Can Designated Initializers be Used to Initialize Base Class Members in C 20?

Designated Initializers in C 20: Limitations Explained

Designated initializers introduced in C 20 allow you to explicitly specify the order of member initialization for aggregates. However, a recent question has arisen regarding their use for derived classes.

In the code example provided:

<code class="cpp">struct Person
{
    std::string name{};
    std::string surname{};
    unsigned age{};
};

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

int main()
{
    Employee e1{.name{"John"}, .surname{"Wick"}, .age{40}, .salary{50000}}; // doesn't compile
    Employee e2 {.salary{55000}}; // warning: missing initializer for base class member
}</code>

The code fails to compile e1 because designated initializers can only be used to initialize non-static members of the derived class directly. In other words, you cannot initialize members of a base class directly with designated initializers.

To resolve this, you can use the following approaches:

• Traditional list initialization:

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

• Nested designated initializer list:

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

Alternatively, as suggested by @Jarod42, you can also specify the initializer list for the base class explicitly:

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

This allows you to initialize both the direct base class members and the derived class members using designated initializers.

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