省略指向结构的指针的前向声明是否有效? [英] Is the omission of a forward declaration for a pointer to a structure valid?

问题描述

我最近遇到了此评论，作者: @Paul Ogilvie :

I recently came across this comment by @Paul Ogilvie:

"您说要定义指向结构的指针，您只需知道结构标签".根据我的经验，这是不必要的.只需声明一个pointer_to_some_type，编译器将为指针保留空间并在赋值时进行类型检查.仅在您要取消引用指针并访问其成员之后，才必须完全知道该类型."

"You say "To define a pointer to a structure you only need to know the structure tag". In my experience that is unnecessary. Just declare a pointer_to_some_type and the compiler will reserve space for a pointer and does type checking on assignment. Only once you want dereference the pointer and access its members, must the type fully be known."

我尝试过:

struct foo {
    int a;
    struct bar* x;
};

struct bar {
    int b;
};

实际上，GCC和Clang都不会抛出任何诊断，这与尝试在不进行前向声明的情况下定义相应结构的对象相反.

and indeed neither GCC nor Clang doesn't throw any diagnostic, as opposed to attempt to defining an object of the respective struct without forward-declaration.

证据

但这让我感到奇怪:这符合标准吗?

But that makes me wonder: Is this standard-compliant?

该标准的引用受到高度赞赏.

Citations from the standard are highly appreciated.

推荐答案

如果表示形式相同，则表示大小相同.

If the representations are the same, that implies that the sizes are the same.

C标准的第6.2.6.1p2节状态:

存储在任何其他对象类型的非位字段对象中的值[ed.不是未签名的字符]由 n×CHAR_BIT 位组成，其中 n 是对象的大小该类型(以字节为单位).该值可以复制到一个类型的对象中 unsigned char [n] (例如，通过 memcpy )；结果字节集为称为值的对象表示形式.位字段由 m 位组成，其中 m 是指定的大小对于位域.对象表示形式是该位字段包含 m 位，该位包含该位字段.相同的Tw o值(NaN除外)对象表示比较相等，但比较的值相等可能具有不同的对象表示形式.

Values stored in non-bit-field objects of any other object type [ed. not unsigned char] consist of n×CHAR_BIT bits, where n is the size of an object of that type, in bytes. The value may be copied into an object of type unsigned char [n] (e.g., by memcpy); the resulting set of bytes is called the object representation of the value. Values stored in bit-fields consist of m bits, where m is the size specified for the bit-field. The object representation is the set of m bits the bit-field comprises in the addressable storage unit holding it. Tw o values (other than NaNs) with the same object representation compare equal, but values that compare equal may have different object representations.

作为示例，gcc下的 int 和 short 都使用二进制补码，但是它们没有相同的对象表示形式，因为一个是4个字节，另一个是是2个字节.

As an example, an int and a short under gcc both use two's complement but they don't have the same object representation because one is 4 bytes and the other is 2 bytes.

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