为什么不会发生内存泄漏或访问冲突？
Why does not MemoryLeak or AccessViolation occur?
我发现了两个让我很好奇的案例。
首先是为什么在使用JSON库时不会发生内存泄漏
If GetValue returns 一个未实现接口的 TJSONValue。为什么我的 "if" 没有引发内存泄漏？
procedure TFrmApp.btnJSONClick(Sender: TObject);
var
JSONObject: TJSONObject;
JSONStr: string;
begin
JSONStr := '{"colors":[{"name":"red", "hex":"#f00"}]}';
JSONObject := TJSONObject.ParseJSONValue(JSONStr) as TJSONObject;
try
// If GetValue returns a TJSONValue that does not implement an interface.
// Why does my "if" not raise a memory leak?
if JSONObject.GetValue('colors') <> nil then
memo.Lines.Add('Colors exist.')
else
memo.Lines.Add('Colors not found.');
finally
JSONObject.Free;
end;
end;
其二是为什么在访问Cds的字段时不会发生访问冲突
如果提供数据的CD已经从内存中释放。为什么不发生访问冲突？
procedure TFrmApp.btnDataSetClick(Sender: TObject);
var
Cds: TClientDataSet;
begin
Cds := TClientDataSet.Create(Self);
try
Cds.Data := Self.GetData;
// If the Cds that provided the Data has been released from memory.
// Why does not access violation occur?
memo.Lines.Add('Name: ' + Cds.FieldByName('VendorName').AsString);
memo.Lines.Add('City: ' + Cds.FieldByName('City').AsString);
finally
Cds.Free;
end;
end;
function TFrmApp.GetData: OleVariant;
var
Cds: TClientDataSet;
begin
Cds := TClientDataSet.Create(Self);
try
Cds.LoadFromFile(TDirectory.GetCurrentDirectory + '\data.xml');
Result := Cds.Data;
finally
Cds.Free;
end;
end;
示例项目：
.DPR
program TestMemoryLeak;
uses
Vcl.Forms,
uApp in 'uApp.pas' {FrmApp};
{$R *.res}
begin
ReportMemoryLeaksOnShutdown := true;
Application.Initialize;
Application.MainFormOnTaskbar := True;
Application.CreateForm(TFrmApp, FrmApp);
Application.Run;
end.
uAPP.pas
unit uApp;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, System.JSON, REST.Json, Datasnap.DBClient, Data.DB,
System.IOUtils;
type
TFrmApp = class(TForm)
btnJSON: TButton;
btnDataSet: TButton;
memo: TMemo;
procedure btnJSONClick(Sender: TObject);
procedure btnDataSetClick(Sender: TObject);
private
{ Private declarations }
function GetData: OleVariant;
public
{ Public declarations }
end;
var
FrmApp: TFrmApp;
implementation
{$R *.dfm}
procedure TFrmApp.btnJSONClick(Sender: TObject);
var
JSONObject: TJSONObject;
JSONStr: string;
begin
JSONStr := '{"colors":[{"name":"red", "hex":"#f00"}]}';
JSONObject := TJSONObject.ParseJSONValue(JSONStr) as TJSONObject;
try
// If GetValue returns a TJSONValue that does not implement an interface.
// Why does my "if" not raise a memory leak?
if JSONObject.GetValue('colors') <> nil then
memo.Lines.Add('Colors exist.')
else
memo.Lines.Add('Colors not found.');
finally
JSONObject.Free;
end;
end;
procedure TFrmApp.btnDataSetClick(Sender: TObject);
var
Cds: TClientDataSet;
begin
Cds := TClientDataSet.Create(Self);
try
Cds.Data := Self.GetData;
// If the Cds that provided the Data has been released from memory.
// Why does not access violation occur?
memo.Lines.Add('Name: ' + Cds.FieldByName('VendorName').AsString);
memo.Lines.Add('City: ' + Cds.FieldByName('City').AsString);
finally
Cds.Free;
end;
end;
function TFrmApp.GetData: OleVariant;
var
Cds: TClientDataSet;
begin
Cds := TClientDataSet.Create(Self);
try
Cds.LoadFromFile(TDirectory.GetCurrentDirectory + '\data.xml');
Result := Cds.Data;
finally
Cds.Free;
end;
end;
end.
uApp.dfm
object FrmApp: TFrmApp
Left = 0
Top = 0
Caption = 'FrmApp'
ClientHeight = 245
ClientWidth = 516
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -11
Font.Name = 'Tahoma'
Font.Style = []
OldCreateOrder = False
Position = poScreenCenter
PixelsPerInch = 96
TextHeight = 13
object btnJSON: TButton
Left = 24
Top = 30
Width = 75
Height = 25
Caption = 'JSON'
TabOrder = 0
OnClick = btnJSONClick
end
object btnDataSet: TButton
Left = 24
Top = 96
Width = 75
Height = 25
Caption = 'DataSet'
TabOrder = 1
OnClick = btnDataSetClick
end
object memo: TMemo
Left = 176
Top = 8
Width = 321
Height = 229
TabOrder = 2
end
end
data.xml
<?xml version="1.0" standalone="yes"?>
<DATAPACKET Version="2.0">
<METADATA>
<FIELDS>
<FIELD attrname="VendorNo" fieldtype="r8"/>
<FIELD attrname="VendorName" fieldtype="string" WIDTH="30"/>
<FIELD attrname="City" fieldtype="string" WIDTH="20"/>
<FIELD attrname="State" fieldtype="string" WIDTH="20"/>
</FIELDS>
<PARAMS DEFAULT_ORDER="1" PRIMARY_KEY="1" LCID="1033"/>
</METADATA>
<ROWDATA>
<ROW VendorNo="2014" VendorName="Cacor Corporation" City="Southfield" State="OH"/>
<ROW VendorNo="2641" VendorName="Underwater" City="Indianapolis" State="IN" />
<ROW VendorNo="2674" VendorName="J.W. Luscher Mfg." City="Berkely" State="MA"/>
<ROW VendorNo="3511" VendorName="Scuba Professionals" City="Rancho Dominguez"/>
<ROW VendorNo="3819" VendorName="Divers' Supply Shop" City="Macon" State="GA"/>
</ROWDATA>
</DATAPACKET>
why no memory leak occurs when using the JSON library.
If GetValue returns a TJSONValue that does not implement an interface. Why does my "if" not raise a memory leak?
TJSONObject.GetValue() returns 指向 TJSONValue 对象的指针，该对象由 TJSONObject 内部拥有。 TJSONValue 将在 TJSONObject 被释放时被释放，这是您在过程结束时执行的操作。 GetValue() 不会为输出分配任何新内存，因此没有任何内容可供您释放，因此不会发生泄漏。
why access violation does not occur when accessing the fields of the Cds.
If the Cds that provided the Data has been released from memory. Why does not access violation occur?
您正在将源 ClientDataSet 的 Data 属性 分配给一个 OleVariant，然后将其分配给另一个 ClientDataSet 的 Data 属性。 OleVariant 对分配给它的任何数据进行 复制 （并且在 [arrays of] 接口对象的情况下，它会增加它们的引用计数），所以原始的 ClientDataSet 是否是否释放没有区别，因为 OleVariant 是独立的并管理它持有的数据。
我发现了两个让我很好奇的案例。
首先是为什么在使用JSON库时不会发生内存泄漏
If GetValue returns 一个未实现接口的 TJSONValue。为什么我的 "if" 没有引发内存泄漏？
procedure TFrmApp.btnJSONClick(Sender: TObject);
var
JSONObject: TJSONObject;
JSONStr: string;
begin
JSONStr := '{"colors":[{"name":"red", "hex":"#f00"}]}';
JSONObject := TJSONObject.ParseJSONValue(JSONStr) as TJSONObject;
try
// If GetValue returns a TJSONValue that does not implement an interface.
// Why does my "if" not raise a memory leak?
if JSONObject.GetValue('colors') <> nil then
memo.Lines.Add('Colors exist.')
else
memo.Lines.Add('Colors not found.');
finally
JSONObject.Free;
end;
end;
其二是为什么在访问Cds的字段时不会发生访问冲突
如果提供数据的CD已经从内存中释放。为什么不发生访问冲突？
procedure TFrmApp.btnDataSetClick(Sender: TObject);
var
Cds: TClientDataSet;
begin
Cds := TClientDataSet.Create(Self);
try
Cds.Data := Self.GetData;
// If the Cds that provided the Data has been released from memory.
// Why does not access violation occur?
memo.Lines.Add('Name: ' + Cds.FieldByName('VendorName').AsString);
memo.Lines.Add('City: ' + Cds.FieldByName('City').AsString);
finally
Cds.Free;
end;
end;
function TFrmApp.GetData: OleVariant;
var
Cds: TClientDataSet;
begin
Cds := TClientDataSet.Create(Self);
try
Cds.LoadFromFile(TDirectory.GetCurrentDirectory + '\data.xml');
Result := Cds.Data;
finally
Cds.Free;
end;
end;
示例项目：
.DPR
program TestMemoryLeak;
uses
Vcl.Forms,
uApp in 'uApp.pas' {FrmApp};
{$R *.res}
begin
ReportMemoryLeaksOnShutdown := true;
Application.Initialize;
Application.MainFormOnTaskbar := True;
Application.CreateForm(TFrmApp, FrmApp);
Application.Run;
end.
uAPP.pas
unit uApp;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, System.JSON, REST.Json, Datasnap.DBClient, Data.DB,
System.IOUtils;
type
TFrmApp = class(TForm)
btnJSON: TButton;
btnDataSet: TButton;
memo: TMemo;
procedure btnJSONClick(Sender: TObject);
procedure btnDataSetClick(Sender: TObject);
private
{ Private declarations }
function GetData: OleVariant;
public
{ Public declarations }
end;
var
FrmApp: TFrmApp;
implementation
{$R *.dfm}
procedure TFrmApp.btnJSONClick(Sender: TObject);
var
JSONObject: TJSONObject;
JSONStr: string;
begin
JSONStr := '{"colors":[{"name":"red", "hex":"#f00"}]}';
JSONObject := TJSONObject.ParseJSONValue(JSONStr) as TJSONObject;
try
// If GetValue returns a TJSONValue that does not implement an interface.
// Why does my "if" not raise a memory leak?
if JSONObject.GetValue('colors') <> nil then
memo.Lines.Add('Colors exist.')
else
memo.Lines.Add('Colors not found.');
finally
JSONObject.Free;
end;
end;
procedure TFrmApp.btnDataSetClick(Sender: TObject);
var
Cds: TClientDataSet;
begin
Cds := TClientDataSet.Create(Self);
try
Cds.Data := Self.GetData;
// If the Cds that provided the Data has been released from memory.
// Why does not access violation occur?
memo.Lines.Add('Name: ' + Cds.FieldByName('VendorName').AsString);
memo.Lines.Add('City: ' + Cds.FieldByName('City').AsString);
finally
Cds.Free;
end;
end;
function TFrmApp.GetData: OleVariant;
var
Cds: TClientDataSet;
begin
Cds := TClientDataSet.Create(Self);
try
Cds.LoadFromFile(TDirectory.GetCurrentDirectory + '\data.xml');
Result := Cds.Data;
finally
Cds.Free;
end;
end;
end.
uApp.dfm
object FrmApp: TFrmApp
Left = 0
Top = 0
Caption = 'FrmApp'
ClientHeight = 245
ClientWidth = 516
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -11
Font.Name = 'Tahoma'
Font.Style = []
OldCreateOrder = False
Position = poScreenCenter
PixelsPerInch = 96
TextHeight = 13
object btnJSON: TButton
Left = 24
Top = 30
Width = 75
Height = 25
Caption = 'JSON'
TabOrder = 0
OnClick = btnJSONClick
end
object btnDataSet: TButton
Left = 24
Top = 96
Width = 75
Height = 25
Caption = 'DataSet'
TabOrder = 1
OnClick = btnDataSetClick
end
object memo: TMemo
Left = 176
Top = 8
Width = 321
Height = 229
TabOrder = 2
end
end
data.xml
<?xml version="1.0" standalone="yes"?>
<DATAPACKET Version="2.0">
<METADATA>
<FIELDS>
<FIELD attrname="VendorNo" fieldtype="r8"/>
<FIELD attrname="VendorName" fieldtype="string" WIDTH="30"/>
<FIELD attrname="City" fieldtype="string" WIDTH="20"/>
<FIELD attrname="State" fieldtype="string" WIDTH="20"/>
</FIELDS>
<PARAMS DEFAULT_ORDER="1" PRIMARY_KEY="1" LCID="1033"/>
</METADATA>
<ROWDATA>
<ROW VendorNo="2014" VendorName="Cacor Corporation" City="Southfield" State="OH"/>
<ROW VendorNo="2641" VendorName="Underwater" City="Indianapolis" State="IN" />
<ROW VendorNo="2674" VendorName="J.W. Luscher Mfg." City="Berkely" State="MA"/>
<ROW VendorNo="3511" VendorName="Scuba Professionals" City="Rancho Dominguez"/>
<ROW VendorNo="3819" VendorName="Divers' Supply Shop" City="Macon" State="GA"/>
</ROWDATA>
</DATAPACKET>
why no memory leak occurs when using the JSON library.
If GetValue returns a TJSONValue that does not implement an interface. Why does my "if" not raise a memory leak?
TJSONObject.GetValue() returns 指向 TJSONValue 对象的指针，该对象由 TJSONObject 内部拥有。 TJSONValue 将在 TJSONObject 被释放时被释放，这是您在过程结束时执行的操作。 GetValue() 不会为输出分配任何新内存，因此没有任何内容可供您释放，因此不会发生泄漏。
why access violation does not occur when accessing the fields of the Cds.
If the Cds that provided the Data has been released from memory. Why does not access violation occur?
您正在将源 ClientDataSet 的 Data 属性 分配给一个 OleVariant，然后将其分配给另一个 ClientDataSet 的 Data 属性。 OleVariant 对分配给它的任何数据进行 复制 （并且在 [arrays of] 接口对象的情况下，它会增加它们的引用计数），所以原始的 ClientDataSet 是否是否释放没有区别，因为 OleVariant 是独立的并管理它持有的数据。