<xstd/ints/memory/align_down.hpp>

xstd::align_down

// (1)
template<alignable T>
  constexpr auto align_down(T value, size_t alignment) noexcept(nothrow_alignable<T>) -> T;

// (2)
template<class T>
  auto align_down(T* pointer, size_t alignment) noexcept -> T*;

Rounds down to a multiple of a power-of-two alignment.

1. For an alignable value.
2. For a pointer, through its address.

Parameters

Parameter	Meaning
value	the value to round down
pointer	the pointer whose address is rounded down
alignment	a power of two

Constraints: (1) T models alignable.

Preconditions: (1) alignment is a power of two.

Return value

1. Returns: The greatest multiple of alignment not greater than value, as a T, computed as value - (value & m) for m = T(alignment - 1). A narrow type is returned as itself. It is a constant expression.
2. Effects: Equivalent to return reinterpret_cast<T*>(align_down(reinterpret_cast<uintptr_t>(pointer), alignment));.

Exceptions

1. The exception specification is nothrow_alignable<T>: noexcept for size_t and uint8_t, and not for absl::uint128.
2. As for align_up's pointer form.

Remarks

(2) As for align_up's pointer form.

Notes

• (1) It is exact at the maximum of size_t.
• (2) It returns a T*.

See also

Entity	Relation
align_up	rounds up instead
is_aligned	holds of what it returns, and of value exactly when it leaves it unchanged
alignable	what (1) asks of T
boost::alignment::align_down	the Boost.Align function it agrees with