如何从基类变量（C ++，Visual Studio 2013）访问公共嵌套类方法？ [英] How can I access public nested class methods from base class variables (C++, Visual Studio 2013)?

问题描述

我有一个嵌套类

 int testfunction（）
 {
 CDCDirectControls :: Graphic :: Button Ÿ; 
 y.isNew.Set（）; 
 CDCDirectControls x1; 
 x1.Graphic.Button.isNew.Set（）; //不编译错误
 
返回1; 
} 

我得到以下输出：

 1> ------ Build build：Project：bmpLoad，Configuration：Debug Win32 ------ 
 1> CDCDirectControls.cpp 
 1> c：\ users \stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h（93）：warning C4832：token'。 '在UDT'CDCDirectControls :: Graphic'
 1>之后是非法的。 c：\users\stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h（44）：参见'CDCDirectControls :: Graphic'的声明
 1> c：\users\stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h（93）：错误C2274：'function-style cast'：非法作为右侧'。'运算符
 1> c：\ usersrs \stephen\desktop\stephen\cppprojects \samplemfc \bmpload\bmpload \ cdcdirectcontrols.h（93）：错误C2228：左边的'.Button'必须有class / struct / union 
 1> c：\ usersrs \stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h（93 ）：错误C2228：'。isNew'左边必须有class / struct / union 
 1> c：\ usersrs \stephen \ desktop \stephen \ cppprojects \samplemfc \ bmpload \bmpload \cdcdirectcontrols .h（93）：错误C2228：'。''的左边必须有class / struct / union 
 1>生成代码... 
 1>跳过...（未检测到相关更改）
 1> bmpLoadView.cpp 
 ==========构建：0成功，1个失败，0个最新，0跳过========== 

我已将错误固定到标有不编译错误的行。



更新＃1：



我在类中定义了变量itGraphic，itsButton等等然后它没有编译器错误就像这样工作。但有没有更好的方法来使用调用变量？是否有更好的语法？



否则，当我从cpp文件中拆分头时，我需要小心将变量定义放在cpp端。< br $> b $ b

  int  testfunction（）
 {
 CDCDirectControls :: Graphic :: Button y; 
 y.isNew.Set（）; 
 CDCDirectControls x1; 
 x1.itsGraphic.itsButton.isNew.Set（）; 
 
 
  return   1 ; 
} 

问题：



1）如果有一个基类的变量，访问嵌套类方法的正确方法是什么？



1a）是否有方法独立于在类中定义变量？ （我想可以定义一个指针;但是有什么比这更清晰吗？）

 x1.itsGraphic-> itsButton-> isNew.Set（）; //当itsGraphic被定义为类中Graphic的指针时，编译得很好，但有没有更简洁的方法从基类访问嵌套类方法？

2）是从基类变量引用嵌套类的唯一方法 - 创建变量然后访问变量？



参考文献：



嵌套类声明 [ 1 ]



初始化嵌套类 - C ++论坛 [ 2 ]

解决方案

< blockquote>如果您有特定类型的对象，则无法访问从该对象派生的类的成员。



但是常见的情况是你有一个对象的引用应该在别处使用（例如作为参数传递给另一个函数）使用基类的类型。在这种情况下，您可以将传递的引用强制转换为实际类型或使用虚函数。



Casting（ dynamic_cast转换 - cppreference.com [ ^ ]）要求您知道实时类型的运行时类型信息（RTTI）（请参阅类型支持（基本类型，RTTI，类型特征）） - cppreference.com [ ^ ]和运行时类型信息 - MSDN [ ^ ]）或在基地有一名成员对于每个派生类都是唯一的类。



使用虚拟函数是更智能的解决方案。



你的派生类应该提供一个函数 isNew（），将其添加为虚拟（虚拟函数说明符 - cppreference.com [ ^ ] ）到基类和派生类。如果基类是抽象的（不用作变量或成员），它可以是纯虚函数（参见抽象类 - cppreference .com [ ^ ]）以确保它在所有派生类中实现。

I have a nested class

int testfunction()
{
	CDCDirectControls::Graphic::Button y;
	y.isNew.Set();
	CDCDirectControls x1;
	x1.Graphic.Button.isNew.Set();  // does not compile with error

	return 1;
}

I get the following output:

1>------ Build started: Project: bmpLoad, Configuration: Debug Win32 ------
1>  CDCDirectControls.cpp
1>c:\users\stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h(93): warning C4832: token '.' is illegal after UDT 'CDCDirectControls::Graphic'
1>          c:\users\stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h(44) : see declaration of 'CDCDirectControls::Graphic'
1>c:\users\stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h(93): error C2274: 'function-style cast' : illegal as right side of '.' operator
1>c:\users\stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h(93): error C2228: left of '.Button' must have class/struct/union
1>c:\users\stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h(93): error C2228: left of '.isNew' must have class/struct/union
1>c:\users\stephen\desktop\stephen\cppprojects\samplemfc\bmpload\bmpload\cdcdirectcontrols.h(93): error C2228: left of '.Set' must have class/struct/union
1>  Generating Code...
1>  Skipping... (no relevant changes detected)
1>  bmpLoadView.cpp
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

I have pinned the error down to the line marked with "does not compile with error".

Update #1:

I defined variables inside the class itsGraphic, itsButton, and so forth and then it works like this without a compiler error. But is there a better way of doing it with the calling variable? Is there a better syntax?

Otherwise, when I split the header from the cpp file, I need to be careful to put the variable definition on the cpp side.

int testfunction()
{
	CDCDirectControls::Graphic::Button y;
	y.isNew.Set();
	CDCDirectControls x1;
	x1.itsGraphic.itsButton.isNew.Set();


	return 1;
}

Questions:

1) If one has a variable of the base class, what is the proper way to access the nested class methods?

1a) Is there a method independent of defining a variable in the class? (I guess a pointer could be defined; but is there anything cleaner than that?)

x1.itsGraphic->itsButton->isNew.Set(); //compiles fine when itsGraphic is defined as a pointer to Graphic in the class, but is there a cleaner way to access the nested class methods from the base class?

2) Is the only way to reference the nested class from the base class variable - to create a variable and then access the variable?

References:

Nested Class Declarations[1]

Initializing nested classes - C++ Forum[2]

解决方案

If you have an object of a specific type, you can not access members of classes that are derived from that object.

But a common situation is that you have a reference to an object that should be used elsewhere (e.g. passed as parameter to another function) using the type of a base class. In this case you can cast the passed reference to the real type or use virtual functions.

Casting (dynamic_cast conversion - cppreference.com[^]) requires that you know the real type by run-time type information (RTTI) (see Type support (basic types, RTTI, type traits) - cppreference.com[^] and Run-Time Type Information - MSDN[^]) or having a member in the base class that is unique for each derived class.

Using virtual functions is the smarter solution.

When your derived classes should provide a function isNew(), add that as virtual (virtual function specifier - cppreference.com[^]) to the base class and the derived classes. If the base class is abstract (not used itself as variable or member) it can be a pure virtual function (see Abstract Classes - cppreference.com[^]) to ensure that it is implemented in all derived classes.

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