How Can You Safely Type Punning in Modern C ?

The Modern, Correct Approach to Type Punning in C

The traditional approach to type punning, as exemplified by the fast inverse square root function, involved reinterpreting the bit pattern of one type as another using low-level casts. However, this approach is fraught with pitfalls such as:

• Unspecified behavior: Undefined results can occur depending on hardware and compiler.
• Strict aliasing violations: Casting between incompatible types can lead to errors.
• Lifetime issues: Punned objects may be destroyed earlier than intended.
• Endianness and alignment problems: Assumptions about byte ordering and data alignment can fail.

Modern Mechanisms for Type Punning

In modern C , there are several safer and more reliable mechanisms for type punning:

1. std::bit_cast(x) (C 20)

std::bit_cast copies the bit pattern of x into a new object of type T. It is the recommended method for type punning because it ensures:

• Bit-level preservation: The bit pattern is preserved exactly.
• Correct alignment and endianness: The resulting object respects the alignment and endianness requirements of T.
• Runtime safety: Throws an exception if the conversion is not possible.

2. std::memcpy(&y, &x, x.size())

Using std::memcpy to copy bytes between memory locations is another safe option. It is suitable when:

• The size of the source and destination types match.
• The memory layout is not platform-dependent.
• The lifetime of the source object is managed.

3. Placement new with std::launder (C 17)

This technique can be used to create a new object of type T using the memory of an existing object x:

new (&x) T;
return *std::launder(reinterpret_cast<T*>(&x));

It is similar to std::bit_cast but allows for modifying the memory content before casting.

4. std::byte and reinterpret_cast

std::byte represents a single byte, which can be used to reinterpret the bit pattern of other types:

return *reinterpret_cast<T*>(reinterpret_cast<std::byte*>(&x));

This method is similar to the original reinterpret_cast, but it allows for explicit control over byte ordering and alignment.

Rewriting the Fast Inverse Square Root Function

Using std::bit_cast, the fast inverse square root function can be rewritten as follows:

float fast_inverse_square_root(float number)
{
    // Assuming sizeof(long) == sizeof(float) on target platform
    return std::bit_cast<float>(0x5f3759df - (
        std::bit_cast<long>(number) >> 1
    ));
}

This version is safe, performant, and adheres to modern C best practices.

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