Classes and Structs (C++ Component Extensions)
The latest version of this topic can be found at Classes and Structs (C++ Component Extensions).
Declares a class or struct whose object lifetime is administered automatically. When the object is no longer accessible or goes out of scope, Visual C++ automatically discards the memory that is allocated to the object.
All Runtimes
Syntax
class_access
ref class
name
modifier : inherit_accessbase_type {};
class_accessref structnamemodifier : inherit_accessbase_type {};
class_accessvalue classnamemodifier : inherit_accessbase_type {};
class_accessvalue structnamemodifier : inherit_accessbase_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) or hidden (with new (new slot in vtable)). 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.
For more information on classes and structs, see
Windows Runtime
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.
Requirements
Compiler option: /ZW
Common Language Runtime
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.
Requirements
Compiler option: /clr