I want to store a file pointer in QT Creator software for which I am using QSettings. I tried
settings.setValue("name", variable);
But I am not able to store it. How can I store it in QSettings, or if not possible this way, then what are other ways to do it?
Related
I'm currently in the code generation phase of building a compiler for a Java-like language. I'm trying to understand how to implement dynamic dispatch for virtual methods.
I get how to build a virtual function table for every class and store a pointer to it in every object. What I don't get is- when generating code for a function call, how do you know what the offset is for that function in the table?
Thanks.
How do you know what anything is in your language? You write it down somewhere while parsing.
What I did in one of my toy languages was to keep a "vtable size" for each class, and when you add a new method to a class, you write down the vtable size as the offset of the method somewhere (i.e. you create a lookup table that maps method name to info about it, like its parameter types and its offset in the vtable), then add to the size to account for the newly-added method.
Of course, this assumes your language actually uses a vtable, like for example C++. If you use messaging in the style of Smalltalk or Objective-C, this table that you build actually gets saved to your compiled product and just used directly. Now a table look-up is slower than just accessing an offset directly, but also has the advantage that a caller does not need to know the type of an object to call a method on it, and you can easily add methods to objects without having to recompile the entire program.
Of the two choices I have to access the value of a control which is the most efficient?
getComponent("ControlName").getValue();
or
dataSource.getItemValue("FieldName");
I find that on occasion the getComponent does not seem to return the current value, but accessing the dataSource seems to be more reliable. So does it make much difference from a performance perspective which one is used?
The dataSource.getValue seems to work everywhere that I have tried it. However, when working with rowData I still seem to need to do a rowData.getColumnValue("Something"). rowData.getValue("Something") fails.
Neither. The fastest syntax is dataSource.getValue ("FieldName"). The getItemValue method is only reliable on the document data source, whereas the getValue method is not only also available on view entries accessed via a view data source (although in that context you would pass it the programmatic name of a view column, which is not necessarily the same name as a field), but will also be available on any custom data sources that you develop or install (e.g. third-party extension libraries). Furthermore, it does automatic type conversion that you'd have to do yourself if you used getItemValue instead.
Even on very simple pages, dataSource.getValue ("FieldName") is 5 times as fast as getComponent ("id").getValue (), because, as Fredrik mentions, first it has to find the component, and then ask it what the value is... which, behind the scenes, just asks the data source anyway. So it will always be faster to just ask the data source yourself.
NOTE: the corresponding write method is dataSource.setValue ("FieldName", "NewValue"), not dataSource.replaceItemValue ("FieldName", "NewValue"). Both will work, but setValue also does the same type conversion that getValue does, so you can pass it data that doesn't strictly conform to the old Domino Java API and it usually just figures out what the value needs to be converted to in order to be "safe" for Domino to store.
I would say that the most efficient way is to get the value directly from the datasource.
Because if you use getComponent("ControlName").getValue(); you will do a get on the component first and then a getValue from that. So do a single get from the datasource is more efficient if you ask me.
I come from a Java and .NET background.
In VB6 it appears that you do not always have to create an instance of a class when using it. For example, when using the Printer class you can simply say Printer.print instead of having to create an instance first i.e. Dim printer As New Printer and then running printer.Print. I know that Printer is a system object in VB6, but why don't you have to create an instance?
Visual Basic traditionally had a large number of pre-defined identifiers that are directly recognized by the compiler. The Printer object is one of those. Under the hood, this is implemented with the [appobject] attribute but that's carefully hidden in the language. The runtime creates an instance of the COM coclass automatically, much like the As New syntax. The DAO DBEngine object would be an example of one that isn't predefined in the language parser.
This is over and done with in VB.NET, a truly object oriented language with a large class library, much like Java. There is no Printer object anymore, you're supposed to use the PrintDocument class. The Printer object is still supported for legacy code, available in the Microsoft.VisualBasic.PowerPacks.Printing.Compatibility.VB6 namespace. It however requires the New keyword to create an instance of it.
Be careful investing a lot of time and energy in VB6, it is in all respects a badly outdated language.
VB6 isn't an object oriented language in the same way as you would expect if you are used to newer languages. VB6 will do implicit instantiation and you can treat certain things as if it were static. For example, you can declare a variable of a form, but you don't have to. You can directly call a form and manipulate it without declaring it. In the case of the printer, it can't be explicitly declared and instantiated, but VB6 already has one available.
I used the isolated storage before to save text files, xml files and images. However, is it possible to save variables of type IEnumerable using IsolatedStorage or any other resource in windows phone 7??
Thanks,
You are misunderstanding core concepts.. There is no such thing as "saving variables", you save objects. Your variable points to an object, and that objects implements IEnumerable. Is On WP7, it is the object's actual class that determines whether that object can be serialized and stored on the ISO directly. If that actual collection class does not support serialization, you will have to re-wrap all its current elements into a List/Array/Dictionary/Stack/Queue - literally whatever what supports being serialized - and store that instead of.
Once you have an serializable collection, then your code for saving gets reduced to something as trivial as:
IsolatedStorageSettings.ApplicationSettings["blah"] = your_serializable_collection;
IsolatedStorageSettings.ApplicationSettings.Save();
and in general, that's it. Retrieving is similar:
var items = (SomeCollection)IsolatedStorageSettings.ApplicationSettings["blah"];
where SomeCollection may be an IEnumerable, a List/Array/Dictionary/Stack/Queue - whatever you had put there and whatever is implemented by the actual collection class.
If you want, you may use IsolatedStorageFile and write files directly, but unless you have a good reason to - there's no point in it, as using the common dictionary is far simplier.
In my other post you'll find some links:
How to do isolated storage in Wp7?
Use for saving/loading of data List which are serializable out of the box. Last time i tried deserialize an IEnumerable I got errors...
In iOS SDK, NSDictionary has got writeToFile and dictionaryWithContentsOfFile methods to write a dictionary into a file and to read the contents of a file as dictionary. Is it possible to do the same in WP7 (C#) ?
Yes, but you have to serialize the dictionaary and write to a file yourself.
Alternatively you could add to IsoaltedStorageSettings and let the framework do the serialization for you.
I believe the SilverlightSerializer library is capable of serializing a dictionary; you could serialize your dictionary to a byte array and write it to Isolated Storage.
SilverlightSerializer examples: http://whydoidoit.com/silverlight-serializer/
Or try SharpSerializer, there's a WP7 version available (and a NuGet package): http://www.sharpserializer.com/en/tutorial/index.html
#saikamesh, Here is a thread that covers XML serialization of dictionary in .NET. Why isn't there an XML-serializable dictionary in .NET?. It lists a number of different approaches. You can then write the output string to a file in isolated storage.
You can find mappings of iPhone classes to Windows Phone classes at iPhone and Android to WP7 mapping site
Isolated storage provides a file system. Serialisation is an orthogonal problem.
It is possible to write any object graph into IsolatedStorageSettings provided every object in the graph is DataContract serialisable. Many common framework types are not, for example GeoCoordinate.
Arguably, IsolatedStorageSettings is a dictionary. But the caveat stands regarding DataContract seralisability.
It is equally possible and a lot smarter to write your dictionary to a file, because the more you store in ISS the longer it takes to instantiate. This can seriously affect app load and resume times. You will still have to manage serialisation yourself to the extent that unsupported classes are involved. Your biggest handicap would be the absence of BinaryFormatter from the framework (I don't know whether Mango adds it.)