学习string类型的一点记录

tmp : string[1] 这句代码是什么时候意思？

看看Delphi的帮助：

A ShortString is 0 to 255 characters long. While the length of a ShortString can change dynamically, its memory is a statically allocated 256 bytes; the first byte stores the length of the string, and the remaining 255 bytes are available for characters. If S is a ShortString variable, Ord(S[0]), like Length(S), returns the length of S; assigning a value to S[0], like calling SetLength, changes the length of S. ShortString is maintained for backward compatibility only.

The Delphi language supports short-string types--in effect, subtypes of ShortString--whose maximum length is anywhere from 0 to 255 characters. These are denoted by a bracketed numeral appended to the reserved word string. For example,

var MyString: string[100];

creates a variable called MyString whose maximum length is 100 characters. This is equivalent to the declarations

type CString = string[100];
var MyString: CString;

Variables declared in this way allocate only as much memory as the type requires--that is, the specified maximum length plus one byte. In our example, MyString uses 101 bytes, as compared to 256 bytes for a variable of the predefined ShortString type.

When you assign a value to a short-string variable, the string is truncated if it exceeds the maximum length for the type.

The standard functions High and Low operate on short-string type identifiers and variables. High returns the maximum length of the short-string type, while Low returns zero.

注意下面两行：

type CString = string[100];
var MyString: CString;

这两行中定义的MyString有多长呢？按照上面的说法，MyString有101个字节，如果给MyString[0]赋值，或调用SetLength, 就改变了MyString的长度，不过，MyString最长是256个字节，而且只能放255个字符，因为MyString[0]是存放MyString的长度的，不能放其它字符。