Question

可能重复：
C++ Virtual/Pure Virtual Explained

虚拟功能与纯虚函数有什么区别？

Answer 1

虚拟函数可以在派生类中重写 必须在派生类中重写纯虚函数。具体来说，除非至少有一个派生类重写该虚函数，否则无法使用纯虚函数实例化一个类。

Answer 2

来自Wikipedia：

纯虚函数或纯虚方法是需要由非抽象的派生类实现的虚函数。包含纯虚方法的类称为“抽象”;它们不能直接实例化，如果所有继承的纯虚方法都已由该类或父类实现，则只能直接实例化抽象类的子类。纯虚方法通常具有声明（签名）而没有定义（实现）。

Answer 3

纯虚函数是一个必须由派生类重新定义的虚函数，因此通常对它没有基类中的实现（尽管可以提供，由派生类通过范围解析运算符调用;感谢@Mark Ransom和@Toolbox指出它。

如果一个类具有纯虚方法，则无法实例化，因此任何派生自抽象类（=具有纯虚方法的类）的类都是可实例化的，必须实际定义此类方法。

示例：

#include <iostream>

using std::cout;
using std::endl;

class BaseClass
{
public:

    // A "normal" virtual function which provides a default implementation
    virtual void OnlyVirtual()
    {
        cout<<"BaseClass::OnlyVirtual"<<endl;
    }

    // A "normal" pure virtual function: must be redefined, no default
    // implementation
    virtual void PureVirtual()=0;

    // A pure virtual function that provides a default implementation that can
    // be called only explicitly via scope-resolution operator
    virtual void PureVirtualWImpl()=0;
};

void BaseClass::PureVirtualWImpl()
{
    cout<<"BaseClass::PureVirtualWImpl"<<endl;
}

class Derived0 : public BaseClass
{
public:
    // Define the pure virtual function
    virtual void PureVirtual()
    {
        cout<<"Derived0::PureVirtual"<<endl;
    }

    // notice that, even if there's an implementation of PureVirtualWImpl in
    // BaseClass, since it's marked as pure, Derived0 cannot still be
    // instantiated
};

class Derived1 : public Derived0
{

public:
    // PureVirtual is already defined by the parent class Derived0

    // I must define also PureVirtualWImpl if I want to instantiate this class
    virtual void PureVirtualWImpl()
    {
        cout<<"Derived1::PureVirtualWImpl"<<endl;
    }
};

class Derived2 : public Derived1
{
public:

    // Obviously I can redefine the "normal" virtual function
    virtual void OnlyVirtual()
    {
        cout<<"Derived2::OnlyVirtual"<<endl;
    }

    // Redefine PureVirtual
    virtual void PureVirtual()
    {
        cout<<"Derived2::PureVirtual"<<endl;
    }

    // Just for fun I can redefine PureVirtualWImpl to call the base normally-
    // unaccessible implementation
    virtual void PureVirtualWImpl()
    {
        BaseClass::PureVirtualWImpl();
    }
};

void TestClass(BaseClass & C)
{
    C.OnlyVirtual();
    C.PureVirtual();
    C.PureVirtualWImpl();
}

int main()
{
    // BaseClass b; // <--- compilation error
    // Derived0 d0; // <--- compilation error
    Derived1 d1;
    Derived2 d2;
    TestClass(d1);
    TestClass(d2);
}

输出：

BaseClass::OnlyVirtual
Derived0::PureVirtual
Derived1::PureVirtualWImpl
Derived2::OnlyVirtual
Derived2::PureVirtual
BaseClass::PureVirtualWImpl

虚函数和纯虚函数有什么区别？

3 个答案: