c++ - Designated initializers in C++20

Question

I've got a question about one of the c++20 feature, designated initializers (more info about this feature here)

#include <iostream>

constexpr unsigned DEFAULT_SALARY {10000};

struct Person
{
    std::string name{};
    std::string surname{};
    unsigned age{};
};

struct Employee : Person
{
    unsigned salary{DEFAULT_SALARY};
};

int main()
{
    std::cout << std::boolalpha << std::is_aggregate_v<Person> << '
'; // true is printed
    std::cout << std::boolalpha << std::is_aggregate_v<Employee> << '
'; // true is printed

    Person p{.name{"John"}, .surname{"Wick"}, .age{40}}; // it's ok
    Employee e1{.name{"John"}, .surname{"Wick"}, .age{40}, .salary{50000}}; // doesn't compile, WHY ?

    // For e2 compiler prints a warning "missing initializer for member 'Employee::<anonymous>' [-Wmissing-field-initializers]"
    Employee e2 {.salary{55000}}; 
}

This code was compiled with gcc 9.2.0 and -Wall -Wextra -std=gnu++2a flags.

As you can see above, both structs, Person and Employee are aggregates but initialization of Employee aggregate isn't possible using designated initializers.

Could someone explain me why ?

Answer 1

According to the C++ 20 Standard (9.3.1 Aggregates. p. #3)

(3.1) — If the initializer list is a designated-initializer-list, the aggregate shall be of class type, the identifier in each designator shall name a direct non-static data member of the class, and the explicitly initialized elements of the aggregate are the elements that are, or contain, those members.

So you may not use the designated initializer list to initialize data members of base classes.

Use instead the usual list initialization like

Employee e1{ "John", "Wick", 40, 50000 };

or

Employee e1{ { "John", "Wick", 40 }, 50000 };

or as @Jarod42 pointed in a comment you can write

Employee e1{ { .name{"John"}, .surname{"Wick"}, .age{40} }, 50000 };

In this case the direct base class is initialized by a designated initializer list while the class Employe in whole is initialised by a non-designated initializer list.