Designated Initializers with Private Members

Designated initializers, introduced in C++20, are a convenient way to initialize objects by specifying the member names. However, they can only be used with aggregate types, which do not include private or protected members.

Let's consider an example with an aggregate type:

1#include <iostream>
2
3struct Vec3 {
4  int x{0};
5  int y{0};
6  int z{0};
7};
8
9int main() {
10  Vec3 vector{.x = 1, .y = 2, .z = 3};  
11  std::cout << "x=" << vector.x
12    << ", y=" << vector.y
13    << ", z=" << vector.z;
14}

1x=1, y=2, z=3

In this case, Vec3 is an aggregate type because it has no private members, non-public base classes, or user-defined constructors. This allows us to use designated initializers to initialize its members.

However, if Vec3 had private or protected members, it would no longer be considered an aggregate, and designated initializers would not work:

1#include <iostream>
2
3struct Vec3 {
4private:
5  int x{0};
6  int y{0};
7  int z{0};
8
9public:
10  Vec3(int a, int b, int c)
11    : x{a}, y{b}, z{c} {}
12
13  void Log() {
14    std::cout << "x=" << x
15      << ", y=" << y
16      << ", z=" << z << '\n';
17  }
18};
19
20int main() {
21  Vec3 vector{.x = 1, .y = 2, .z = 3}; 
22  vector.Log();
23}

1error: 'Vec3': designated initialization can only be used to initialize aggregate class types

This code will not compile because Vec3 is no longer an aggregate type due to the private members. Designated initializers cannot be used in this scenario.

To initialize types with private or protected members, you must use constructors. Constructors allow you to control how the members are initialized and can enforce encapsulation rules.

Here is how you can modify the Vec3 struct to use a constructor instead:

1#include <iostream>
2
3struct Vec3 {
4 private:
5  int x{0};
6  int y{0};
7  int z{0};
8
9 public:
10  Vec3(int a, int b, int c)
11    : x{a}, y{b}, z{c} {}
12
13  void Log() {
14    std::cout << "x=" << x
15      << ", y=" << y
16      << ", z=" << z;
17  }
18};
19
20int main() {
21  Vec3 vector{1, 2, 3};  
22  vector.Log();
23}

1x=1, y=2, z=3

In summary, designated initializers are great for aggregate types but cannot be used with types that have private or protected members. For such types, constructors are the appropriate way to handle initialization.