Designated Initializers and Inheritance: Why Can\'t I Initialize an Employee with a Designated Initializer?

Designated Initializers in C 20: An Enigma with Employee Initialization

C 20's designated initializers allow precise member initialization for classes considered aggregates. However, the question arises when a class inherits from an aggregate base class, as exemplified by the code provided:

<code class="cpp">#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> << '\n'; // true is printed
    std::cout << std::boolalpha << std::is_aggregate_v<Employee> << '\n'; // 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}};
}</code>

In this case, both Person and Employee are considered aggregates, yet Employee cannot be initialized with designated initializers. The explanation lies in the C 20 Standard's regulation regarding designated initializers:

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

This means that designated initializers can only initialize data members directly declared in the class, not members inherited from a base class.

Therefore, initializing Employee using designated initializers as e1 is incorrect. Instead, the usual list initialization must be used:

<code class="cpp">Employee e1{ "John", "Wick", 40, 50000 };</code>

Alternatively, since Employee directly inherits from Person, one can use a nested designated initializer for the base class:

<code class="cpp">Employee e1{ { .name{"John"}, .surname{"Wick"}, .age{40} }, 50000 };</code>

This approach correctly initializes the Person base class with designated initializers while employing a regular initializer for the Employee class.

The above is the detailed content of Designated Initializers and Inheritance: Why Can\'t I Initialize an Employee with a Designated Initializer?. For more information, please follow other related articles on the PHP Chinese website!