如何初始化 CArray<CClass*> 的默认值具有空 CArray 的函数参数?

How can I initialize the default value of a CArraylt;CClass*gt; function parameter with an empty CArray?(如何初始化 CArraylt;CClass*gt; 的默认值具有空 CArray 的函数参数?)

问题描述

我知道我可以用 std::vector 做得更好，但是我正在搞乱的应用程序已经在很多相关的方面有一堆 CArray 参数函数...我暂时不会全部更改！

I know I could do this better with std::vector, but the application I am messing with, has already a bunch of CArray parameters on a lot of related functions ... and I will not change them all at the moment!

我只是想定义一个空的 CArray - pointers 到 CClass 的数组，所以问题 不能在 CClass 构造函数上——作为函数参数的默认值.

I simply want to define an empty CArray<CClass*> — array of pointers to CClass, so the problem can not be on the CClass constructor — as the default value of a function parameter.

方法 1

如果我尝试使用 赋值运算符 和 默认构造函数:

If I try to do it with assignment operator and default constructor:

void Function(CArray<CClass*> parameter = CArray<CClass*>());

我得到错误:

1>C:Program Files (x86)Microsoft Visual Studio 10.0VCatlmfcincludeafxTempl.h(262): error C2248: 'CObject::CObject' : cannot access private member declared in class 'CObject'
1>          C:Program Files (x86)Microsoft Visual Studio 10.0VCatlmfcincludeafx.h(535) : see declaration of 'CObject::CObject'
1>          C:Program Files (x86)Microsoft Visual Studio 10.0VCatlmfcincludeafx.h(510) : see declaration of 'CObject'
1>          This diagnostic occurred in the compiler generated function 'CArray<TYPE>::CArray(const CArray<TYPE> &)'
1>          with
1>          [
1>              TYPE=CClass *
1>          ]

方法 2

我也尝试了复制构造函数:

void Function(CArray<Class*> parameter(CArray<CClass*>()));

我得到了错误:

 >File.cpp(line X): error C2664: 'FunctionClass::Function' : cannot convert parameter 1 from 'CArray<TYPE>' to 'CArray<TYPE> (__cdecl *)(CArray<TYPE> (__cdecl *)(void))'
1>          with
1>          [
1>              TYPE=CClass*
1>          ]
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

• 第 X 行:包含向 Function 提供参数的调用，如图所示:pFunctionClass->Function(parameter);

1>CFunctionClass.cpp(Y 行):错误 C2511:'void CFunctionClass::Function(CArray)':在 'CShoePlaceDoc' 中找不到重载的成员函数1> 与1> [1> 类型=C类*1>]1> FunctionClass.h(line A) : 见 'CFunctionClass' 的声明

1>CFunctionClass.cpp(line Y): error C2511: 'void CFunctionClass::Function(CArray)' : overloaded member function not found in 'CShoePlaceDoc' 1> with 1> [ 1> TYPE=CClass* 1> ] 1> FunctionClass.h(line A) : see declaration of 'CFunctionClass'

• Y行包含Function实现头，如图:`void CFunctionClass::Function(CArray parameter)
• line Y contains the Function implementation header, as shown: `void CFunctionClass::Function(CArray parameter)

1>File.cpp(Z 行):错误 C2660:'CClassFunction::Function':函数不接受 0 个参数

1>File.cpp(line Z): error C2660: 'CClassFunction::Function' : function does not take 0 arguments

• 第 Z 行:包含对 Function 的调用，但未提供参数，如图所示:pClassFunction->Function();
• line Z: contains a call to Functionwithout supplying it parameters, as shown: pClassFunction->Function();

该方法不起作用，但它得到了一个结论:不可能使用复制构造函数来为函数参数分配默认值.

The approach didn't work, but it got its way towards a conclusion: It is not possible to use a copy-constructor for assigning a default value for a function parameter.

方法 3

如果我尝试使用 lambda:

void Function(CArray<CClass*> parameter = [] () -> CArray<CClass*>{ return CArray<CClass*> (); } );

，那么我会得到这两个错误的多个输出:

, then I will get multiple outputs of these two errors:

1>FunctionClass.h(line A): error C2440: 'default argument' : cannot convert from '`anonymous-namespace'::<lambda2>' to 'CArray<TYPE>'
1>          with
1>          [
1>              TYPE=CClass*
1>          ]
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>FunctionClass.h(line B): fatal error C1903: unable to recover from previous error(s); stopping compilation

• A 行:方法声明
• B行:关闭包含Function方法的FunctionClass类的}

问题的根源

问题的根源似乎是CArray是一个直接派生自CObject的类，将赋值运算符声明为私有:

The root cause of the problem seems to be the fact that CArray is a class directly derived from CObject, which declares the assignment operator as private:

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

那么，如何提供一个空数组作为参数的默认值?

So, how can I supply an empty array as default value for the parameter?

推荐答案

有了这个声明

void Function(CArray<CClass*> parameter /*...*/);

你不能.如您所见，调用此函数将调用 CObject 的私有复制构造函数.

You can't. Calling this function will invoke the private copy constructor of CObject as you have noticed.

你可以做的是在你的类中添加一个 static CArray<CClass*> 的对象，并使用对它的引用来初始化函数.这样它将为空(只要您不填充它...)，您可以执行 .IsEmpty() 检查一下.

What you could do, is to add a object of static CArray<CClass*> in your class and initialize the function with a reference to it. This way it will be empty (as long as you do not populate it...) and you can perform a .IsEmpty() check on it.

private:
    static CArray<CClass*> myObj;
//...
void Function(CArray<CClass*> &parameter = myObj);

或者将其初始化为0.这样你只需通过if (parameter) 或if (NULL == parameter) 来检查它.

Or initialize it to 0. This way you simply check it by if (parameter) or if (NULL == parameter).

void Function(CArray<CClass*> *parameter = NULL);

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