从c ++ struct字段获取单个字段的大小 [英] Getting the size of an indiviual field from a c++ struct field
问题描述
简短版本是:如何了解c ++字段的单个字段的大小(以位为单位)?
The short version is: How do I learn the size (in bits) of an individual field of a c++ field?
为了阐明,字段的示例我在说:
To clarify, an example of the field I am talking about:
struct Test {
unsigned field1 : 4; // takes up 4 bits
unsigned field2 : 8; // 8 bits
unsigned field3 : 1; // 1 bit
unsigned field4 : 3; // 3 bits
unsigned field5 : 16; // 16 more to make it a 32 bit struct
int normal_member; // normal struct variable member, 4 bytes on my system
};
Test t;
t.field1 = 1;
t.field2 = 5;
// etc.
要获得整个Test对象的大小很容易,只是说
To get the size of the entire Test object is easy, we just say
sizeof(Test); // returns 8, for 8 bytes total size
我们可以通过
sizeof(((Test*)0)->normal_member); // returns 4 (on my system)
我想知道如何获得个别字段,说Test :: field4。上面的例子对一个普通的结构成员不起作用。有任何想法吗?或者有人知道一个原因,为什么它不能工作?我相信sizeof不会有帮助,因为它只返回字节大小,但如果有人知道,否则我都耳朵。
I would like to know how to get the size of an individual field, say Test::field4. The above example for a normal struct member does not work. Any ideas? Or does someone know a reason why it cannot work? I am fairly convinced that sizeof will not be of help since it only returns size in bytes, but if anyone knows otherwise I'm all ears.
谢谢! >
Thanks!
推荐答案
您可以计算运行时的大小fwiw,例如:
You can calculate the size at run time, fwiw, e.g.:
//instantiate
Test t;
//fill all bits in the field
t.field1 = ~0;
//extract to unsigned integer
unsigned int i = t.field1;
... TODO use contents of i to calculate the bit-width of the field ...
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