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 ofConcepts TS).

[edit]Core language changes

[edit]reflexpr-specifier

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

reflexpr-operand can be one of following:

wherefunction-call-expression is

andfunctional-type-conv-expression are following sorts of expressions which performexplict cast:

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

For areflexpr-operand of form(expression), theexpression shall be a (possibly multi-parenthesized)function-call-expression orfunctional-type-conv-expression.

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

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

[edit]Meta-object types

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

It is unspecified whether repeatedly applyingreflexpr 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 toreflexpr through type traits or type transformations on it.

[edit]Overload resolution

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

Ifpostfix-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

[edit]Reflection-related

Analias is a name introduced by atypedef declaration, analias-declaration, or ausing-declaration.

An entity or aliasB isreflection-related to an entity or aliasA if

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

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

Informally speaking, the case thatB is reflection-related toA means thatB participates in the declaration or definition ofA.

Zero or more successive applications of type transformations that yield meta-object types to the type denoted by areflexpr-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

[edit]Miscellaneous

• An expression used asreflexpr-operand is anunevaluated expressions andpotentially constant evaluated.
• For the purpose of determination of variablescaptured in a lambda expression by a capture-default, areflexpr operand is not considered to be an unevaluated operand.
• A function or variable of staticstorage duration reflected by meta-object typeT isodr-used by the specializationstd::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 isdependent if it is denoted by a reflexpr-specifier, and the operand
  • is atype-dependent expression or a (possibly parenthesized)functional-type-conv-expression with at least one type-dependent immediate subexpression, or
  • designates a dependent type or a member of anunknown specialization or avalue-dependent constant expression.

[edit]Keywords

reflexpr

[edit]Predefined feature testing macros

[edit]Library support

[edit]Concepts

[edit]Meta-object operations

[edit]Library feature testing macros

[edit]Satisfaction of concepts

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

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

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

[edit]See also

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