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Extensions for reflection

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C++
 
Experimental
Technical Specification
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals TS v2)
Library fundamentals 3 (library fundamentals TS v3)
Extensions for parallelism (parallelism TS)
Extensions for parallelism 2 (parallelism TS v2)
Extensions for concurrency (concurrency TS)
Extensions for concurrency 2 (concurrency TS v2)
Concepts (concepts TS)
Ranges (ranges TS)
Reflection (reflection TS)
Mathematical special functions (special functions TR)
Experimental Non-TS
Pattern Matching
Linear Algebra
std::execution
Contracts
2D Graphics
 
Extensions for reflection
Concepts
Meta-object operations
Variable
FunctionParameter
Callable
Variable and Callable
Namespace and Callable
ParenthesizedExpression
FunctionCallExpression
FunctionalConversion
Variable and Function
 

The C++ Extensions for Reflection, ISO/IEC TS 23619:2021, specifies modifications to the core language and defines new components for the C++ standard library listed on this page.

The Reflection TS is based on the C++20 standard (except that the definition of concepts are specified in the style of Concepts TS).

Contents

Core language changes

reflexpr-specifier

A reflexpr-specifier is of form reflexpr ( reflexpr-operand ), and specifies a meta-object type (see below).

reflexpr-operand can be one of following:

:: (1)
type-id (2)
nested-name-specifier(optional) namespace-name (3)
id-expression (4)
( expression ) (5)
function-call-expression (6)
functional-type-conv-expression (7)

where function-call-expression is

postfix-expression ( expression-list(optional) )

and functional-type-conv-expression are following sorts of expressions which perform explict cast:

simple-type-specifier ( expression-list(optional) ) (1)
typename-specifier ( expression-list(optional) ) (2)
simple-type-specifier braced-init-list (3)
typename-specifier braced-init-list (4)

The operand to the reflexpr-specifier shall be a type, namespace, enumerator, variable, data member, function parameter, captured entity, function-call-expression or functional-type-conv-expression, and parenthesized expression. reflexpr(::) reflects the global namespace.

For a reflexpr-operand of form ( expression ), the expression shall be a (possibly multi-parenthesized) function-call-expression or functional-type-conv-expression.

If an unparenthesized operand can be treated as either a type-id or a functional-type-conv-expression, then it is treated as a type-id. Parenthesizes can be used for disambiguation between function-style cast and a type-id. For example, given a class type X with default constructor, reflexpr(X()) reflects the function type X(), and reflexpr((X())) reflects the expression X().

If the operand designates both an alias and a class name, the type represented by the reflexpr-specifier reflects the alias and satisfies reflect::Alias.

If the operand designates a name whose declaration is enclosed in a block scope and the named entity is neither captured nor a function parameter, the program is ill-formed.

Meta-object types

A meta-object type is an unnamed, incomplete namespace-scope class type. A type satisfies the concept reflect::Object if and only if it is a meta-object type. Meta-object types may satisfy other concepts, depending on the operand to reflexpr.

It is unspecified whether repeatedly applying reflexpr to the same operand yields the same type or a different type. If a meta-object type reflects an incomplete class type, certain type transformations cannot be applied.

A meta-object type allows inspection of some properties of the operand to reflexpr through type traits or type transformations on it.

Overload resolution

If the postfix-expression of the function-call-expression is of class type, i.e. e in the function-call-expression e(args) is of class type, then the user-defined conversion function of the type of the postfix-expression (e) shall not be used.

If postfix-expression is not of class type, it shall name a function that is the unique result of overload resolution. 

struct Functor {
    void operator()(int) const;
 
    using fptr_t = void(*)(std::nullptr_t);
    operator fptr_t() const;
};
 
using Meta0 = reflexpr(Functor{}(0));          // OK
// using Meta1 = reflexpr(Functor{}(nullptr)); // error: conversion function used

Reflection-related

An alias is a name introduced by a typedef declaration, an alias-declaration, or a using-declaration.

An entity or alias B is reflection-related to an entity or alias A if

  1. A and B are the same entity or alias,
  2. A is a variable or enumerator and B is the type of A,
  3. A is an enumeration and B is the underlying type of A,
  4. A is a class and B is a member or base class of A,
  5. A is a non-template alias that designates the entity B,
  6. A is not the global namespace and B is an enclosing class or namespace of A,
  7. A is the parenthesized expression ( B ),
  8. A is a lambda capture of the closure type B,
  9. A is the closure type of the lambda capture B,
  10. B is the type specified by the functional-type-conv-expression A,
  11. B is the function selected by overload resolution for a function-call-expression A,
  12. B is the return type, a parameter type, or function type of the function A, or
  13. B is reflection-related to an entity or alias X and X is reflection-related to A.

Reflection-relation relationship is reflexive and transitive, but not symmetric.

Informally speaking, the case that B is reflection-related to A means that B participates in the declaration or definition of A.

Zero or more successive applications of type transformations that yield meta-object types to the type denoted by a reflexpr-specifier enable inspection of entities and aliases that are reflection-related to the operand; such a meta-object type is said to reflect the respective reflection-related entity or alias. 

struct X;
struct B {
    using X = ::X;
    typedef X Y;
};
struct D : B {
    using B::Y;
};
// ::X, but not B::X or B::Y is reflection-related to D::Y

Miscellaneous

  • An expression used as reflexpr-operand is an unevaluated expressions and potentially constant evaluated.
  • For the purpose of determination of variables captured in a lambda expression by a capture-default, a reflexpr operand is not considered to be an unevaluated operand.
  • A function or variable of static storage duration reflected by meta-object type T is odr-used by the specialization std::experimental::reflect::get_pointer<T>, as if by taking the address of an id-expression nominating the function or variable.
  • There can be more than one definition of a meta-object type, as long as all operations on this type yield the same constant expression results.
  • A type is dependent if it is denoted by a reflexpr-specifier, and the operand

Keywords

reflexpr

Predefined feature testing macros

__cpp_reflection
(reflection TS)
 a value of at least 201902 indicates that the Reflection TS is supported
(macro constant)

Library support

Concepts

Defined in header <experimental/reflect>
Defined in namespace std::experimental::reflect
Defined in inline namespace std::experimental::reflect::v1
(reflection TS)
 specifies that a type is a meta-object type
(concept)
(reflection TS)
 specifies that a meta-object type is a meta-object sequence type
(concept)
(reflection TS)
 specifies that a meta-object type reflects a template parameter scope
(concept)
(reflection TS)
 specifies that a meta-object type reflects an entity or alias with an associated (possibly empty) name
(concept)
(reflection TS)
 specifies that a meta-object type reflects a type alias, namespace alias, or an alias introduced by a using-declaration
(concept)
(reflection TS)
 specifies that a meta-object type reflects a member-declaration of a class
(concept)
(reflection TS)
 specifies that a meta-object type reflects an enumerator
(concept)
(reflection TS)
 specifies that a meta-object type reflects a variable or data member
(concept)
(reflection TS)
 specifies that a meta-object type satisfies RecordMember, Enumerator, or Variable, or reflects a namespace other than the global namespace
(concept)
(reflection TS)
 specifies that a meta-object type reflects an entity with a type
(concept)
(reflection TS)
 specifies that a meta-object type reflects a namespace
(concept)
(reflection TS)
 specifies that a meta-object type reflects the global namespace
(concept)
(reflection TS)
 specifies that a meta-object type reflects a non-union class type
(concept)
(reflection TS)
 specifies that a meta-object type reflects an enumeration type
(concept)
(reflection TS)
 specifies that a meta-object type reflects a class type
(concept)
(reflection TS)
 specifies that a meta-object type reflects a namespace, class, enumeration, function, closure type, a template parameter scope
(concept)
(reflection TS)
 specifies that a meta-object type reflects a type
(concept)
(reflection TS)
 specifies that a meta-object type reflects an enumerator or a constexpr variable
(concept)
(reflection TS)
 specifies that a meta-object type reflects a direct base class obtained from get_base_classes
(concept)
(reflection TS)
 specifies that a meta-object type reflects a function parameter
(concept)
(reflection TS)
 specifies that a meta-object type reflects a function (including constructors and destructors)
(concept)
(reflection TS)
 specifies that a meta-object type reflects an expression
(concept)
(reflection TS)
 specifies that a meta-object type reflects a parenthesized expression
(concept)
(reflection TS)
 specifies that a meta-object type reflects a function-call-expression
(concept)
(reflection TS)
 specifies that a meta-object type reflects a functional-type-conv-expression
(concept)
(reflection TS)
 specifies that a meta-object type reflects a function (excluding constructors and destructors)
(concept)
(reflection TS)
 specifies that a meta-object type reflects a member function (excluding constructors and destructors)
(concept)
(reflection TS)
 specifies that a meta-object type reflects a special member function
(concept)
(reflection TS)
 specifies that a meta-object type reflects a constructor
(concept)
(reflection TS)
 specifies that a meta-object type reflects a destructor
(concept)
(reflection TS)
 specifies that a meta-object type reflects an operator function or a conversion function
(concept)
(reflection TS)
 specifies that a meta-object type reflects a conversion function
(concept)
(reflection TS)
 specifies that a meta-object type reflects the closure type of a non-generic lambda
(concept)
(reflection TS)
 specifies that a meta-object type reflects a lambda capture
(concept)

Meta-object operations

Defined in header <experimental/reflect>
Defined in namespace std::experimental::reflect
Defined in inline namespace std::experimental::reflect::v1
Object operations
(reflection TS)
 checks if two meta-object types reflect the same entity or alias
(class template)
(reflection TS)
 obtains the presumed line number of the declaration of the reflected entity or alias
(class template)
(reflection TS)
 obtains the implementation-defined column number of the declaration of the reflected entity or alias
(class template)
(reflection TS)
 obtains the presumed file name of the declaration of the reflected entity or alias
(class template)
ObjectSequence operations
(reflection TS)
 obtains the size of a meta-object sequence
(class template)
(reflection TS)
 obtains the meta-object type with specified index in a sequence
(class template)
(reflection TS)
 applies a template to the meta-object sequence
(class template)
Named operations
(reflection TS)
 checks if the reflected entity or alias is unnamed
(class template)
(reflection TS)
 obtains the unqualified name of the reflected entity or alias
(class template)
(reflection TS)
 obtains the implementation-defined display name of the reflected entity or alias
(class template)
Alias operations
(reflection TS)
 obtains the meta-object type reflecting the associated entity of the reflected alias
(class template)
Type operations
(reflection TS)
 obtains the meta-object type reflecting the type of the reflected entity or alias
(class template)
(reflection TS)
 obtains the type of the reflected entity or alias
(class template)
(reflection TS)
 checks if the meta-object type reflects an enumeration type
(class template)
(reflection TS)
 checks if the meta-object type reflects a union type
(class template)
(reflection TS)
 checks if the meta-object type reflects a non-union class type whose declaration uses class or struct respectively
(class template)
ScopeMember operations
(reflection TS)
 obtains the meta-object type reflecting the scope of the reflected entity or alias
(class template)
Base operations
(reflection TS)
 obtains the meta-object type reflecting the base class in the given base class relationship
(class template)
RecordMember and Base operations
(reflection TS)
 checks if the reflected member or base class is public
(class template)
(reflection TS)
 checks if the reflected member or base class is protected
(class template)
(reflection TS)
 checks if the reflected member or base class is private
(class template)
Record operations
(reflection TS)
 obtains a meta-object sequence type whose elements reflect public, accessible, or all data members of the reflected class
(class template)
(reflection TS)
 obtains a meta-object sequence type whose elements reflect public, accessible, or all member functions of the reflected class
(class template)
(reflection TS)
 obtains a meta-object sequence type whose elements reflect all constructors of the reflected class
(class template)
(reflection TS)
 obtains a meta-object sequence type whose elements reflect all operator functions and conversion functions declared in the reflected class
(class template)
(reflection TS)
 obtains the meta-object type reflecting the destructor of the reflected class
(class template)
(reflection TS)
 obtains a meta-object sequence type whose elements reflect public, accessible, or all nested type or member typedefs of the reflected class
(class template)
(reflection TS)
 obtains a meta-object sequence type whose elements reflect public, accessible, or all base classes of the reflected class
(class template)
Enum operations
(reflection TS)
 checks whether the reflected enumeration is scoped
(class template)
(reflection TS)
 obtains a meta-object sequence type whose elements reflects the enumerators of the reflected enumeration
(class template)
(reflection TS)
 obtains the meta-object type reflecting the underlying type of the reflected enumeration
(class template)
Variable operations
(reflection TS)
 obtains the value of the reflected variable which is a constant expression
(class template)
(reflection TS)
 checks if the variable is declared with thread_local
(class template)
FunctionParameter operations
(reflection TS)
 checks whether the reflected parameter has a default argument
(class template)
Callable operations
(reflection TS)
 obtains a meta-object sequence type whose elements reflects the parameters of the reflected function
(class template)
(reflection TS)
 checks whether the parameter list of the reflected function contains an ellipsis parameter
(class template)
(reflection TS)
 checks whether the reflected function is non-throwing
(class template)
(reflection TS)
 checks whether the reflected function is deleted
(class template)
Variable and Callable operations
(reflection TS)
 checks if the reflected variable or function is constexpr
(class template)
Namespace and Callable operations
(reflection TS)
 checks whether the reflected namespace or function is inline
(class template)
ParenthesizedExpression operations
(reflection TS)
 obtains the meta-object type reflecting the unparenthesized expression of the reflected parethesized expression
(class template)
FunctionCallExpression operations
(reflection TS)
 obtains the meta-object type reflecting the function in the reflected function-call-expression
(class template)
FunctionalTypeConversion operations
(reflection TS)
 obtains the meta-object type reflecting the constructor in reflected functional-type-conv-expression
(class template)
Variable and Function operations
(reflection TS)
 get the address of the reflected variable or function, or the pointer-to-member value to the reflected non-static member
(class template)
MemberFunction operations
(reflection TS)
 checks if the reflected member function is declared with const, volatile, &, or && qualifier respectively
(class template)
(reflection TS)
 checks if the reflected member function overrides a member function of base class
(class template)
Record and MemberFunction operations
(reflection TS)
 checks if the reflected class or member function is marked with final
(class template)
Variable and MemberFunction operations
(reflection TS)
 checks if the reflected variable is of static storage duration, or the reflected member function is static
(class template)
SpecialMemberFunction operations
(reflection TS)
 checks if the reflected special member function is implicitly declared
(class template)
(reflection TS)
 checks if the reflected special member function is defaulted in its first declaration
(class template)
Constructor and ConversionOperator operations
(reflection TS)
 checks if the reflected constructor or conversion function is declared with explicit
(class template)
MemberFunction and Destructor operations
(reflection TS)
 checks if the reflected member function is virtual
(class template)
(reflection TS)
 checks if the reflected member function is pure virtual
(class template)
Lambda operations
(reflection TS)
 obtains a meta-object sequence type whose elements reflect the captures of the reflected closure type
(class template)
(reflection TS)
 checks if the capture-default of the lambda expression of the reflected closure type is = or & respectively
(class template)
(reflection TS)
 checks if the operator() of the reflected closure type is declared with const
(class template)
LambdaCapture operations
(reflection TS)
 checks if the reflected lambda capture is explicitly captured
(class template)
(reflection TS)
 checks if the reflected lambda capture is an init-capture
(class template)

Library feature testing macros

Defined in header <experimental/reflect>
__cpp_lib_reflection
(reflection TS)
 a value of at least 201902 indicates that the support library of the Reflection TS is supported
(macro constant)

Satisfaction of concepts

The following table lists that whether a meta-object type reflecting an operand satisfies concepts introduced by the Reflection TS.

Category reflexpr operands Satisfied concepts
Type class-name designating a union reflect::Union
class-name designating a closure type reflect::Lambda
class-name designating a non-union class reflect::Record
enum-name reflect::Enum
template type-parameter reflect::Type, reflect::Alias
decltype-specifier reflect::Type, reflect::Alias
type-name introduced by a using-declaration reflect::Type, reflect::Alias, reflect::ScopedMember
any other typedef-name reflect::Type, reflect::Alias
any other type-id reflect::Type
Namespace namespace-alias reflect::Namespace, reflect::Alias
the global namespace reflect::GlobalScope
any other namespace reflect::Namespace
Expression the name of a data member reflect::Variable
the name of a variable reflect::Variable
the name of an enumerator reflect::Enumerator
the name of a function parameter reflect::FunctionParameter
the name of a captured entity reflect::LambdaCapture
parenthesized expression reflect::ParenthesizedExpression
function-call-expression reflect::FunctionCallExpression
functional-type-conv-expression reflect::FunctionalTypeConversion

If the operand of the form id-expression is a constant expression, the type specified by the reflexpr-specifier also satisfies reflect::Constant.

If the reflexpr-operand designates a class member, the type represented by the reflexpr-specifier also satisfies reflect::RecordMember.

See also

 contains some type’s information, the class returned by the typeid operator
(class) [edit]
(C++11)
 Compile-time type information utilities[edit]
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