我从单元DBXCommon.pas(在Delphi XE中)获取访问冲突.当我查看代码时,我会看到以下内容(在感叹号处)：
function TDBXConnectionFactory.GetConnection(const DBXContext: TDBXContext;
const ConnectionProperties: TDBXProperties): TDBXConnection;
var
ConnectionBuilder: TDBXConnectionBuilder;
DelegatePath: TDBXDelegateItem;
Connection: TDBXConnection;
CombinedProperties: TDBXProperties;
begin
//...
ConnectionBuilder := TDBXConnectionBuilder.Create;
Connection := nil;
try
//..lots of setting ConnectionBuilder properties
ConnectionBuilder.FInputPassword := CombinedProperties[TDBXPropertyNames.Password];
Connection := ConnectionBuilder.CreateConnection;
Connection.Open;
Result := Connection;
!! Connection := nil;
finally
!! Connection.Free;
ConnectionBuilder.Free;
end;
end;
但是我在DBXCommon.pas中看到更像这样的构造(首先分配Nil,然后是Free).这是一些我不知道的构造,或者每次调用这段代码时这是否真的导致访问冲突？
解决方法
在空引用上调用Free始终是安全的.去看看TObject.Free的实现,看看为什么.
此代码是工厂函数的示例.它的工作是创建一个类的新实例,但是如果它失败了,它需要确保它在抛出异常时不会泄漏半创建的实例,因此它调用Free.当它确定它会成功时,它会将结果的所有权转移给调用者.它仍然称为Free,但如果已经转移了所有权,那么它最终会在空引用上调用Free,并且没有任何损害.此代码转移所有权：
Result := Connection; Connection := nil;
我编写工厂函数的方式将取消单独的Connection变量.我直接在Result中构造结果,但如果有异常则释放它,如下所示：
function TDBXConnectionFactory.GetConnection(const DBXContext: TDBXContext;
const ConnectionProperties: TDBXProperties): TDBXConnection;
var
ConnectionBuilder: TDBXConnectionBuilder;
DelegatePath: TDBXDelegateItem;
Connection: TDBXConnection;
CombinedProperties: TDBXProperties;
begin
//...
ConnectionBuilder := TDBXConnectionBuilder.Create;
try
//..lots of setting ConnectionBuilder properties
ConnectionBuilder.FInputPassword := CombinedProperties[TDBXPropertyNames.Password];
Result := ConnectionBuilder.CreateConnection;
try
Result.Open;
except
Result.Free;
raise;
end;
finally
ConnectionBuilder.Free;
end;
end;
这具有相同的效果.