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This tutorial uses C++/CX. Microsoft has released C++/WinRT: an entirely standard modern C++17 language projection for Windows Runtime (WinRT) APIs. For more information on this language, please see C++/WinRT.

With Microsoft Visual Studio, you can use C++/CX to develop an app that runs on Windows 10 with a UI that's defined in Extensible Application Markup Language (XAML).

Note

This tutorial uses Visual Studio Community 2019. If you are using a different version of Visual Studio, it may look a little different for you.

It is possible to compile C programs with it, but you'll see some issues and incompleteness.Moreover, it lacks many features familiar with more modern IDEs. For that reason, the tools included - notably the compiler and debugger - are slightly obsolete.Plus, the latest versions were released before the newest version of C of 2017, so it's lagging.

Before you start

• To complete this tutorial, you must use Visual Studio Community, or one of the non-Community versions of Visual Studio, on a computer that's running Windows 10. To download, see Get the tools.
• We assume you have a basic understanding of C++/CX, XAML, and the concepts in the XAML overview.
• We assume you're using the default window layout in Visual Studio. To reset to the default layout, on the menu bar, choose Window > Reset Window Layout.

Comparing C++ desktop apps to Windows apps

If you're coming from a background in Windows desktop programming in C++, you'll probably find that some aspects of writing apps for the UWP are familiar, but other aspects require some learning.

What's the same?

• You can use the STL, the CRT (with some exceptions), and any other C++ library as long as the code only calls Windows functions that are accessible from the Windows Runtime environment.

• If you're accustomed to visual designers, you can still use the designer built into Microsoft Visual Studio, or you can use the more full-featured Blend for Visual Studio. If you're accustomed to coding UI by hand, you can hand-code your XAML.

• You're still creating apps that use Windows operating system types and your own custom types.

• You're still using the Visual Studio debugger, profiler, and other development tools.

• You're still creating apps that are compiled to native machine code by the Visual C++ compiler. UWP apps in C++/CX don't execute in a managed runtime environment.

What's new?

• The design principles for UWP apps and Universal Windows apps are very different from those for desktop apps. Window borders, labels, dialog boxes, and so on, are de-emphasized. Content is foremost. Great Universal Windows apps incorporate these principles from the very beginning of the planning stage.

• You're using XAML to define the entire UI. The separation between UI and core program logic is much clearer in a Windows Universal app than in an MFC or Win32 app. Other people can work on the appearance of the UI in the XAML file while you're working on the behavior in the code file.

• You're primarily programming against a new, easy-to-navigate, object-oriented API, the Windows Runtime, although on Windows devices Win32 is still available for some functionality.

• You use C++/CX to consume and create Windows Runtime objects. C++/CX enables C++ exception handling, delegates, events, and automatic reference counting of dynamically created objects. When you use C++/CX, the details of the underlying COM and Windows architecture are hidden from your app code. For more information, see C++/CX Language Reference.

• Your app is compiled into a package that also contains metadata about the types that your app contains, the resources that it uses, and the capabilities that it requires (file access, internet access, camera access, and so forth).

• In the Microsoft Store and Windows Phone Store your app is verified as safe by a certification process and made discoverable to millions of potential customers.

Hello World Store app in C++/CX

Our first app is a 'Hello World' that demonstrates some basic features of interactivity, layout, and styles. We'll create an app from the Windows Universal app project template. If you've developed apps for Windows 8.1 and Windows Phone 8.1 before, you might remember that you had to have three projects in Visual Studio, one for the Windows app, one for the phone app, and another with shared code. The Windows 10 Universal Windows Platform (UWP) makes it possible to have just one project, which runs on all devices, including desktop and laptop computers running Windows 10, devices such as tablets, mobile phones, VR devices and so on.

We'll start with the basics:

• How to create a Universal Windows project in Visual Studio.

• How to understand the projects and files that are created.

• How to understand the extensions in Visual C++ component extensions (C++/CX), and when to use them.

First, create a solution in Visual Studio

1. In Visual Studio, on the menu bar, choose File > New > Project...

2. In the Create a new project dialog box, select Blank App (Universal Windows - C++/CX). If you don't see this option, make sure you have the Universal Windows App Development Tools installed. See Get set up for more information.

3. Choose Next, and then enter a name for the project. We'll name it HelloWorld.

4. Choose the Create button.

Note

If this is the first time you have used Visual Studio, you might see a Settings dialog asking you to enable Developer mode. Developer mode is a special setting that enables certain features, such as permission to run apps directly, rather than only from the Store. For more information, please read Enable your device for development. To continue with this guide, select Developer mode, click Yes, and close the dialog.

Your project files are created.

Before we go on, let's look at what's in the solution.

About the project files

Every .xaml file in a project folder has a corresponding .xaml.h file and .xaml.cpp file in the same folder and a .g file and a .g.hpp file in the Generated Files folder, which is on disk but not part of the project. You modify the XAML files to create UI elements and connect them to data sources (DataBinding). You modify the .h and .cpp files to add custom logic for event handlers. The auto-generated files represent the transformation of the XAML markup into C++/CX. Don't modify these files, but you can study them to better understand how the code-behind works. Basically, the generated file contains a partial class definition for a XAML root element; this class is the same class that you modify in the *.xaml.h and .cpp files. The generated files declare the XAML UI child elements as class members so that you can reference them in the code you write. At build time, the generated code and your code are merged into a complete class definition and then compiled.

Let's look first at the project files.

• App.xaml, App.xaml.h, App.xaml.cpp: Represent the Application object, which is an app's entry point. App.xaml contains no page-specific UI markup, but you can add UI styles and other elements that you want to be accessible from any page. The code-behind files contain handlers for the OnLaunched and OnSuspending events. Typically, you add custom code here to initialize your app when it starts and perform cleanup when it's suspended or terminated.
• **MainPage.xaml, MainPage.xaml.h, MainPage.xaml.cpp:**Contain the XAML markup and code-behind for the default 'start' page in an app. It has no navigation support or built-in controls.
• pch.h, pch.cpp: A precompiled header file and the file that includes it in your project. In pch.h, you can include any headers that do not change often and are included in other files in the solution.
• Package.appxmanifest: An XML file that describes the device capabilities that your app requires, and the app version info and other metadata. To open this file in the Manifest Designer, just double-click it.
• **HelloWorld_TemporaryKey.pfx:**A key that enables deployment of the app on this machine, from Visual Studio.

A first look at the code

If you examine the code in App.xaml.h, App.xaml.cpp in the shared project, you'll notice that it's mostly C++ code that looks familiar. However, some syntax elements might not be as familiar if you are new to Windows Runtime apps, or you've worked with C++/CLI. Here are the most common non-standard syntax elements you'll see in C++/CX:

Ref classes

Almost all Windows Runtime classes, which includes all the types in the Windows API--XAML controls, the pages in your app, the App class itself, all device and network objects, all container types--are declared as a ref class. (A few Windows types are value class or value struct). A ref class is consumable from any language. In C++/CX, the lifetime of these types is governed by automatic reference counting (not garbage collection) so that you never explicitly delete these objects. You can create your own ref classes as well.

All Windows Runtime types must be declared within a namespace and unlike in ISO C++ the types themselves have an accessibility modifier. The public modifier makes the class visible to Windows Runtime components outside the namespace. The sealed keyword means the class cannot serve as a base class. Almost all ref classes are sealed; class inheritance is not broadly used because Javascript does not understand it.

ref new and ^ (hats)

You declare a variable of a ref class by using the ^ (hat) operator, and you instantiate the object with the ref new keyword. Thereafter you access the object's instance methods with the -> operator just like a C++ pointer. Static methods are accessed with the :: operator just as in ISO C++.

In the following code, we use the fully qualified name to instantiate an object, and use the -> operator to call an instance method.

Typically, in a .cpp file we would add a using namespace Windows::UI::Xaml::Media::Imaging directive and the auto keyword, so that the same code would look like this:

Properties

A ref class can have properties, which, just as in managed languages, are special member functions that appear as fields to consuming code.

Delegates

Just as in managed languages, a delegate is a reference type that encapsulates a function with a specific signature. They are most often used with events and event handlers

Adding content to the app

Let's add some content to the app.

Step 1: Modify your start page

1. In Solution Explorer, open MainPage.xaml.

2. Create controls for the UI by adding the following XAML to the root Grid, immediately before its closing tag. It contains a StackPanel that has a TextBlock that asks the user's name, a TextBox element that accepts the user's name, a Button, and another TextBlock element.

3. At this point, you have created a very basic Universal Windows app. To see what the UWP app looks like, press F5 to build, deploy, and run the app in debugging mode.

The default splash screen appears first. It has an image—AssetsSplashScreen.scale-100.png—and a background color that are specified in the app's manifest file. To learn how to customize the splash screen, see Adding a splash screen.

When the splash screen disappears, your app appears. It displays the main page of the App.

It doesn't do much—yet—but congratulations, you've built your first Universal Windows Platform app!

To stop debugging and close the app, return to Visual Studio and press Shift+F5.

For more information, see Run a Store app from Visual Studio.

In the app, you can type in the TextBox, but clicking the Button doesn't do anything. In later steps, you create an event handler for the button's Click event, which displays a personalized greeting.

Step 2: Create an event handler

1. In MainPage.xaml, in either XAML or design view, select the 'Say Hello' Button in the StackPanel you added earlier.

2. Open the Properties Window by pressing F4, and then choose the Events button ().

3. Find the Click event. In its text box, type the name of the function that handles the Click event. For this example, type 'Button_Click'.

4. Press Enter. The event handler method is created in MainPage.xaml.cpp and opened so that you can add the code that's executed when the event occurs.

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At the same time, in MainPage.xaml, the XAML for the Button is updated to declare the Click event handler, like this:

You could also have simply added this to the xaml code manually, which can be helpful if the designer doesn't load. If you enter this manually, type 'Click' and then let IntelliSense pop up the option to add a new event handler. That way, Visual Studio creates the necessary method declaration and stub.

The designer fails to load if an unhandled exception occurs during rendering. Rendering in the designer involves running a design-time version of the page. It can be helpful to disable running user code. You can do this by changing the setting in the Tools, Options dialog box. Under XAML Designer, uncheck Run project code in XAML designer (if supported).

5. In MainPage.xaml.cpp, add the following code to the Button_Click event handler that you just created. This code retrieves the user's name from the nameInputTextBox control and uses it to create a greeting. The greetingOutputTextBlock displays the result.

6. Set the project as the startup, and then press F5 to build and run the app. When you type a name in the text box and click the button, the app displays a personalized greeting.

Step 3: Style the start page

Choosing a theme

It's easy to customize the look and feel of your app. By default, your app uses resources that have a light style. The system resources also include a light theme. Let's try it out and see what it looks like.

To switch to the dark theme

1. Open App.xaml.

2. In the opening Application tag, edit the RequestedTheme property and set its value to Dark:

   Here's the full Application tag with the dark theme :

3. Press F5 to build and run it. Notice that it uses the dark theme.

Which theme should you use? Whichever one you want. Here's our take: for apps that mostly display images or video, we recommend the dark theme; for apps that contain a lot of text, we recommend the light theme. If you're using a custom color scheme, use the theme that goes best with your app's look and feel. In the rest of this tutorial, we use the Light theme in screenshots.

Note The theme is applied when the app is started and can't be changed while the app is running.

Using system styles

Right now, in the Windows app the text is very small and difficult to read. Let's fix that by applying a system style.

To change the style of an element

1. In the Windows project, open MainPage.xaml.

2. In either XAML or design view, select the 'What's your name?'TextBlock that you added earlier.

3. In the Properties window (F4), choose the Properties button () in the upper right.

4. Expand the Text group and set the font size to 18 px.

5. Expand the Miscellaneous group and find the Style property.

6. Click the property marker (the green box to the right of the Style property), and then, on the menu, choose System Resource > BaseTextBlockStyle.

   BaseTextBlockStyle is a resource that's defined in the ResourceDictionary in Program FilesWindows Kits10Includewinrtxamldesigngeneric.xaml.

   On the XAML design surface, the appearance of the text changes. In the XAML editor, the XAML for the TextBlock is updated:

7. Repeat the process to set the font size and assign the BaseTextBlockStyle to the greetingOutputTextBlock element.

   Tip Although there's no text in this TextBlock, when you move the pointer over the XAML design surface, a blue outline shows where it is so that you can select it.

   Your XAML now looks like this:

8. Press F5 to build and run the app. It now looks like this:

Step 4: Adapt the UI to different window sizes

Now we'll make the UI adapt to different screen sizes so it looks good on mobile devices. To do this, you add a VisualStateManager and set properties that are applied for different visual states.

To adjust the UI layout

1. In the XAML editor, add this block of XAML after the opening tag of the root Grid element.

2. Debug the app on the local machine. Notice that the UI looks the same as before unless the window gets narrower than 641 device-independent pixels (DIPs).

3. Debug the app on the mobile device emulator. Notice that the UI uses the properties you defined in the narrowState and appears correctly on the small screen.

If you've used a VisualStateManager in previous versions of XAML, you might notice that the XAML here uses a simplified syntax.

The VisualState named wideState has an AdaptiveTrigger with its MinWindowWidth property set to 641. This means that the state is to be applied only when the window width is not less than the minimum of 641 DIPs. You don't define any Setter objects for this state, so it uses the layout properties you defined in the XAML for the page content.

The second VisualState, narrowState, has an AdaptiveTrigger with its MinWindowWidth property set to 0. This state is applied when the window width is greater than 0, but less than 641 DIPs. (At 641 DIPs, the wideState is applied.) In this state, you do define some Setter objects to change the layout properties of controls in the UI:

• You reduce the left margin of the contentPanel element from 120 to 20.
• You change the Orientation of the inputPanel element from Horizontal to Vertical.
• You add a top margin of 4 DIPs to the inputButton element.

Summary

Congratulations, you've completed the first tutorial! It taught how to add content to Windows Universal apps, how to add interactivity to them, and how to change their appearance.

Next steps

If you have a Windows Universal app project that targets Windows 8.1 and/or Windows Phone 8.1, you can port it to Windows 10. There is no automatic process for this, but you can do it manually. Start with a new Windows Universal project to get the latest project system structure and manifest files, copy your code files into the project's directory structure, add the items to your project, and rewrite your XAML using the VisualStateManager according to the guidance in this topic. For more information, see Porting a Windows Runtime 8 project to a Universal Windows Platform (UWP) project and Porting to the Universal Windows Platform (C++).

If you have existing C++ code that you want to integrate with a UWP app, such as to create a new UWP UI for an existing application, see How to: Use existing C++ code in a Universal Windows project.

C++ provides the following classes to perform output and input of characters to/from files:

• ofstream: Stream class to write on files
• ifstream: Stream class to read from files
• fstream: Stream class to both read and write from/to files.

These classes are derived directly or indirectly from the classes istream and ostream. We have already used objects whose types were these classes: cin is an object of class istream and cout is an object of class ostream. Therefore, we have already been using classes that are related to our file streams. And in fact, we can use our file streams the same way we are already used to use cin and cout, with the only difference that we have to associate these streams with physical files. Let's see an example:
This code creates a file called example.txt and inserts a sentence into it in the same way we are used to do with cout, but using the file stream myfile instead.
But let's go step by step:

Open a file

The first operation generally performed on an object of one of these classes is to associate it to a real file. This procedure is known as to open a file. An open file is represented within a program by a stream (i.e., an object of one of these classes; in the previous example, this was myfile) and any input or output operation performed on this stream object will be applied to the physical file associated to it.
In order to open a file with a stream object we use its member function open:
open (filename, mode);

Where filename is a string representing the name of the file to be opened, and mode is an optional parameter with a combination of the following flags:

ios::in	Open for input operations.
ios::out	Open for output operations.
ios::binary	Open in binary mode.
ios::ate	Set the initial position at the end of the file. If this flag is not set, the initial position is the beginning of the file.
ios::app	All output operations are performed at the end of the file, appending the content to the current content of the file.
ios::trunc	If the file is opened for output operations and it already existed, its previous content is deleted and replaced by the new one.

All these flags can be combined using the bitwise operator OR (). For example, if we want to open the file example.bin in binary mode to add data we could do it by the following call to member function open:

Each of the open member functions of classes ofstream, ifstream and fstream has a default mode that is used if the file is opened without a second argument:

class	default mode parameter
ofstream	ios::out
ifstream	ios::in
fstream	ios::in ios::out

For ifstream and ofstream classes, ios::in and ios::out are automatically and respectively assumed, even if a mode that does not include them is passed as second argument to the open member function (the flags are combined).
For fstream, the default value is only applied if the function is called without specifying any value for the mode parameter. If the function is called with any value in that parameter the default mode is overridden, not combined.
File streams opened in binary mode perform input and output operations independently of any format considerations. Non-binary files are known as text files, and some translations may occur due to formatting of some special characters (like newline and carriage return characters).
Since the first task that is performed on a file stream is generally to open a file, these three classes include a constructor that automatically calls the open

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member function and has the exact same parameters as this member. Therefore, we could also have declared the previous myfile object and conduct the same opening operation in our previous example by writing:
Combining object construction and stream opening in a single statement. Both forms to open a file are valid and equivalent.
To check if a file stream was successful opening a file, you can do it by calling to member is_open. This member function returns a bool value of true in the case that indeed the stream object is associated with an open file, or false otherwise:

Closing a file

When we are finished with our input and output operations on a file we shall close it so that the operating system is notified and its resources become available again. For that, we call the stream's member function close. This member function takes flushes the associated buffers and closes the file:
Once this member function is called, the stream object can be re-used to open another file, and the file is available again to be opened by other processes.
In case that an object is destroyed while still associated with an open file, the destructor automatically calls the member function close.

Text files

Text file streams are those where the ios::binary flag is not included in their opening mode. These files are designed to store text and thus all values that are input or output from/to them can suffer some formatting transformations, which do not necessarily correspond to their literal binary value.
Writing operations on text files are performed in the same way we operated with cout:

Reading from a file can also be performed in the same way that we did with cin:
This last example reads a text file and prints out its content on the screen. We have created a while loop that reads the file line by line, using getline. The value returned by getline is a reference to the stream object itself, which when evaluated as a boolean expression (as in this while-loop) is true if the stream is ready for more operations, and false if either the end of the file has been reached or if some other error occurred.

Checking state flags

The following member functions exist to check for specific states of a stream (all of them return a bool value):

bad()

Returns true if a reading or writing operation fails. For example, in the case that we try to write to a file that is not open for writing or if the device where we try to write has no space left.

fail()

Returns true in the same cases as bad(), but also in the case that a format error happens, like when an alphabetical character is extracted when we are trying to read an integer number.

eof()

Returns true if a file open for reading has reached the end.

good()

It is the most generic state flag: it returns false in the same cases in which calling any of the previous functions would return true. Note that good and bad are not exact opposites (good checks more state flags at once).

The member function clear() can be used to reset the state flags.

get and put stream positioning

All i/o streams objects keep internally -at least- one internal position:
ifstream, like istream, keeps an internal get position with the location of the element to be read in the next input operation.
ofstream, like ostream, keeps an internal put position with the location where the next element has to be written.
Finally, fstream, keeps both, the get and the put position, like iostream.
These internal stream positions point to the locations within the stream where the next reading or writing operation is performed. These positions can be observed and modified using the following member functions:

tellg() and tellp()

These two member functions with no parameters return a value of the member type streampos, which is a type representing the current get position (in the case of tellg) or the put position (in the case of tellp).

seekg() and seekp()

These functions allow to change the location of the get and put positions. Both functions are overloaded with two different prototypes. The first form is:
seekg ( position );
seekp ( position );

Using this prototype, the stream pointer is changed to the absolute position position (counting from the beginning of the file). The type for this parameter is streampos, which is the same type as returned by functions tellg and tellp.
The other form for these functions is:
seekg ( offset, direction );
seekp ( offset, direction );


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Using this prototype, the get or put position is set to an offset value relative to some specific point determined by the parameter direction. offset is of type streamoff. And direction is of type seekdir, which is an enumerated type that determines the point from where offset is counted from, and that can take any of the following values:

ios::beg	offset counted from the beginning of the stream
ios::cur	offset counted from the current position
ios::end	offset counted from the end of the stream

The following example uses the member functions we have just seen to obtain the size of a file:

Notice the type we have used for variables begin and end

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:
streampos is a specific type used for buffer and file positioning and is the type returned by file.tellg(). Values of this type can safely be subtracted from other values of the same type, and can also be converted to an integer type large enough to contain the size of the file.
These stream positioning functions use two particular types: streampos and streamoff. These types are also defined as member types of the stream class:

Type	Member type	Description
streampos	ios::pos_type	Defined as fpos<mbstate_t>. It can be converted to/from streamoff and can be added or subtracted values of these types.
streamoff	ios::off_type	It is an alias of one of the fundamental integral types (such as int or long long).

Each of the member types above is an alias of its non-member equivalent (they are the exact same type). It does not matter which one is used. The member types are more generic, because they are the same on all stream objects (even on streams using exotic types of characters), but the non-member types are widely used in existing code for historical reasons.

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Binary files

For binary files, reading and writing data with the extraction and insertion operators (<< and >>) and functions like getline is not efficient, since we do not need to format any data and data is likely not formatted in lines.
File streams include two member functions specifically designed to read and write binary data sequentially: write and read. The first one (write) is a member function of ostream (inherited by ofstream). And read is a member function of istream (inherited by ifstream). Objects of class fstream have both. Their prototypes are:
write ( memory_block, size );
read ( memory_block, size );

Where memory_block is of type char* (pointer to char), and represents the address of an array of bytes where the read data elements are stored or from where the data elements to be written are taken. The size parameter is an integer value that specifies the number of characters to be read or written from/to the memory block.

In this example, the entire file is read and stored in a memory block. Let's examine how this is done:
First, the file is open with the ios::ate flag, which means that the get pointer will be positioned at the end of the file. This way, when we call to member tellg(), we will directly obtain the size of the file.
Once we have obtained the size of the file, we request the allocation of a memory block large enough to hold the entire file:
Right after that, we proceed to set the get position at the beginning of the file (remember that we opened the file with this pointer at the end), then we read the entire file, and finally close it:

At this point we could operate with the data obtained from the file. But our program simply announces that the content of the file is in memory and then finishes.

Buffers and Synchronization

When we operate with file streams, these are associated to an internal buffer object of type streambuf

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. This buffer object may represent a memory block that acts as an intermediary between the stream and the physical file. For example, with an ofstream, each time the member function put (which writes a single character) is called, the character may be inserted in this intermediate buffer instead of being written directly to the physical file with which the stream is associated.
The operating system may also define other layers of buffering for reading and writing to files.
When the buffer is flushed, all the data contained in it is written to the physical medium (if it is an output stream). This process is called synchronization and takes place under any of the following circumstances:

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• When the file is closed: before closing a file, all buffers that have not yet been flushed are synchronized and all pending data is written or read to the physical medium.
• When the buffer is full: Buffers have a certain size. When the buffer is full it is automatically synchronized.
• Explicitly, with manipulators: When certain manipulators are used on streams, an explicit synchronization takes place. These manipulators are: flush and endl.
• Explicitly, with member function sync(): Calling the stream's member function sync() causes an immediate synchronization. This function returns an int value equal to -1 if the stream has no associated buffer or in case of failure. Otherwise (if the stream buffer was successfully synchronized) it returns 0.

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