C ++：比较基类和派生类的指针 [英] C++: Comparing pointers of base and derived classes

问题描述

在比较指针时，我想了解一些最佳做法的相关资讯，例如：

  class Base {
}; 
 
 class Derived 
：public Base {
}; 
 
 Derived * d = new Derived; 
 Base * b = dynamic_cast< Base *>（d）; 
 
 //比较两个指针时，应该把它们
 //转换成相同的类型还是没有关系？ 
 bool theSame = b == d; 
 //或者，bool theSame = dynamic_cast< Derived *>（b）== d？ 
  

解决方案

如果你想比较任意类层次结构， bet是使它们多态并使用 dynamic_cast

  class Base {
 virtual〜Base（）{} 
}; 
 
 class Derived 
：public Base {
}; 
 
 Derived * d = new Derived; 
 Base * b = dynamic_cast< Base *>（d）; 
 
 //比较两个指针时，应该把它们
 //转换成相同的类型还是没有关系？ 
 bool theSame = dynamic_cast< void *>（b）== dynamic_cast< void *>（d）;考虑有时候你不能使用static_cast或从派生类到基类的隐式转换：
 
  struct A {}; 
 struct B：A {}; 
 struct C：A {}; 
 struct D：B，C {}; 
 
 A * a = ...; 
 D * d = ...; 
 
 / *静态转换A到D将失败，因为对于一个D * / 
 / * dynamic_cast< void *>（a）有多个A将神奇地将您的a转换为D指针，不管
 *它指向的两个A。 
 * / 
  
如果 A 是虚拟继承的，你不能static_cast到 D 。 
 
I'd like some information about best practices when comparing pointers in cases such as this one:
class Base {
};

class Derived
    : public Base {
};

Derived* d = new Derived;
Base* b = dynamic_cast<Base*>(d);

// When comparing the two pointers should I cast them
// to the same type or does it not even matter?
bool theSame = b == d;
// Or, bool theSame = dynamic_cast<Derived*>(b) == d?

解决方案
If you want to compare arbitrary class hierarchies, the safe bet is to make them polymorphic and use dynamic_cast
class Base {
  virtual ~Base() { }
};

class Derived
    : public Base {
};

Derived* d = new Derived;
Base* b = dynamic_cast<Base*>(d);

// When comparing the two pointers should I cast them
// to the same type or does it not even matter?
bool theSame = dynamic_cast<void*>(b) == dynamic_cast<void*>(d);
Consider that sometimes you cannot use static_cast or implicit conversion from a derived to a base class:
struct A { };
struct B : A { };
struct C : A { };
struct D : B, C { };

A * a = ...;
D * d = ...;

/* static casting A to D would fail, because there are multiple A's for one D */
/* dynamic_cast<void*>(a) magically converts your a to the D pointer, no matter
 * what of the two A it points to.
 */
If A is inherited virtually, you can't static_cast to a D either. 

                        
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