Therefore, notice the () after the declaration with the goal of calling for its execution:

const CArray<CItem*>& Items=
    (ItemsInput!= nullptr)?
        *ItemsInput
    :
        [this]() -> const CArray<CItem*>&
        {
            CArray<CItem*> InnerItems;
            GetContainer().GetInnerItems(InnerItems, NULL, true);
            return (InnerItems);
        } ()
;

It is now compiling fine.

But this lead to a new question: Lambda did not automatically deduce return type

来源：https://stackoverflow.com/questions/32103323/how-to-get-alternative-value-from-function-that-gives-wanted-data-via-non-const

标签

c++

mfc

const-correctness

const-cast

const-reference