c++ - void 和 C++ 中其他不完整类型的主要区别是什么？

Question

我在整体编程方面是新手。

我已经尝试阅读该语言的官方标准，但找不到我的问题的任何答案。

所以我需要了解 void 类型和 C++ 中其他不完整类型的主要区别是什么。例如:语言(或代码)中是否有可以使用 void 而不是其他不完整类型的地方，反之亦然？还是 void 在各个方面都像其他不完整类型一样？

Answer 1

void 和另一个不完整类型之间确实有一个非常细微的区别:

您不能引用 void 类型:

struct T;       // incompletely defined type 
                // incompletely defined types and void are incomplete types
T* x=nullptr;   // pointer to incomplete type are valid, same for void pointers
T& tref=*x;     // it's valid to have a reference to an incompletely defined type 
void& vref;     // it's INVALID to have a reference to void, because no object could ever be void

 int f(void &vt);   // INVALID function declaration with invalid parameter: void reference 
 int g(T& vt);      // valid function declaration using a parameter: reference to incomplete type

C++ 标准引号:

3.9/5: A class that has been declared but not defined, or an array of unknown size or of incomplete element type, is an incompletely-defined object type. Incompletely-defined object types and the void types are incomplete types

3.9/8: An object type is a (possibly cv-qualified) type that is not a function type, not a reference type, and not a void type.

8.3.2/1: A declarator that specifies the type “reference to cv void” is ill-formed.

可以将任何表达式转换为void，这显然不是其他不完整类型的情况:

(void)(3 + 5);    // OK explicit conversion to void
(T)(3 + 5);       // Invalid expression to an incomplete type

这在 C++ 标准中有记录:

3.9.1/9: Any expression can be explicitly converted to type cv void