Original

String types

A string represents a sequence of characters. Object Pascal supports the following predefined string types.

Type	Maximum length	Memory required	Used for
ShortString	255 characters	2 to 256 bytes	backward compatibility
AnsiString	~2^31 characters	4 bytes to 2GB	8-bit (ANSI) characters
WideString	~2^30 characters	4 bytes to 2GB	multiuser servers and multi-language applications

AnsiString, sometimes called the long string, is the preferred type for most purposes.

String types can be mixed in assignments and expressions; the compiler automatically performs required conversions. But strings passed by reference to a function or procedure (as var and out parameters) must be of the appropriate type. Strings can be explicitly cast to a different string type (see Typecasts).

The reserved word string functions like a generic type identifier. For example,

var S: string;

creates a variable S that holds a string. In the default {$H+} state, the compiler interprets string (when it appears without a bracketed number after it) as AnsiString. Use the {$H-} directive to turn string into ShortString.

The standard function Length returns the number of characters in a string. The SetLength procedure adjusts the length of a string.

Comparison of strings is defined by the ordering of the characters in corresponding positions. Between strings of unequal length, each character in the longer string without a corresponding character in the shorter string takes on a greater-than value. For example, ‘AB’ is greater than ‘A’, that is, 'AB' > 'A' returns True. Zero-length strings hold the lowest values.

You can index a string variable just as you would an array. If S is a string variable and i an integer expression, S[i] represents the ith character or, strictly speaking, the ith byte in S. For a ShortString or AnsiString, S[i] is of type AnsiChar; for a WideString, S[i] is of type WideChar. The statement MyString[2] := 'A'; assigns the value A to the second character of MyString. The following code uses the standard UpCase function to convert MyString to uppercase.

var I: Integer;

begin

I := Length(MyString);

while I > 0 do

begin

MyString[I] := UpCase(MyString[I]);

I := I - 1;

end;

Be careful indexing strings in this way, since overwriting the end of a string can cause access violations. Also, avoid passing long-string indexes as var parameters, because this results in inefficient code.

You can assign the value of a string constant or any other expression that returns a string to a variable. The length of the string changes dynamically when the assignment is made. Examples:

MyString := 'Hello world!';

MyString := 'Hello ' + 'world';

MyString := MyString + '!';

MyString := ' '; { space }

MyString := ''; { empty string }

Topic groups

译文

串类型

串表示字符序列。Object Pascal提供下列预定义串类型：

类型	最大长度	占用内存	用于
ShortString	255个字符	2到256字节	向后兼容
AnsiString	大约2^31个字符	4字节到2GB	8位（ANSI）字符
WideString	大约2^30个字符	4字节到2GB	多用户服务和多语言应用程序

AnsiString有时也叫做long stirng，大多数情况下首选该类型。

各种串类型可以在赋值语句和表达式中混合适用，编译器自动执行必需的转换。然而，以参数形式传递给函数或过程的串（作为var参数或out参数）必需具有适当的类型。串可以之间转换为不同的串类型（见类型转换）。

保留字string的作用有如一般类型的标识符。例如，

var S: string;

创建了变量S，用于保存串。在默认的编译指示{$H+}状态下，编译器将string（后面没有跟随方括号和数字）解释为AnsiString；使用编译指示{$H-}时，解释为ShortString。

标准函数Length返回串中字符的个数；标准过程SetLength可以调整串的长度。

串的比较由定义为串中相应位置字符顺序比较。在不等长的两个串中，对于较长的串中的每个字符，如果在较短的串中没有相应位置可比较的字符，那么将视为“大于”。例如，串‘AB’大于串‘A’，也就是说，表达式 'AB' > 'A' 返回True。零长度串（空串）保存最小串值。

可以象索引数组那样对串进行索引。如果S是一个串变量，i是一个整数表达式，那么S[i]表示串S中第 i 个字符或者直接地说，是S中的第i个字节。对ShortString或AnsiString，S[i]是AnsiChar类型；对WideString，S[i]是WideChar类型。语句 MyString[2] := ‘A’; 把值 A 赋给MyString的第二个字符。下面的代码用标准函数UpCase将MyString转换为大写。

var I: Integer;

begin

I := Length(MyString);

while I > 0 do

begin

MyString[I] := UpCase(MyString[I]);

I := I - 1;

end;

通过这种方法对串进行索引时要特别小心，因为重写串尾可能导致访问非法。同样，应避免将长串索引作为var参数传递，因为此举将使代码效率降低。

可以将串常量的值和其他任何返回串的表达式赋给串变量。赋值语句完成时，串的长度会动态改变。下面是对串变量赋值的范例：

MyString := 'Hello world!';

MyString := 'Hello ' + 'world';

MyString := MyString + '!';

MyString := ' '; { 空格 }

MyString := ''; { 空串 }

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String types

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串类型

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