DeveloperNote.com

#include <cstddef>
#include <cstdint>
#include <iostream>
#include <new>

struct Packet final
{
    std::uint64_t Data[4];
};

struct A final
{
    std::uint32_t First;
    std::uint32_t Second;
};

struct B final
{
    std::uint32_t First;
    std::uint32_t Second;
};

struct C final
{
    std::uint32_t Value;
};

#include <cstdint>

std::uint32_t toInt(float value)
{
    auto* p = reinterpret_cast<std::uint32_t*>(&value); // The cast itself is permitted.
    return *p; // Undefined behavior: accesses a float object through a std::uint32_t glvalue.
}

#include <bit>
#include <cstdint>
#include <iomanip>
#include <iostream>

std::uint32_t badBits(const float value)
{
  // const prevents writes, but reading a float through a uint32_t pointer
  // still violates the strict-aliasing rule.
  const auto* bits = reinterpret_cast<const std::uint32_t*>(&value);
  return *bits; // Undefined behavior.
}

std::uint32_t goodBits(const float value)
{
  // std::bit_cast copies the object representation without violating
  // strict-aliasing or object-lifetime rules.
  return std::bit_cast<std::uint32_t>(value);
}

int main()
{
  constexpr float value = 1.0F;

  std::cout << std::hex
      << "bad:  0x" << badBits(value) << '\n'
      << "good: 0x" << goodBits(value) << '\n';
}

template <class Result, class... Params>
class Slot
{
public:

    virtual Result operator()(Params ... args) = 0;
};