有关虚函数的问题

2011-08-02 10:43:49 100 阅读 AI 写同款 · GEO 优化AI 选题数据监测一键分发 ›

C++ 虚函数的缺省参数问题前些日子，有个同学问我一个关于虚函数的缺省参数问题。他是从某个论坛上看到的，但是自己没想通，便来找我。现在分享一下这个问题。先看一小段代码： #include using namespace std; class A { public: virtual void Fun(int number = 10) { cout << "A::Fun 阅读详情

public:
//创建主窗口
virtual CTest * CTest() { return new CTest2; }//这一句出了问题

//报错
error C2259: 'CGameClientDlg' : cannot instantiate abstract class
1> due to following members:
1> 'void CGameFrameDlg::OnEventContinueGame(void)' : is abstract
1> d:\vs2008\library\include\gameframedlg.h(120) : see declaration of 'CGameFrameDlg::OnEventContinueGame'

这个问题改怎么解决？

...全文

面试中c++中单继承关于虚函数常遇到的4个问题在讲到虚函数之前，先附一张表（如果急切，直接翻到说虚函数的部分即可）　　基类的私有成员变量，派生类虽能继承，但不能访问，是不可见的。　　基类的保护成员和派生类的唯一不同就是作用域。　　除了析构和所有的构造不可继承外，其他都可继承　　基类和派生类成员方法的关系：重载：（同一作用域），方法名相同，返回值类型，参数列表不同。隐藏：（使用前提：派生类对象调用继承于基类的方法）同名阅读详情

myMugua

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myMugua 2011-08-02

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特别感谢dizuo！每次都是你帮我解决的问题

luciferisnotsatan 2011-08-02

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Error Message
'class' : cannot instantiate abstract class

Code declares an instance of an abstract class or structure.

You cannot instantiate a class or structure with one or more pure virtual functions. To instantiate objects of a derived class, the derived class must override each pure virtual function.

For more information, see Implicitly Abstract Classes.

The following sample generates C2259:

Copy Code

// C2259.cpp

// compile with: /c

class V {

public:

   void virtual func() = 0;

};

class A : public V {};

class B : public V {

public:

   void func();

};

V v;  // C2259, V is an abstract class

A a;  // C2259, A inherits func() as pure virtual

B b;  // OK, B defines func()

Whenever you derive from an interface and implement the interface methods in the derived class with access permissions other than public, you may receive C2259. This occurs because the compiler expects the interface methods implemented in the derived class to have public access. When you implement the member functions for an interface with more restrictive access permissions, the compiler does not consider them to be implementations for the interface methods defined in the interface, which in turn makes the derived class an abstract class.

There are two possible workarounds for the problem:

Make the access permissions public for the implemented methods.

Use the scope resolution operator for the interface methods implemented in the derived class to qualify the implemented method name with the name of the interface.

C2259 can also occur as a result of conformance work that was done in Visual C++ 2005, /Zc:wchar_t is now on by default. In this situation, C2599 can be resolved either by compiling with /Zc:wchar_t-, to get the behavior from previous versions, or preferably, by updating your types so they are compatible. For more information, see Breaking Changes in the Visual C++ 2005 Compiler and /Zc:wchar_t (wchar_t Is Native Type).

The following sample generates C2259:

Copy Code
// C2259b.cpp
// compile with: /c
#include <windows.h>

class MyClass {
public:
// WCHAR now typedef'ed to wchar_t
virtual void func(WCHAR*) = 0;
};

class MyClass2 : MyClass {
public:
void func(unsigned short*);
};

MyClass2 x; // C2259

// OK
class MyClass3 {
public:
virtual void func(WCHAR*) = 0;
virtual void func2(wchar_t*) = 0;
virtual void func3(unsigned short*) = 0;
};

class MyClass4 : MyClass3 {
public:
void func(WCHAR*) {}
void func2(wchar_t*) {}
void func3(unsigned short*) {}
};

MyClass4 y;

The following sample generates C2259:

Copy Code
// C2259c.cpp
// compile with: /clr
interface class MyInterface {
void MyMethod();
};

ref class MyDerivedClass: public MyInterface {
private:
// Uncomment the following line to resolve.
// public:
void MyMethod(){}
// or the following line
// void MyInterface::MyMethod() {};
};

int main() {
MyDerivedClass^ instance = gcnew MyDerivedClass; // C2259
}

The following sample generates C2259:

Copy Code

// C2259d.cpp

// compile with: /clr:oldSyntax

public __gc __interface MyInterface {

   void MyMethod();

};

      

__gc class MyDerivedClass : public MyInterface {

// Uncomment the following line to resolve.

// public:

   void MyMethod() {};

   // or the following line

   // void MyInterface::MyMethod() {};

};

      

int main() {

   MyDerivedClass *instance = new MyDerivedClass();   // C2259

}

myMugua 2011-08-02

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public:

//创建主窗口

virtual CTest * CTest() { return new CTest2; }//这一句出了问题



//报错

error C2259: 'CGameClientDlg' : cannot instantiate abstract class

1> due to following members:

1> 'void CGameFrameDlg::OnEventContinueGame(void)' : is abstract

1> d:\vs2008\library\include\gameframedlg.h(120) : see declaration of 'CGameFrameDlg::OnEventContinueGame'

ryfdizuo 2011-08-02

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看错误提示啊。。。
抽象类不能实例化，
CGameClientDlg中需要实现一下纯虚函数：OnEventContinueGame

bdmh 2011-08-02

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哪个是abstract 类，abstract 类不能实例化

有关虚函数的问题

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