《Delphi 算法与数据结构》: 关于 const

如果参数在函数中不可能修改, 一定要使用 const;
不然, 编译器就会:
假定先修改, 先要备份; 使用前后要增减引用计数; 还要套上 try finally.
指定了 const 就可以避免以上过程从而提高效率.
unit
Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;
type
TForm1 = class
(TForm)
Button1: TButton;
procedure
Button1Click(Sender: TObject);
end
;
var
Form1: TForm1;
implementation
{$R *.dfm}
//判断一个字符串中数字的个数
function
GetNum1(str: string
): Integer;
var
i: Integer;
begin
Result := 0
;
for
i := 1
to
Length(str) do
if
str[i] in
['0'
..'9'
] then
Inc(Result);
end
;
//同样的函数只是给参数加上 const
function
GetNum2(const
str: string
): Integer;
var
i: Integer;
begin
Result := 0
;
for
i := 1
to
Length(str) do
if
str[i] in
['0'
..'9'
] then
Inc(Result);
end
;
{对比测试}
procedure
TForm1.Button1Click(Sender: TObject);
var
s: string
;
n: Cardinal;
i: Integer;
begin
s := 'ABC123'
;
n := GetTickCount;
for
i := 0
to
1000000
do
GetNum1(s);
n := GetTickCount - n;
Text := IntToStr(n) + ' - '
;
n := GetTickCount;
for
i := 0
to
1000000
do
GetNum2(s);
n := GetTickCount - n;
Text := Text + IntToStr(n);
end
;
end
.