Component Extensions for Runtime Platforms

Visual C++ provides language extensions to help you program against runtime platforms. By using Visual C++ component extensions (C++/CX), you can program Windows Store apps and components that compile to native code. Although you can create Windows Store apps by programming directly against the Windows Runtime COM interfaces, by using C++/CX, you can work with constructors, exceptions, and other modern C++ programming idioms. To enable C++ programming in a managed execution environment on the .NET platform, you can use C++/CLI.

Two runtimes, one set of extensions

C++/CX is a subset of C++/CLI. For extensions that are common to C++/CX and C++/CLI, the semantics depend on whether you are targeting the common language runtime (CLR) or the Windows Runtime. To compile your app to run on the Windows Runtime, specify the /ZW compiler option. To compile it to run on the CLR, specify the /clr compiler option. These switches are set automatically when you use Visual Studio to create a project.

For more information about how to create Windows Store apps in C++, see Roadmap for Windows Store apps using C++.

C++/CLI extends the ISO/ANSI C++ standard, and is defined under the Ecma C++/CLI Standard. For more information, see .NET Programming in Visual C++.

Data Type Keywords

The language extensions include aggregate keywords, which are keywords that consist of two tokens separated by white space. The tokens might have one meaning when they are used separately, and another meaning when they are used together. For example, the word "ref" is an ordinary identifier, and the word "class" is a keyword that declares a native class. But when these words are combined to form ref class, the resulting aggregate keyword declares an entity that is known as a runtime class.

The extensions also include context-sensitive keywords. A keyword is treated as context-sensitive depending on the kind of statement that contains it, and its placement in that statement. For example, the token "property" can be an identifier, or it can declare a special kind of public class member.

The following table lists keywords in the C++ language extension.

Keyword	Context sensitive	Purpose	Reference
ref class ref struct	No	Declares a class.	Classes and Structs (C++ Component Extensions)
value class value struct	No	Declares a value class.	Classes and Structs (C++ Component Extensions)
interface class interface struct	No	Declares an interface.	interface class (C++ Component Extensions)
enum class enum struct	No	Declares an enumeration.	enum class (C++ Component Extensions)
property	Yes	Declares a property.	property (C++ Component Extensions)
delegate	Yes	Declares a delegate.	delegate (C++ Component Extensions)
event	Yes	Declares an event.	event (C++ Component Extensions)

Override Specifiers

You can use the following keywords to qualify override behavior for derivation. Although the new keyword is not an extension of C++, it is listed here because it can be used in an additional context. Some specifiers are also valid for native programming. For more information, see How to: Declare Override Specifiers in Native Compilations (C++/CLI).

Keyword	Context Sensitive	Purpose	Reference
abstract	Yes	Indicates that functions or classes are abstract.	abstract (C++ Component Extensions)
new	No	Indicates that a function is not an override of a base class version.	new (new slot in vtable) (C++ Component Extensions)
override	Yes	Indicates that a method must be an override of a base-class version.	override (C++ Component Extensions)
sealed	Yes	Prevents classes from being used as base classes.	sealed (C++ Component Extensions)

Keywords for Generics

The following keywords have been added to support generic types. For more information, see Generics (C++ Component Extensions).

Keyword	Context sensitive	Purpose
generic	No	Declares a generic type.
where	Yes	Specifies the constraints that are applied to a generic type parameter.

Miscellaneous Keywords

The following keywords have been added to the C++ extensions.

Keyword	Context sensitive	Purpose	Reference
finally	Yes	Indicates default exception handlings behavior.	Exception Handling (C++ Component Extensions)
for each, in	No	Enumerates elements of a collection.	for each, in
gcnew	No	Allocates types on the garbage-collected heap. Use instead of new and delete.	ref new, gcnew (C++ Component Extensions)
ref new	Yes	Allocates a Windows Runtime type. Use instead of new and delete.	ref new, gcnew (C++ Component Extensions)
initonly	Yes	Indicates that a member can only be initialized at declaration or in a static constructor.	initonly (C++/CLI)
literal	Yes	Creates a literal variable.	literal (C++ Component Extensions)
nullptr	No	Indicates that a handle or pointer does not point at an object.	nullptr (C++ Component Extensions)

Template Constructs

The following language constructs are implemented as templates, instead of as keywords. If you specify the /ZW compiler option, they are defined in the lang namespace. If you specify the /clr compiler option, they are defined in the cli namespace.

Keyword	Purpose	Reference
array	Declares an array.	Arrays (C++ Component Extensions)
interior_ptr	(CLR only) Points to data in a reference type.	interior_ptr (C++/CLI)
pin_ptr	(CLR only) Points to CLR reference types to temporarily suppress the garbage-collection system.	pin_ptr (C++/CLI)
safe_cast	Determines and executes the optimal casting method for a runtime type.	safe_cast (C++ Component Extensions)
typeid	(CLR only) Retrieves a Type object that describes the given type or object.	typeid (C++ Component Extensions)

Declarators

The following type declarators instruct the runtime to automatically manage the lifetime and deletion of allocated objects.

Operator	Purpose	Reference
^	Declares a handle to an object; that is, a pointer to a Windows Runtime or CLR object that is automatically deleted when it is no longer usable.	Handle to Object Operator (^) (C++ Component Extensions)
%	Declares a tracking reference; that is, a reference to a Windows Runtime or CLR object that is automatically deleted when it is no longer usable.	Tracking Reference Operator (C++ Component Extensions)

Additional Constructs and Related Topics

This section lists additional programming constructs, and topics that pertain to the CLR.

Topic	Description
__identifier (C++/CLI)	(Windows Runtime and CLR) Enables the use of keywords as identifiers.
Variable Argument Lists (...) (C++/CLI)	(Windows Runtime and CLR) Enables a function to take a variable number of arguments.
.NET Framework Equivalents to C++ Native Types (C++/CLI)	Lists the CLR types that are used in place of C++ integral types.
appdomain__declspec modifier	__declspec modifier that mandates that static and global variables exist per appdomain.
C-Style Casts with /clr (C++/CLI)	Describes how C-style casts are interpreted.
__clrcall calling convention	Indicates the CLR-compliant calling convention.
__cplusplus_cli	Predefined Macros
Custom Attributes (C++)	Describes how to define your own CLR attributes.
Exception Handling (C++ Component Extensions)	Provides an overview of exception handling.
Explicit Overrides (C++ Component Extensions)	Demonstrates how member functions can override arbitrary members.
Friend Assemblies (C++)	Discusses how a client assembly can access all types in an assembly component.
Boxing (C++ Component Extensions)	Demonstrates the conditions in which values types are boxed.
Compiler Support for Type Traits (C++ Component Extensions)	Discusses how to detect characteristics of types at compile time.
managed, unmanaged pragmas	Demonstrates how managed and unmanaged functions can co-exist in the same module.
process__declspec modifier	__declspec modifier that mandates that static and global variables exist per process.
Reflection (C++/CLI)	Demonstrates the CLR version of run-time type information.
String (C++ Component Extensions)	Discusses compiler conversion of string literals to String.
Type Forwarding (C++/CLI)	Enables the movement of a type in a shipping assembly to another assembly so that client code does not have to be recompiled.
User-Defined Attributes (C++ Component Extensions)	Demonstrates user-defined attributes.
#using Directive (C++)	Imports external assemblies.
XML Documentation (Visual C++)	Explains XML-based code documentation by using /doc (Process Documentation Comments) (C/C++)

See Also

Other Resources

.NET Programming in Visual C++

Native and .NET Interoperability