类型不正常工作 [英] Typeid not functioning correcly

问题描述

我无法正确获取typeid函数。
我缺少某些

I cannot get typeid function correctly. Am I missing something

代码：

class A
{
     public:
     int a1;
     A()
    {
    }
};


class B: public A
{
    public:
    int b1;
    B()
    {
    }
};


int main()
{
     B tempb;
     A tempa;
     A * ptempa;
     ptempa = &tempb;

     std::cout << typeid(tempb).name() << std::endl;
     std::cout << typeid(tempa).name() << std::endl;
     std::cout << typeid(*ptempa).name() << std::endl;

     return 0;
}

它总是打印：

B类
A类
A类

Class B Class A Class A

我对我的项目使用VS2010

I am using VS2010 for my project

推荐答案

它指向的对象必须是多态的，这样才能正常工作。如果 A 有 virtual 方法比你的代码会按预期工作，例如添加一个虚析构函数， a href =http://rextester.com/PPI9073 =nofollow>演示使用gcc 在这里演示。

The object it points to must be polymorphic for this to work as you expect. If A had virtual methods than your code would have work as expected, for example adding a virtual destructor, which I demo live here using gcc.

href =http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3485.pdf =nofollow> C ++草稿标准部分 5.2.8 段落 说：

Quote form the C++ draft standard section 5.2.8 Type identification paragraph 2 says:

当将typeid应用于类型为
多态类类型（10.3）的glvalue表达式时，结果引用std :: type_info
对象，表示最导出对象的类型（1.8 ）[...]

When typeid is applied to a glvalue expression whose type is a polymorphic class type (10.3), the result refers to a std::type_info object representing the type of the most derived object (1.8) [...]

这适用于我们有 virtual 方法，在您的情况下，您没有多边形类型，因此段落 3 适用：

Which applies to the case where we have a virtual method, in your case you do not have a polymorphic type so paragraph 3 applies:

应用于除
多态类类型的glvalue之外的表达式，结果引用std :: type_info对象
表示表达式的静态类型

When typeid is applied to an expression other than a glvalue of a polymorphic class type, the result refers to a std::type_info object representing the static type of the expression

所以你会得到 static 类型，它是 A

So you will get the static type back which is A.

只是为了更完整一节 10.3 / p>

Just to be a little more complete section 10.3 Virtual functions says:

虚函数支持动态绑定和面向对象的
编程。声明或继承虚函数的类是
，称为多态类。

Virtual functions support dynamic binding and object-oriented programming. A class that declares or inherits a virtual function is called a polymorphic class.

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