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C++ 虚函数（多态，多重继承，菱形继承）

news 2025/7/8 10:44:05

多态的实现依赖于虚函数。类存在虚函数，则编译器会为每个类创建虚函数表。
虚函数的调用过程：创建基类赋值给基类指针 --> 虚函数表指针 --> 虚函数表(数组) 里面进行查询并调用实际的函数。

基本实现

#include <iostream>// 基类
class Base {
public:// 虚函数1virtual void func1() { std::cout << "Base::func1()" << std::endl; }// 虚函数2virtual void func2() { std::cout << "Base::func2()" << std::endl; }
};// 派生类 继承
class Derived : public Base {
public:// 重写void func1() override { std::cout << "Derived::func1()" << std::endl; }
};int main() {Base* ptr = new Derived();// 通过基类指针调用虚函数，运行时动态绑定ptr->func1();  // 输出: Derived::func1()ptr->func2();  // 输出: Base::func2()delete ptr;return 0;
}

多重继承

派生类继承了多个基类，那就存在多个虚函数指针。

#include <iostream>// 基类A
class BaseA {
public:virtual void funcA() { std::cout << "BaseA::funcA()" << std::endl; }virtual void common() { std::cout << "BaseA::common()" << std::endl; }
};// 基类B
class BaseB {
public:virtual void funcB() { std::cout << "BaseB::funcB()" << std::endl; }virtual void common() { std::cout << "BaseB::common()" << std::endl; }
};// 派生类：多重继承自BaseA和BaseB
class Derived : public BaseA, public BaseB {
public:void funcA() override { std::cout << "Derived::funcA()" << std::endl; }void funcB() override { std::cout << "Derived::funcB()" << std::endl; }void common() override { std::cout << "Derived::common()" << std::endl; }
};int main() {Derived d;// 通过不同基类指针调用虚函数BaseA* ptrA = &d;BaseB* ptrB = &d;ptrA->funcA();   // 输出: Derived::funcA()ptrA->common();  // 输出: Derived::common()ptrB->funcB();   // 输出: Derived::funcB()ptrB->common();  // 输出: Derived::common()// 直接通过派生类对象调用d.funcA();       // 输出: Derived::funcA()d.funcB();       // 输出: Derived::funcB()d.common();      // 输出: Derived::common()return 0;
}

菱形继承

继承关系：
A
/
B C
\ /
D
虚基表

当前例子：D里面会有B和C的虚函数表，当调用公共的虚函数时，会进行进行跳转。

#include <iostream>class A {
public:virtual void foo() { std::cout << "A::foo()" << std::endl; }
};class B : virtual public A {
public:void foo() override { std::cout << "B::foo()" << std::endl; }
};class C : virtual public A {
public:void foo() override { std::cout << "C::foo()" << std::endl; }
};class D : public B, public C {
public:void foo() override { std::cout << "D::foo()" << std::endl; }
};int main() {D d;A* ptr = &d;ptr->foo();  // 输出: D::foo()return 0;
}