“this”指针的类型 [英] Type of 'this' pointer

问题描述

如标题中所述，我想知道'this'指针的类型。

As mentioned in the title, I would like to know about the type of 'this' pointer.

我正在处理一个项目，我观察到'this'指针的类型是ClassName * const this在Windows上使用VC ++ 2008。我想知道什么是需要/要求使这个指针成为一个常量指针。感谢。

I'm working on a project and I observed that the type of 'this' pointer is "ClassName * const this" on windows using VC++ 2008. Well I would want to know what is the need/requirement to make the this pointer a constant pointer. Thanks.

推荐答案

此指针的类型是 ClassName * const ClassName * ，这取决于是否在类 ClassName 的非const或const方法中检查。指针此不是一个左值。

The type of this pointer is either ClassName * or const ClassName *, depending on whether it is inspected inside a non-const or const method of the class ClassName. Pointer this is not an lvalue.

class ClassName {
  void foo() {
    // here `this` has `ClassName *` type
  }

  void bar() const {
    // here `this` has `const ClassName *` type
  }
};

上面提到的观察结果是误导性的。指针这个是不是一个左值，这意味着它不可能有 ClassName * const 类型，即不能在 * 右边有 const 。非指针类型的值不能是const或非const。在C ++语言中根本没有这样的概念。你所观察到的必须是特定编译器的内部怪癖。

The observation you mentioned above is misleading. Pointer this is not an lvalue, which means that it cannot possibly have ClassName * const type, i.e. it cannot possible have a const to the right of the *. Non-lvalues of pointer type cannot be const or non-const. There's simply no such concept in C++ language. What you observed must be an internal quirk of the specific compiler. Formally, it is incorrect.

以下是语言规范中的相关引用（强调我的）

Here are the relevant quotes from the language specification (emphasis mine)

9.3.2这个指针

在非静态成员函数这是
a prvalue表达式，其值是调用函数的
对象的地址。 这种类型在
的成员函数中，类X是X *。如果成员函数声明为const，那么
的类型为const X *，如果成员函数声明为volatile，那么
类型为volatile X *，如果成员函数声明为
const volatile，其类型是const volatile X *。 [注意：因此在
a const成员函数中，函数调用的对象
通过const访问路径。 -end note]

In the body of a non-static (9.3) member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*. If the member function is declared const, the type of this is const X*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*. [ Note: thus in a const member function, the object for which the function is called is accessed through a const access path. —end note ]

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