《Cracking the Coding Interview》——第13章：C和C++——题目6

2014-04-25 20:07

题目：为什么基类的析构函数必须声明为虚函数？

解法：不是必须，而是应该，这是种规范。对于基类中执行的一些动态资源分配，如果基类的析构函数不是虚函数，那么 派生类的析构函数在自动调用的时候，不会调用基类的析构函数，这样就会造成资源未释放引起的内存泄漏。

代码：

 // 13.6 If a class is defined as base class, why must its destructor be declared "virtual"?
 // Answer:
 //    If the destructor of base class is declared "virtual", it will be called when a derived class object is being destroyed.
 //    Otherwise it wouldn't. As base class may already have some allocated data or other resources, the destructor of basic class must be called to clear them up.
 //    Missing the base class destructor will leave out those data allocated for base class, causing memory leakage.
 //    Even though there're no dynamically allocated data, the base class should be declared "virtual", as a good practice of coding.
 class A {
 public:
     virtual ~A() {};
 };

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

 int main()
 {
     A *ptr = new B();
     delete ptr;
     // Here ~B() will be called first, ~A() to follow.
     return ;
 }