《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.