std::meta::define_aggregate

 

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Defined in header <meta>
template< std::meta::reflection_range R = std::initializer_list<std::meta::info> > consteval std::meta::info define_aggregate( std::meta::info class_type, R&& member_dsc );		(since C++26)

See More

Defines an aggregate type C from a sequence of data member descriptions member_dsc.

Let C denote the type represented by class_type and r_k be the K ^th reflection value in member_dsc. Let ⊥ (“absurdum symbol” in logic) denote a special value. For every r_k in member_dsc, let the sextuple (T_k, N_k, A_k, W_k, NUA_k, ANN_k) be the corresponding data member description represented by r_k .

A call to this function is a core constant expression if and only if all of the following preconditions are met:

• class_type represents a cv-unqualified class type;
• C is incomplete from every point in the evaluation context;
• std::meta::is_data_member_spec(r_k) is true for every r_k ;
• std::meta::is_complete_type(T_k) is true for every r_k ; and
• for every pair ( r_k , r_m ) where k < m, if N_k is not ⊥ and N_m is not ⊥, then either:

• N_k is not the same identifier as N_m or
• N_k is the identifier _ (LOW LINE).

The define_aggregate produces an injected declaration D that defines C and has properties as follows:

• The target scope of D is the scope to which C belongs.
• The locus of D follows immediately after the core constant expression currently under evaluation.
• The characteristic sequence of D is the sequence of reflection values r_k .
• If C is a specialization of a templated class T, and C is not a local class, then D is an explicit specialization of T.
• For each r_k, there is a corresponding entity M_k with public access belonging to the class scope of D with the following properties:
  • If N_k is ⊥, M_k is an unnamed bit-field. Otherwise, M_k is a non-static data member whose name is the identifier N_k .
  • The type of M_k is T_k .
  • M_k is declared with the attribute [[no_unique_address]] if and only if NUA_k is true.
  • If W_k is not ⊥, M_k is a bit-field whose width is that value. Otherwise, M_k is not a bit-field.
  • If A_k is not ⊥, M_k has the alignment-specifier alignas(A_k). Otherwise, M_k has no alignment-specifier.
  • For every reflection r in ANN_k , M_k has an annotation whose underlying constant is r.
• For every r_m in member_dsc such that k < m, the declaration corresponding to r_k precedes the declaration corresponding to r_m .

If C is a specialization of a templated class and it has not been instantiated, C is treated as an explicit specialization.

Parameters

class_type	-	reflected incomplete class type to complete
member_dsc	-	range of data member descriptions to inject

Return value

class_type that is complete (contains injected members), as described above.

Notes

C can be a union type.

Example

Run this code

#include <meta>
#include <print>

struct MyAggregate; // declares an incomplete type

consteval
{
    // Define the members
    std::meta::info members[]
    {
        std::meta::data_member_spec(^^int, {.name = "id"}),
        std::meta::data_member_spec(^^double, {.name = "value"})
    };

    // Pass the reflection of the incomplete class
    std::meta::define_aggregate(^^MyAggregate, members);
}

// MyAggregate is now a complete aggregate type with “id” and “value” fields.

int main()
{
    MyAggregate a{.id = 42, .value = 1.618};
    std::println("a.id = {}, a.value = {}", a.id, a.value);
}

Output:

a.id = 42, a.value = 1.618

See also

is_data_member_spec (C++26)	checks if reflection represents a data member description (function)
data_member_spec (C++26)	creates a data member description for use with injected declarations (function)