CODEWITHSUNDEEP
c++aggregatec++20designated-initializer
MateuszGierczak
MateuszGierczak

Reputation: 426

Designated initializers in C++20

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> << '\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}}; 
}

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 ?

Upvotes: 42

Views: 30601

Answers (3)

SunlayGGX
SunlayGGX

Reputation: 121

An old question, but since all newer were closed duplicating this one, I'll answer here : All previous answers are quite good suggesting a mix, but kind of incomplete.

By adding multiple inheritance to the mix, you can achieve exactly what you are looking for with C++20:

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

struct Employee_Data_POD
{
    unsigned salary{DEFAULT_SALARY};
};

struct Employee : Person, Employee_Data_POD {};

Now, you can just declare your Employee like that :

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

Upvotes: 0

Vlad from Moscow
Vlad from Moscow

Reputation: 310920

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.

Upvotes: 35

Jarod42
Jarod42

Reputation: 217085

You might have several fields with same name from different bases,

so logically, you should provide name of the wanted base, but it seems there is no way to do it.

// Invalid too:
Employee e1{.Person.name{"John"}, .Person.surname{"Wick"}, .Person.age{40}, .salary{50000}};
Employee e2{.Person{.name{"John"}, .surname{"Wick"}, .age{40}}, .salary{50000}};

In addition C++ designated initialization is more constrained than C:

Note: out-of-order designated initialization, nested designated initialization, mixing of designated initializers and regular initializers, and designated initialization of arrays are all supported in the C programming language, but are not allowed in C++.

Upvotes: 13

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