条款39:避免在继承体系中做向下转型(downcast)动作

1,先看个例子:
class Person { ... };
class BankAccount
{
public:
  BankAccount(const Person *primaryOwner,
              const Person *jointOwner);
  virtual ~BankAccount();
  virtual void makeDeposit(double amount) = 0;
  virtual void makeWithdrawal(double amount) = 0;
  virtual double balance() const = 0;
  ...
};

class SavingsAccount: public BankAccount
{
public:
  SavingsAccount(const Person *primaryOwner,
                 const Person *jointOwner);
  ~SavingsAccount();
  void creditInterest();                // add interest to account
  ...
};

使用:
list<BankAccount*> allAccounts;
for (list<BankAccount*>::iterator p = allAccounts.begin(); p != allAccounts.end(); ++p)
{
  (*p)->creditInterest();      // error!
}
错误:creditInterest在BankAccounts中不存在.

可行的纠正:
for (list<BankAccount*>::iterator p = allAccounts.begin(); p != allAccounts.end(); ++p)
{
  static_cast<SavingsAccount*>(*p)->creditInterest(); //downcast
}

注:转型(cast)之于程序员,犹如苹果之于夏娃.

2,如果又有一个新的账户加入.
class CheckingAccount: public BankAccount
{
public:
  void creditInterest();    // add interest to account
  ...
};

我们这时候必须这么做:
for (list<BankAccount*>::iterator p = allAccounts.begin(); p != allAccounts.end(); ++p)
{
  if (*p points to a SavingsAccount)
    static_cast<SavingsAccount*>(*p)->creditInterest();
  else
    static_cast<CheckingAccount*>(*p)->creditInterest();
}

根据型别做事,这不是C++的精神,应该使用虚拟函数.
class BankAccount { ... };      // as above

// new class representing accounts that bear interest
class InterestBearingAccount: public BankAccount
{
public:
  virtual void creditInterest() = 0; //提供接口
  ...
};
class SavingsAccount: public InterestBearingAccount
{
  ...                           // as above
};
class CheckingAccount: public InterestBearingAccount
{
  ...                           // as above
};

如下图所示:



这时候,你可以这么使用:
for (list<BankAccount*>::iterator p = allAccounts.begin(); p != allAccounts.end(); ++p)
{
  static_cast<InterestBearingAccount*>(*p)->creditInterest();
}

3,还有另外一种解决的办法:
class BankAccount
{
public:
  virtual void creditInterest() {} //基类提供一个什么都不做的缺省实现
  ...
};
class SavingsAccount: public BankAccount { ... };
class CheckingAccount: public BankAccount { ... };

list<BankAccount*> allAccounts;// look ma, no cast!

for (list<BankAccount*>::iterator p = allAccounts.begin(); p != allAccounts.end(); ++p)
{
  (*p)->creditInterest();
}

4,总结:为了摆脱downcast,不论花多少努力都是值得的.
但是偶尔还是无法摆脱downcast.
例如:当我们无权改变BankAccount, SavingsAccount, 或 allAccounts的定义.
这时候,相对于static_cast,还有个比较好的办法:"safe downcasting"(安全向下转型)
使用dynamic_cast,如果失败,会传回null指针.

回到一开始的地方:

class Person { ... };
class BankAccount
{
public:
  BankAccount(const Person *primaryOwner,
              const Person *jointOwner);
  virtual ~BankAccount();
  virtual void makeDeposit(double amount) = 0;
  virtual void makeWithdrawal(double amount) = 0;
  virtual double balance() const = 0;
  ...
};

class SavingsAccount: public BankAccount
{
public:
  void creditInterest();                // add interest to account
  ...
};

class CheckingAccount: public BankAccount
{
public:
  void creditInterest();                // add interest to account
  ...
};

使用:
list<BankAccount*> allAccounts; 

void error(const string& msg);     

//至少可以保证downcast失败时,可以侦测到.
for (list<BankAccount*>::iterator p = allAccounts.begin(); p != allAccounts.end(); ++p)
{
  if (SavingsAccount *psa = dynamic_cast<SavingsAccount*>(*p))
  {
    psa->creditInterest();
  }
  else if (CheckingAccount *pca = dynamic_cast<CheckingAccount*>(*p))
  {
    pca->creditInterest();
  }
  // uh oh — unknown account type
  else
  {
    error("Unknown account type!");
  }
}

注:downcast必然导致if-then-else风格,比起虚函数,拙劣之极.
万不得已,不要这么使用.