如何从函数中获得替代值，通过非const输出参数赋予所需的数据，以便为其分配引用到const的变量？ [英] How to get alternative value from function that gives wanted data via non-const output parameter for assigning reference-to-const variable to it?

问题描述

注释的代码工作，但它不是一个参考，所以它有更多的计算成本。

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);
        }
    ;

    //...
 }

取消注释的代码是我尝试使用三元运算符和lambda表达式的方法的最大程度，但是直到现在它是不成功的，给出错误：

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

重要要注意的事情：

• 我在lambda捕获时传递 this 访问 GetInnerItems 方法


• GetInnerItems 它是 CArray 类型的第一个参数，它是非常量，永远不能为const。



• 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.

因此，在我看来，问题是 GetInnerItems 的工作方式，与三元运算符无关。假设我不能改变它的签名返回一个 CArray ，因为 CArray 派生自 CObject 其中声明：

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
//...
}

are：

• 如何将替代值指定给 reference-to-const 项变量保持const正确性？

• How can I assign the alternative value to the reference-to-const Items variable keeping const-correctness?

如果您成功地使用

If you successfully answered the first question using a const_cast, can you now give a solution without it?

推荐答案

我声明了lambda函数，但我忘了调用它。因此，需要括号。因此，请注意在声明后调用其执行的（）：

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);
        } ()
;

现在编译正常。

但这会导致一个新问题： Lambda没有自动推导返回类型

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

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