引用不完全类型 [英] References to incomplete types

问题描述

根据C ++ 03标准，是否有引用不完全类型的有效性？我不知道任何实现引用作为任何非空指针，因此这样的代码应该工作。但是，我不知道对不完全类型的引用是否符合标准。

According to the C++03 standard, is it valid to have references to incomplete types? I'm not aware of any implementation that implements references as any other than non-null pointers, so such code ought to work. However, I wonder whether references to incomplete types are standard conforming.

我会感谢带有引用和C ++标准引用的答案。

I would appreciate answers with quotes and references to the C++ standard.

推荐答案

根据我所知，C ++标准没有明确说明你可以引用一个不完整的类型。但标准中有一个部分指定了必须完成的时间：

The C++ standard doesn't explicitly say, as far as I know, that you can have a reference to an incomplete type. But the standard does have a section that specifies when a type must be complete:

定义规则[basic.def.odr]

4。翻译中需要一个类的定义
unit如果类的使用方式要求类类型为
完成。 [示例：下面的完整翻译单元是
格式正确，即使它从未定义 X ：

4. Exactly one definition of a class is required in a translation unit if the class is used in a way that requires the class type to be complete. [Example: the following complete translation unit is well-formed, even though it never defines X:

    struct X;     // declare X as a struct type
    struct X* x1; // use X in pointer formation
    X* x2;        // use X in pointer formation

-end example >注意：声明和表达式的规则
描述了在哪些上下文中需要完整的类类型。类别
T 必须完成，如果：

—end example] [Note: the rules for declarations and expressions describe in which contexts complete class types are required. A class type T must be complete if:

• T 类型定义（3.1，5.3.4）或


• 将左值转换为右值转换应用于左值引用
  类型 T （4.1）的对象，或


• 一个表达式被转换（隐式或显式）到
  类型 T （第4，5.2.3，5.2.7，5.2.9，5.4）或


• 一个不是空指针常量的表达式，并且具有类型
  而不是 void * 的表达式转换为 T 或引用
  到 T 使用隐式转换（第4节）， dynamic_cast （5.2.7）
  或a static_cast （5.2.9）或


• 应用于类型
  T （5.2.5）或


• 的表达式 typeid 运算符（5.2.8）或 sizeof 运算符（5.3.3）是
应用于 T 或


• 定义具有 T 类型的返回类型或参数类型的函数
  （3.1）或调用（5.2.2）或


• 分配类型 T 的左值）。 ]

• an object of type T is defined (3.1, 5.3.4), or
• an lvalue-to-rvalue conversion is applied to an lvalue referring to an object of type T (4.1), or
• an expression is converted (either implicitly or explicitly) to type T (clause 4, 5.2.3, 5.2.7, 5.2.9, 5.4), or
• an expression that is not a null pointer constant, and has type other than void * is converted to the type pointer to T or reference to T using an implicit conversion (clause 4), a dynamic_cast (5.2.7) or a static_cast (5.2.9), or
• a class member access operator is applied to an expression of type T (5.2.5), or
• the typeid operator (5.2.8) or the sizeof operator (5.3.3) is applied to an operand of type T, or
• a function with a return type or argument type of type T is defined (3.1) or called (5.2.2), or
• an lvalue of type T is assigned to (5.17). ]

似乎在其他情况下，包括对不完全类型引用的声明，可以不完整。

It appears that in every other case, including declarations of references to incomplete types, the type can be incomplete.

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