List Initialization with constexpr and constinit

List initialization works well with constexpr and constinit specifiers, enhancing compile-time evaluation and constant initialization in C++20.

These specifiers help ensure that variables are initialized at compile-time, leading to safer and more efficient code.

constexpr

The constexpr specifier indicates that the value of a variable or function can be evaluated at compile-time. You can use list initialization with constexpr to define compile-time constants:

1#include <iostream>
2
3struct Vec3 {
4  int x, y, z;
5};
6
7constexpr Vec3 CreateVector(int a, int b, int c) {
8  return {a, b, c};  
9}
10
11int main() {
12  constexpr Vec3 vector = CreateVector(1, 2, 3);  
13  std::cout << "x=" << vector.x
14    << ", y=" << vector.y
15    << ", z=" << vector.z;
16}

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

In this example, the CreateVector function is constexpr, meaning it can be evaluated at compile-time. The vector variable is also constexpr, ensuring its value is determined during compilation.

constinit

The constinit specifier, introduced in C++20, guarantees that a variable is initialized at compile-time. Unlike constexpr, constinit can be used with non-constant expressions, as long as the initialization happens at compile-time:

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

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

Here, constinit ensures that vector is initialized at compile-time. If the initialization expression were not a constant expression, the code would not compile.

Combining constexpr and List Initialization

You can combine constexpr with list initialization to create complex compile-time constants:

1#include <iostream>
2
3struct Vec3 {
4  int x, y, z;
5};
6
7struct Transform {
8  Vec3 position;
9  Vec3 rotation;
10  Vec3 scale;
11};
12
13constexpr Transform CreateTransform() {
14  return {{0, 0, 0}, {0, 0, 0}, {1, 1, 1}};  
15}
16
17int main() {
18  constexpr Transform transform = CreateTransform();
19  std::cout
20    << "Position: x=" << transform.position.x
21    << ", y=" << transform.position.y
22    << ", z=" << transform.position.z;
23}

1Position: x=0, y=0, z=0

This example demonstrates a Transform structure initialized using list initialization within a constexpr function. The resulting transform variable is a compile-time constant.

In summary, list initialization interacts seamlessly with constexpr and constinit, enabling efficient and safe compile-time initialization of variables and constants.

Using these specifiers, you can ensure that your variables are initialized correctly and efficiently, leveraging the full power of compile-time evaluation in C++20.