为什么内部的Lua字符串存储他们做的方式是什么？ [英] Why does internal Lua strings store the way they do?

问题描述

我是想将存储一堆常量一个简单的字符串表，我想：嘿！Lua中，让我使用一些有功能！

I was wanting a simple string table that will store a bunch of constants and I thought "Hey! Lua does that, let me use some of there functions!"

这主要是在lstring.h / lstring.c文件（我用5.2）

This is mainly in the lstring.h/lstring.c files (I am using 5.2)

我将显示code我很好奇第一。它从lobject.h

I will show the code I am curious about first. Its from lobject.h

/*
** Header for string value; string bytes follow the end of this structure
*/
typedef union TString {
  L_Umaxalign dummy;  /* ensures maximum alignment for strings */
  struct {
    CommonHeader;
    lu_byte reserved;
    unsigned int hash;
    size_t len;  /* number of characters in string */
  } tsv;
} TString;


/* get the actual string (array of bytes) from a TString */
#define getstr(ts)  cast(const char *, (ts) + 1)

/* get the actual string (array of bytes) from a Lua value */
#define svalue(o)       getstr(rawtsvalue(o))

正如所看到的，该数据被存储在结构的外侧。要获取字节流，你把T​​String的大小，加1，而你得到了的char *指针。

As you see, the data is stored outside of the structure. To get the byte stream, you take the size of TString, add 1, and you got the char* pointer.

这不是坏的编码有关系吗？它已经钻入米我的C类进行明确定义的结构。我知道我会在这里搅拌窝，但是你真的失去了那么多的速度/空间定义一个结构，报头数据，而不是数据定义一个指针值？

Isn't this bad coding though? Its been DRILLED into m in my C classes to make clearly defined structures. I know I might be stirring a nest here, but do you really lose that much speed/space defining a structure as header for data rather than defining a pointer value for that data?

推荐答案

这个想法可能是，你分配头和数据，而不是两个中的一个大块的数据：

The idea is probably that you allocate the header and the data in one big chunk of data instead of two:

TString *str = (TString*)malloc(sizeof(TString) + <length_of_string>);

除了具有只需一个电话来的malloc /免费的，你还可以减少内存碎片并增加内存的本地化。

In addition to having just one call to malloc/free, you also reduce memory fragmentation and increase memory localization.

但回答你的问题，是的，这类黑客通常是一个不好的做法，并应与极端小心。如果你这样做，你可能会想隐藏下的宏/内联函数层。

But answering your question, yes, these kind of hacks are usually a bad practice, and should be done with extreme care. And if you do, you'll probably want to hide them under a layer of macros/inline functions.

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