以不同方式处理子类/避免代码重复

Question

我想知道如何在下面给出的场景中避免代码重复。

(有个问题：
How do I check if an object's type is a particular subclass in C++?
答案是这是不可能的，我猜即使使用动态演员表成员访问也是不可能的。）

所以我想知道您如何避免在不同的方法中使用几乎相同的代码，而只有一种方法会有一些额外的操作。

class Basic {
    ...
}

class Advanced : public Basic {
    ...
}


AnotherClass::lengthy_method(Basic *basic) {
    // do a lot of processing, access members of basic

    if (*basic is actually of class Advanced)
        // do one or a few specific things with members of Advanced

    // more processing just using Basic

    if (*basic is actually of class Advanced)
        // do one or a few specific things with members of Advanced

    // more processing just using Basic
}

同样从设计的角度来看，AnotherClass::lengthy_method 不想在Basic 或Advanced 中定义，因为它并不真正属于它们中的任何一个。它只是在他们的同类上运作。

我很好奇语言专家知道什么，我希望有一个不错的解决方案，至少可以通过 C++11 的一些功能。

Answer 1

这里可以使用dynamic_cast，只要将要访问的Advanced成员声明为public，或者AnotherClass声明为Advanced的friend即可：

AnotherClass::lengthy_method(Basic *basic) {
    // do a lot of processing, access members of basic

    Advanced *adv = dynamic_cast<Advanced*>(basic);
    if (adv != NULL) {
        // use adv as needed...
    }

    // more processing just using Basic

    if (adv != NULL) {
        // use adv as needed...
    }

    // more processing just using Basic
}

另一种选择是使用多态性而不是 RTTI。在Basic 中公开一些额外的虚方法，它们什么都不做，然后让Advanced 覆盖它们：

class Basic {
    ...
    virtual void doSomething1() {}
    virtual void doSomething2() {}
}

class Advanced : public Basic {
    ...
    virtual void doSomething1();
    virtual void doSomething2();
}

void Advanced::doSomething1() {
    ...
}

void Advanced::doSomething2() {
    ...
}    

AnotherClass::lengthy_method(Basic *basic) {
    // do a lot of processing, access members of basic

    // do one or a few specific things with members of Advanced
    basic->doSomething1();

    // more processing just using Basic

    // do one or a few specific things with members of Advanced
    basic->doSomething2();

    // more processing just using Basic
}