真C静态局部变量替换? [英] True C static local variable replacement?
问题描述
仅尝试在ObjectPascal / Delphi中实现C / C ++静态局部变量的类似功能。
让我们在C中具有以下功能:
just trying to achieve similar functionality of C/C++ static local variables in ObjectPascal/Delphi. Let's have a following function in C:
bool update_position(int x, int y)
{
static int dx = pow(-1.0, rand() % 2);
static int dy = pow(-1.0, rand() % 2);
if (x+dx < 0 || x+dx > 640)
dx = -dx;
...
move_object(x+dx, y+dy);
...
}
使用类型化常量作为静态对象的等效ObjectPascal代码变量替换无法编译:
Equivalent ObjectPascal code using typed constants as a static variable replacement fails to compile:
function UpdatePosition(x,y: Integer): Boolean;
const
dx: Integer = Trunc( Power(-1, Random(2)) ); // error E2026
dy: Integer = Trunc( Power(-1, Random(2)) );
begin
if (x+dx < 0) or (x+dx > 640) then
dx := -dx;
...
MoveObject(x+dx, y+dy);
...
end;
[DCC错误] test_f.pas(332):E2026需要常量表达式
那么有一种方法可以一次性初始化 local 变量吗?
So is there some way for a one-time pass initialized local variable ?
推荐答案
在Delphi中没有C静态变量的直接等效项。
There's no direct equivalent of a C static variable in Delphi.
可写的类型常量(请参见user1092187的答案)几乎是等效的。它具有相同的作用域和实例化属性,但不允许使用C或C ++静态变量进行一次性初始化。无论如何,我认为应该将可写类型常量作为古朴的历史注脚。
A writeable typed constant (see user1092187's answer) is almost equivalent. It has the same scoping and instancing properties, but does not allow the one-time initialization that is possible with a C or C++ static variable. In any case it is my opinion that writeable typed constants should be shunned as a quaint historical footnote.
您可以使用全局变量。
var
dx: Integer;
dy: Integer
function UpdatePosition(x,y: Integer): Boolean;
begin
if (x+dx < 0) or (x+dx > 640) then
dx := -dx;
...
MoveObject(x+dx, y+dy);
...
end;
您必须在初始化中进行一次性初始化
部分:
initialization
dx := Trunc( Power(-1, Random(2)) );
dy := Trunc( Power(-1, Random(2)) );
当然,与C静态变量的有限范围不同,这会使全局名称空间一团糟。在现代的Delphi中,您可以将其全部包装在一个类中,并使用类方法,类vars,类构造器,以避免污染全局名称空间。
Of course this make a mess of the global namespace unlike the limited scope of a C static variable. In modern Delphi you can wrap it all up in a class and use a combination of class methods, class vars, class constructors to avoid polluting the global namespace.
type
TPosition = class
private class var
dx: Integer;
dy: Integer;
private
class constructor Create;
public
class function UpdatePosition(x,y: Integer): Boolean; static;
end;
class constructor TPosition.Create;
begin
dx := Trunc( Power(-1, Random(2)) );
dy := Trunc( Power(-1, Random(2)) );
end;
class function TPosition.UpdatePosition(x,y: Integer): Boolean;
begin
// your code
end;
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