C+语法是什么意思

What does C++ syntax struct A::B:A {}; mean? Where is this name definition (or access) described in the C++ standard?

c++语法结构A:::B:A {};的意思吗?c++标准中描述的名称定义(或访问)在哪里?

#include <iostream>

struct B;

struct A {
    struct B;
};

struct A::B:A {
};

int main() {
    A::B::A::B b;
    std::cout<<"Sizeof A::B::A::B is " << sizeof(A::B::A::B)<<std::endl;
    return 0;
}

2 个解决方案

#1

116

This definition

这个定义

struct A {
    struct B;
};

Defines a struct A with a declaration of a nested struct B¹. The fully qualified name of B is A::B, you could say B is inside the "namespace" of A. Then this:

使用嵌套结构B1的声明定义结构a。B的全限定名是A::B，可以说B在A的“名称空间”内，然后是:

struct A::B : A { // Note I added spaces
};

Is the definition of A::B, and the single : specifies that it is derived from A.

是A::B的定义，单数:指定它是从A衍生而来的。

Now, the interesting part is A::B::A::B. Let's dissect it:

现在，有趣的部分是。让我们仔细分析它:

A::B names the nested structure.
A::B命名嵌套结构。
A::B::A accesses the injected class name A inside B. The injection is due to the inheritance.
A:::A访问注入的类名B中的A。注入是由于继承。
A::B::A::B names the nested structure B in A again.
B: A: A: B在A中重新命名了嵌套结构B。

And you can continue ad-infinitum, or at least until your compiler meets its translation limit².

你可以无限地继续，或者至少直到你的编译器满足它的翻译限制2。

A fun intellectual exercise, but avoid like the plague in actual code.

这是一个有趣的智力练习，但要避免像实际代码中的瘟疫。

[class.qual]/1 explains how the lookup works

(类。1解释查找是如何工作的

If the nested-name-specifier of a qualified-id nominates a class, the name specified after the nested-name-specifier is looked up in the scope of the class ([class.member.lookup]), except for the cases listed below. The name shall represent one or more members of that class or of one of its base classes (Clause [class.derived]).

如果限定id的nested-name说明符指定了一个类，则在类的范围([class.member.lookup])中查找nested-name说明符之后指定的名称，但下面列出的情况除外。该名称应表示该类或其基类之一的一个或多个成员(子句[class.派生])。

And the text above allows us to name the base class because [class]/2

上面的文本允许我们命名基类，因为[class]/2

The class-name is also inserted into the scope of the class itself; this is known as the injected-class-name. For purposes of access checking, the injected-class-name is treated as if it were a public member name.

类名也被插入到类本身的范围中;这被称为注入类名。出于访问检查的目的，注入类名被视为公共成员名。

The above clearly says that starting a fully qualified name with A:: allows you to specify a member or a base class. Since A has no bases, you can only specify A::B (a "member type"). But A::B also nominates a class. So we may specify a base or member of that as well with A::B::, which allows us to name A::B::A. Now rinse and repeat.

上面明确指出，使用::开始一个完全限定名可以指定成员或基类。由于A没有基数，所以只能指定A::B(“成员类型”)。但是A: B也提名一个班。因此，我们也可以用a:::B:::来指定基地或成员，这样我们就可以命名a::::: a。现在清洗和重复。

_{¹ - Note it's a completely other B. Not at all related to the global struct B.
² - A recommended minimum of 256 according to [implimits]/2.36}

1 -注意这是一个完全不同的b，与全局结构完全无关。2 -根据[implimit]/2.36，建议最小256

#2

First of all struct B; is a forward declaration of struct B in global namespace. It might be confusing because it is actually not relevant in this example. This global B can be accessed as ::B or as just B.

首先是结构B;是全局命名空间中的struct B的正向声明。它可能会令人困惑，因为在这个例子中它实际上并不相关。本全球B可被访问为:B或仅为B。

struct A {
    struct B;
};

Is a definition of struct A in global namespace with a forward declaration of nested struct B (not the same as previously declared B in global namespace). This nested B can be accessed as ::A::B or A::B.

是全局名称空间中的struct a的定义，带有嵌套struct B的正向声明(与全局名称空间中先前声明的B不同)。这个嵌套B可以被访问为::A::B或A::B。

struct A::B:A {
};

Is a definition of nested struct B of struct A that inherits from A (with access specifier omitted). It can be rewritten to:

是继承自a的结构体的嵌套结构体B的定义(省略了访问说明符)。可以改写为:

struct A::B
:   public A
{
};

Note that writing definition of nested struct B inside of A definition like this won't work:

注意，在这样的定义中编写嵌套结构体B的定义是行不通的:

struct A {
    struct B: A { // error: A is incomplete at this point
    };
};

And finally A::B::A is referring to the base class of nested struct B, that is to A, so A::B::A::B is equivalent to just A::B.

最后A:::B::A是指嵌套结构体B的基类，即A的基类，所以A:::B::A::B就等于A::B。

#1

116