class A {
public:
    virtual ~A() { }

    virtual void print() const {
        std::cout << "In A" << std::endl;
    }
};

class B : public A {
public:
    B() { }
    virtual ~B() { }

    virtual void print() const {
        std::cout << "In B" << std::endl;
    }
};

void doSomething(A* a) {
    a->print();
}

void doSomething(const A a) {
    a.print();
}

int main()
{
    A* a = new B();
    doSomething(a);
    doSomething(B());
    return 0;
}

为什么这个输出：

In B
In A

但当你改变doSomething to doSomething(const A& a)时,它会输出：

In B
In B