为什么没有static_cast就失败？ [英] Why does this fail without static_cast?

问题描述

编译 有效，但编译 失败，并显示错误。

为什么会发生这种情况？

  A类{
 public：
 A（）{} 
}; 
 
 class B：public A {
 public：
 B（）{} 
}; 
 
 void f（）{
 A * a = new A（）; 
 B * b = static_cast< B *>（a）; 
} 
 
 void g（）{
 A * a = new A（）; 
 B * b = a; 
} 
  

解决方案

A static_cast 强制执行可能不安全的转化。

  B * b = static_cast< B * ;（一个）; 
  

如果 a 指向一个 A 对象，实际上是 B 对象的基类子对象，但它不会。强制转换。

  B * b = a; 
 

这里没有转换，并且（正确）没有从基类指针到派生类的隐式转换类指针。指向一个派生类的指针总是可以转换为一个指向一个基类的指针，因为派生类对象总是包含一个基类子对象，但不是每个基类实例都是一个特定派生类类型的子对象。 p>


Compiling f works, but compiling g fails with an error.

Why does this happen?

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

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

void f() {
  A* a = new A();
  B* b = static_cast<B*>(a);
}

void g() {
  A* a = new A();
  B* b = a;
}

解决方案

A static_cast forces a conversion that is potentially unsafe.

B* b = static_cast<B*>(a);

This would be valid if a pointed to an A object that actually was the base class sub-object of a B object, however it doesn't. The cast forces the conversion.

B* b = a;

There is no cast here and there is (correctly) no implicit conversion allowed from base class pointer to derived class pointer. A pointer to a derived class can always be converted to a pointer to a base class because a derived class object always contains a base class sub-object but not every base class instance is a sub-object of a particular derived class type.

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