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Archive for the ‘C++/CX’ Category

In VC++ 2013, you can have a boxed object as a member of a value type. Example code below.

value class Member sealed
{
public:
    String^ name;
    IBox<int>^ age;
};

Here’s some code showing how the type can be used.

Member member;
member.name = "Nish";
member.age = 80;
int age = member.age->Value;
member.age = nullptr;

Even in normal boxing/unboxing, when you use a safe_cast on a boxed object, eventually what happens is that a safe_cast to IBox<T> is performed and then the Value property accessed. Example, the following two snippets are semantically identical.

int y = 11;

// snippet 1
Object^ obj = y;
int y3 = safe_cast<int>(obj);

// snippet 2
IBox<int>^ boxedY = safe_cast<IBox<int>^>(obj);
int y2 = boxedY->Value;

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Object::ToString was not virtual in VS 2012 and thus could not be overridden, and this was a bit of an inconvenience when databinding with XAML. Tarek Madkour’s Build 2013 talk on what’s new in VS 2013 for C++ Developers had a slide about overriding ToString. While that was exciting to see, I was a tad disappointed when I could not get it to compile under the VS 2013 preview build. The following code fails to compile.

ref class Person sealed
{
public:
    virtual String^ ToString() override
    {
        return "Test override";
    }
};

The error thrown is : Error 1 error C3668: ‘Person::ToString’ : method with override specifier ‘override’ did not override any base class methods

I am hoping that it just never made it to the preview release and that the RTM version will have it, as it can be quite handy to be able to override ToString, and not just in XAML scenarios.

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C# projects Windows::Foundation::DateTime as System.DateTimeOffset, whereas C++ just exposes the raw structure. This can be rather unnerving if you are trying to port some C# code to C++/CX. All you have to work with is the UniversalTime property which is a long long that MSDN defines as the number of 100-nanosecond intervals prior to or after midnight on January 1, 1601. Even a simple task as displaying a formatted date/time string is difficult as ToString merely returns the fully qualified name of the type.

Fortunately enough, Windows::Globalization::DateTimeFormatting includes the DateTimeFormatter class that can be used to get readable date/time display strings from a DateTime structure. Here’s some sample code that shows how you can use this.

auto dateFormatter = DateTimeFormatter::LongDate;
auto timeFormatter = DateTimeFormatter::LongTime;

for(auto item : feed->Items)
{   
  listBox->Items->Append(
    dateFormatter->Format(item->PublishedDate) + " " +
    timeFormatter->Format(item->PublishedDate) + " - " +  
    item->Title->Text);         
}

Obviously, it’d have been heaps better if the compiler gave us easier to use projections, but it’s still not that bad. A common practice is to convert this to standard C++ or ATL/Win32 date structures and formats if you want to do additional date/time processing on the object.

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It’s been a long break for me from blogging, but I intend to make up for that over the next few months. I’ll primarily be blogging on topics mostly related to using Visual C++ to develop Windows Store applications. So if that’s the sort of thing that interests you, do check back once in a while.

Putting the BindableAttribute on a C++ ref class makes the class available for databinding. Unfortunately, if you add a very simple ref class and then mark it as [Bindable], you’ll get some weird compiler error messages. Example – consider this simple bindable class defined in Restaurant.h.

namespace ViewModels
{
  [Windows::UI::Xaml::Data::Bindable] 
  public ref class Restaurant sealed
  {
  public:
  };
}

Assume there’s a Restaurant.cpp that includes this file. You’ll get these compiler errors (or something close to it).

Error 1 error C3083: 'ViewModels': the symbol to the left 
    of a '::' must be a type ...\xamltypeinfo.g.cpp 
Error 2 error C2039: 'Restaurant' : is not a member 
    of 'ViewModels' ...\xamltypeinfo.g.cpp 
Error 3 error C2061: syntax error : identifier 
    'Restaurant' ...\xamltypeinfo.g.cpp

The not so obvious fix for these errors is to include Restaurant.h in any of your xxx.xaml.h files (right after the include to the xxx.g.h file would be a good place). This seems to be due to how the XAML compiler ties into the whole build process. Not a major hassle, but more something to be aware of.

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When migrating apps or libraries that use sockets to WinRT, the absence of Winsock is often one of the first hurdles for many C++ devs. The suggested alternative to Winsock is to use the Windows.Networking.Sockets namespace. For a full list of alternate APIs that replace existing ones, see:

Be aware that you will most likely not get a one-to-one mapping for various API functions and structures. So you should fully expect to re-design and workaround that and use alternate approaches to achieving the same functionality/usability.

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This is slightly related to the previous FAQ on connecting to localhost. While Metro apps can use contracts to communicate at some level with other apps, that is not equivalent to the traditional concept of inter-process communication. A metro app cannot communicate with a desktop app or even with another metro application on the same machine.

With desktop apps, the metro app cannot take for granted that it’s running or available, and thus it makes sense to not allow that. But you may be wondering why IPC is blocked between two metro apps. Well, metro apps can be in a suspended state at any given time, and there’s no sure way to predict its state. So even if there are two apps running at the same time, neither app can be sure if the other app’s ready to accept data or a command. This is quite likely why they decided to design it so that metro apps cannot talk to each other.

So, what’s the workaround? Again, the only reliable way an app can talk to another app on the same machine is to use a a 3rd service that’s running on the network, or to use a cliched phrase, use the cloud.

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This is probably one of the most frequently asked questions on WinRT in the forums. The answer is – no, you cannot communicate with localhost. You can communicate with a service running on the local network, the intranet, or the internet. But you cannot connect to the loop back address. This is by design. It’s a security measure, and also a side-effect of how a Metro app cannot take any assumptions about the machine it’s running on. And expecting a localhost service to be running is such an assumption. So what’s the alternative? Use the cloud :-)

Note: you can connect to localhost for debugging, but your app will not be approved for publishing to the store.

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