Classes and Structs (C++ Component Extensions)

Syntax

class_access ref class     name modifier :  inherit_access base_type {};
class_access ref struct    name modifier :  inherit_access base_type {};
class_access value class   name modifier :  inherit_access base_type {};
class_access value struct name modifier :  inherit_access base_type {};

Parameters

class_access (optional)

The accessibility of the class or struct outside the assembly. Possible values are public and private (private is the default). Nested classes or structs cannot have a class_access specifier.

name

The name of the class or struct.

modifier (optional)

abstract and sealed are valid modifiers.

inherit_access (optional)

The accessibility of base_type. The only permitted accessibility is public (public is the default).

base_type (optional)

A base type. However, a value type cannot act as a base type.

For more information, see the language-specific descriptions of this parameter in the Windows Runtime and Common Language Runtime sections.

Remarks

The default member accessibility of an object declared with ref class or value class is private. And the default member accessibility of an object declared with ref struct or value struct is public.

When a reference type inherits from another reference type, virtual functions in the base class must explicitly be overridden (with override (C++ Component Extensions)) or hidden (with new (new slot in vtable) (C++ Component Extensions)). The derived class functions must also be explicitly marked as virtual.

To detect at compile time whether a type is a ref class or ref struct, or a value class or value struct, use __is_ref_class (type), __is_value_class (type), or __is_simple_value_class (type). For more information, see Compiler Support for Type Traits (C++ Component Extensions).

For more information on classes and structs, see

• Instantiating Classes and Structs

• Semantics of the this Pointer in Value and Reference Types

• C++ Stack Semantics for Reference Types

• Classes, Structures, and Unions

• public and private on Native Classes

• Implicitly Abstract Classes

• Define Static Constructors in a Class or Struct

• Copy Constructor May Not Be Generated

• Hide-by-Signature Functions in Reference Types

• Destructors and Finalizers in Visual C++

• Type and Member Visibility

• User-Defined Operators

• User-Defined Conversions

• How to: Wrap Native Class for Use by C#

• Generic Classes (C++/CLI)

Remarks

See Ref classes and structs and Value classes and structs.

Parameters

base_type (optional)

A base type. A ref class or ref struct can inherit from zero or more interfaces and zero or one ref types. A value class or value struct can only inherit from zero or more interfaces.

When you declare an object by using the ref class or ref struct keywords, the object is accessed by a handle to an object; that is, a reference-counter pointer to the object. When the declared variable goes out of scope, the compiler automatically deletes the underlying object. When the object is used as a parameter in a call or is stored in a variable, a handle to the object is actually passed or stored.

When you declare an object by using the value class or value struct keywords, the object lifetime of the declared object is not supervised. The object is like any other standard C++ class or struct.

Remarks

The following table lists differences from the syntax shown in the All Runtimes section that are specific to C++/CLI.

Parameters

base_type (optional)

A base type. A ref class or ref struct can inherit from zero or more managed interfaces and zero or one ref types. A value class or value struct can only inherit from zero or more managed interfaces.

The ref class and ref struct keywords tell the compiler that the class or structure is to be allocated on the heap. When the object is used as a parameter in a call or is stored in a variable, a reference to the object is actually passed or stored.

The value class and value struct keywords tells the compiler that the value of the allocated class or structure is passed to functions or stored in members.

Concepts