C++ constexpr experiments

CMakeLists.txt

cmake_minimum_required(VERSION 3.15)
project(constexpr LANGUAGES CXX)
add_executable(${PROJECT_NAME})
target_sources(${PROJECT_NAME} PRIVATE main.cpp)
target_compile_features(${PROJECT_NAME} PRIVATE cxx_std_17)

main.cpp

#include <cstddef>
#include <iostream>
#include <memory>
#include <type_traits>

void print_buffer(const std::byte (&buffer)[12])
{
    for (int64_t i = 0; i < std::size(buffer); i++) {
        printf("%02hhX", buffer[i]);
    }

    printf("%c", '\n');
}

// <1> - constructor not constexpr, without '{}' when defined
// <2> - constructor not constexpr, with '{}' when defined
// <3> - constructor constexpr, defaulted, without '{}' when defined
// <4> - constructor constexpr, init list, user provided,
//       without '{}' when defined
// <5> - constructor constexpr, user provided, without '{}' when defined
// <6> - constructor constexpr, user provided, member initializers, without '{}'
//       when defined
// <7> - constructor constexpr, defaulted, member initializers, without '{}'
//       when defined

struct vec3
{
    vec3() noexcept = default; // <1,2>
    // constexpr vec3() noexcept = default; // <3, 7>
    // constexpr vec3() : x(0), y(0), z(0) {} // <4>
    // constexpr vec3() noexcept {} // <5,6>

    float x, y, z; // <1,2,3,4,5>
    // float x = 0, y = 0, z = 0; // <6,7>
};

int main(int argc, char** argv)
{
    alignas(std::alignment_of<vec3>::value)
        std::byte vec3_buffer[sizeof(vec3)] = {
            std::byte{0xc0}, std::byte{0xff}, std::byte{0xee}, std::byte{0xc0},
            std::byte{0xff}, std::byte{0xee}, std::byte{0xc0}, std::byte{0xff},
            std::byte{0xee}, std::byte{0xc0}, std::byte{0xff}, std::byte{0xee}};

    // before
    print_buffer(vec3_buffer);

    // equivalent to 'vec3 v;'
    auto v = new (vec3_buffer) vec3; // <1,3,4,5,6,7>
    // equivalent to 'vec3 v{};'
    // auto v = new (vec3_buffer) vec3{}; // <2>

    // after
    print_buffer(vec3_buffer);

    return 0;
}