Delphi中函数定义和声明的位置

　　当函数（或过程）A定义在函数（或过程）B之前，那么函数B就可以调用函数A，并且编译成功，例如下面的

procedure TForm1.btn1Click(Sender: TObject); 和   function showstr: string;

unit Test;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    btn1: TButton;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

function showstr: string;
begin
  Result:= 'lala';
end;

procedure TForm1.btn1Click(Sender: TObject);
begin
   ShowMessage(showstr);
end;

end.

　

　　　当函数（或过程）A定义在函数（或过程）B之后，那么如果函数B调用函数A，在编译时会报错，如下面的例程

还是procedure TForm1.btn1Click(Sender: TObject); 和   function showstr: string;

unit Test;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    btn1: TButton;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.btn1Click(Sender: TObject);
begin
   ShowMessage(showstr);
end;

function showstr: string;      //showstr在TForm1.btn1Click后定义，且没有在它之前进行声明
begin
  Result:= 'lala';
end;

end.

　　要想当函数（或过程）A定义在函数（或过程）B之后，并且函数B调用函数A，在编译时不报错，那就需要现在B前面声明A，然后再在B中调用A，这时候无论A在B之前定义还是在B之后定义，都能够编译成功。如下面的例程

unit Test;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

function showstr: string;    //首先在interface里面声明

type
  TForm1 = class(TForm)
    btn1: TButton;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

function showstr: string;
begin
  Result:= 'lala';
end;

procedure TForm1.btn1Click(Sender: TObject);
begin
   ShowMessage(showstr);
end;

end.