`R'的nmcnt`和'type'；s`sxpinfo`header

R手册第1.1.2条规定：

“sxpinfo”头定义为以下64位C结构：

 struct sxpinfo_struct {
     /* Type and namedcnt in first byte */
     unsigned int nmcnt : 3;   /* count of "names" referring to object */
     unsigned int type : 5;    /* discussed above */
     /* Garbage collector stuff - keep in one byte to maybe speed up access */
     unsigned int gccls : 3;   /* node class for garbage collector */
     unsigned int gcgen : 1;   /* old generation number - may be best first */
     unsigned int mark : 1;    /* marks object as in use in garbage collector */
     /* Object flag */
     unsigned int obj : 1;     /* set if this is an S3 or S4 object */
     /* Flags to synchronize with helper threads */
     unsigned int in_use: 1;   /* whether contents may be in use by a helper */
     unsigned int being_computed : 1;  /* whether helper may be computing this */
     /* "general purpose" field, used for miscellaneous purposes */
     unsigned int gp : 16;     /* The "general purpose" field */
     union {
       struct {                /* field below is for vectors only */
         R_len_t truelength;   /* for old stuff - may someday be defunct... */
       } vec;
       struct {                /* fields below are for non-vectors only */
         /* Debugging */
         unsigned int debug : 1;  /* function/environment is being debugged */
         unsigned int rstep : 1;  /* function is to be debugged, but only once */
         unsigned int trace : 1;  /* function is being traced */
         /* Symbol binding */
         unsigned int basec : 1;       /* sym has base binding in global cache */
         unsigned int spec_sym : 1;    /* this is a "special" symbol */
         unsigned int no_spec_sym : 1; /* environment has no special symbols */
         unsigned int unused : 2; /* not yet used */
         /* Primitive operations */
         unsigned char var1, var2;/* variants for evals of fast primitive args */
         unsigned char pending_ok;/* whether args can have computation pending */
       } nonvec;                  /*  - only one bit, but maybe faster as byte */
     } u;
 };

这就给我留下了两个问题，关于

nmcnt

和

type

字段（第一个字节）：

为什么你是25岁−2

SEXPTYPEs

当2⁵=64&gg；25岁

是否有2³=8个可引用给定（如S4）对象的名称的限制

---

提前感谢。

关于1：如果手册中没有提到这一点，您必须询问作者（开发人员）。我的猜测是：5位（而不是4位）为将来的扩展留出了空间（即39个进一步的值，可以在不破坏数据兼容性的情况下进行分配）。在其他合理的地方，似乎不需要额外的位。（顺便说一句。我不觉得64>>25。我首先对移位运算符

>

感到困惑，直到我想起非C程序员可能会将其用于“远大于”…）顺便说一句。链接中的

结构spxinfo_struct

与您问题中的示例代码相比有些不同。我会认为这是危险的，因为这意味着错误的比特可以在你的身边访问。@谢夫我粘贴了<代码>信息-R -iTS（我的副本），这可以解释不同之处。（答案也是可怕的）