20 General utilities library [utilities]

20.10 Memory [memory]

20.10.6 Pointer alignment [ptr.align]

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void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
1
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Preconditions:
  • (1.1)
    alignment is a power of two
  • (1.2)
    ptr represents the address of contiguous storage of at least space bytes
2
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Effects: If it is possible to fit size bytes of storage aligned by alignment into the buffer pointed to by ptr with length space, the function updates ptr to represent the first possible address of such storage and decreases space by the number of bytes used for alignment.
Otherwise, the function does nothing.
3
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Returns: A null pointer if the requested aligned buffer would not fit into the available space, otherwise the adjusted value of ptr.
4
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[Note 1:
The function updates its ptr and space arguments so that it can be called repeatedly with possibly different alignment and size arguments for the same buffer.
— end note]
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template<size_t N, class T> [[nodiscard]] constexpr T* assume_aligned(T* ptr);
5
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Mandates: N is a power of two.
6
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Preconditions: ptr points to an object X of a type similar ([conv.qual]) to T, where X has alignment N ([basic.align]).
7
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Returns: ptr.
8
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Throws: Nothing.
9
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[Note 2:
The alignment assumption on an object X expressed by a call to assume_­aligned might result in generation of more efficient code.
It is up to the program to ensure that the assumption actually holds.
The call does not cause the compiler to verify or enforce this.
An implementation might only make the assumption for those operations on X that access X through the pointer returned by assume_­aligned.
— end note]