《Delphi 算法与数据结构》学习与感悟[4]: 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.

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测试效果图:

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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.