什么是对象的动态类型 [英] What is dynamic type of object

问题描述

我认为动态类型意味着使用 new 动态分配对象。在下面的情况下，你说 p 指向动态类型还是静态类型的对象？在标准中，它不说动态类型是动态对象。

1.3.3 - 最导出的对象的类型），通过一个左值表达式表示
的左值。 [示例：如果其
静态类型是指向类B的指针的指针（8.3.1）p指向从B（子句10）导出的类
D的对象，则动态类型的表达式* p
是D.参考文献（8.3.2）同样对待。 ]

此外，以下引用是什么意思

右值表达式的动态类型是其静态类型

  class Base { 
 virtual void foo（）{} 
}; 
 
类派生：public Base {
 void foo（）{} 
}; 
 
 int main（）
 {
 Derived d; 
 Base * p =& d; 
} 
  

解决方案

我认为动态类型意味着动态分配对象
使用new。

$ b

动态类型是一个对象的真实类型，它可以通过指向它的实际类型的基本类型的引用（包含指针）访问。

也就是说，如果我们有：

  class A {
 
} 
 
 class B：public A {}; 
 
 
 B l; 
 A& k = 1; 
  

这里k是对A类型对象的引用，但是引用对象的实际类型，它的动态类型是B.

此处动态的含义是仅在运行时知道。

What i think is that dynamic type means dynamically allocated object using new. In the following case, do you say p points to dynamic type or static type of object? In standard, it doesn't say about dynamic type being dynamic object.

1.3.3 - The type of the most derived object (1.8) to which the lvalue denoted by an lvalue expression refers. [Example: if a pointer (8.3.1) p whose static type is "pointer to class B" is pointing to an object of class D, derived from B (clause 10), the dynamic type of the expression *p is "D." References (8.3.2) are treated similarly. ]

Also what does it the following quote mean

The dynamic type of an rvalue expression is its static type

class Base {
    virtual void foo(){}
};

class Derived : public Base {
    void foo(){}
};

int main()
{
    Derived d;
    Base *p = &d;
}

解决方案

What i think is that dynamic type means dynamically allocated object using new.

Nope.

The dynamic type is the real type of an object that might be accessed via a reference (pointer included) that point to a base type of its real type.

That is, if we have :

class A {

};

class B : public A { };


B l;
A& k = l;

Here k is a reference to an object of type A, but the real type of the referred object, its dynamic type, is B.

Here "dynamic" has the meaning of "known only at run-time".

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