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Mixing Pascal strings and null-terminated strings
You can mix long strings (AnsiString
values) and null-terminated strings (PChar values) in expressions and
assignments, and you can pass PChar values to functions or procedures
that take long-string parameters. The assignment S := P, where S is a string variable
and P is a PChar expression, copies a null-terminated string into a long
string.
In a binary operation, if one operand is a
long string and the other a PChar, the PChar operand is converted
to a long string.
You can cast a PChar value
as a long string. This is useful when you want to perform a string operation on
two PChar values. For example,
S := string(P1) + string(P2);
You can also cast a long string as
a null-terminated string. The following rules apply.
· If S is a
long-string expression, PChar(S) casts S as a null-terminated string; it
returns a pointer to the first character in S.
On Windows: For example, if Str1
and Str2 are long strings, you could call the Win32 API MessageBox
function like this:
MessageBox(0, PChar(Str1), PChar(Str2),
MB_OK);
(The declaration of MessageBox
is in the Windows interface unit.)
On Linux: For example, if Str
is a long string, you could call the opendir system function like this:
opendir(PChar(Str));
(The declaration of opendir
is in the Libc interface unit.)
· You can also
use Pointer(S) to cast a long string to an untyped pointer. But if S is empty,
the typecast returns nil.
· When you cast a
long-string variable to a pointer, the pointer remains valid until the variable
is assigned a new value or goes out of scope. If you cast any other long-string
expression to a pointer, the pointer is valid only within the statement where the
typecast is performed.
· When you cast a
long-string expression to a pointer, the pointer should usually be considered
read-only. You can safely use the pointer to modify the long string only when
all of the following conditions are satisfied.
· The expression
cast is a long-string variable.
· The string is
not empty.
· The string is
unique, that is, has a reference count of one. To guarantee that the string is
unique, call the SetLength, SetString, or UniqueString
procedure.
· The string has
not been modified since the typecast was made.
· The characters
modified are all within the string. Be careful not to use an out-of-range index
on the pointer.
The same rules apply when mixing WideString values with PWideChar values.
See also
Using pointers, arrays, and string constants
Working with null-terminated strings
译文
混合Pascal串和空结束串
在赋值语句和表达式中可以混合使用长串（AnsiString值）和空结束串（PChar值），也可以传递PChar值到那些接受长串参数的函数或过程。对于长串变量S和PChar表达式P，赋值语句S := P将赋值空结束串到长串中。在二进制运算中，如果一个操作数是长串而另一个是PChar，那么PChar操作数将被变换为长串，然后在运算。
可以将PChar值转换为长串，当对两个PChar值执行串运算时是很有用的。例如，
S := string(P1) + string(P2);
也可以将长串转换为空结束串，规则如下：
· 如果S是一个长串表达式，那么PChar(S)将S转换为一个空结束串；它返回串S中第一个字符的指针。
对于Windows：例如，如果Str1和Str2都是长串，那么可以如下调用Win32 API函数MessageBox：
MessageBox(0, PChar(Str1), PChar(Str2),
MB_OK);
（MessageBox声明于Windows接口单元）
对于Linux：例如，如果Str是一个长串，那么可以如下调用系统函数opendir：
opendir(PChar(Str));
（opendir声明于Libc接口单元）
· 还可以用Pointer(S)将长串转换为一个无类型指针。如果S为空，那么类型转换返回空指针nil。
· 转换长串变量到指针时，指针一直维持有效状态，直到变量被赋予新值或程序执行点已经离开作用域。如果转换其他任何长串表达式到指针，那么指针仅在类型转换被执行的语句中有效。
· 转换长串表达式到指针时，指针通常被认为是只读的。仅当下列条件全部满足时，可以安全地使用指针修改长串：
· 被转换的表达式是一个长串变量。
· 串非空。
· 串是唯一的（即引用计数为1）。要保证串唯一，可以调用SetLength、SetString或UniqueString等过程。
· 自类型转换完成以来串未曾被修改。
· 修改过的字符都在串内。注意不要使用超出范围的指针索引。
相同的规则适用于混合WideString值和PWideChar值。