NAME¶

std::align - std::align

Synopsis¶

Defined in header <memory>
void* align( std::size_t alignment,

std::size_t size, (since C++11)
void*& ptr,

std::size_t& space );

Given a pointer ptr to a buffer of size space, returns a pointer aligned by the
specified alignment for size number of bytes and decreases space argument by the
number of bytes used for alignment. The first aligned address is returned.

The function modifies the pointer only if it would be possible to fit the wanted
number of bytes aligned by the given alignment into the buffer. If the buffer is too
small, the function does nothing and returns nullptr.

The behavior is undefined if alignment is not a power of two.

Parameters¶

alignment - the desired alignment
size - the size of the storage to be aligned
ptr - pointer to contiguous storage (a buffer) of at least space bytes
space - the size of the buffer in which to operate

Return value¶

The adjusted value of ptr, or null pointer value if the space provided is too small.

Example¶

Demonstrates the use of std::align to place objects of different type in memory.

// Run this code

#include <iostream>
#include <memory>

template<std::size_t N>
struct MyAllocator
{
char data[N];
void* p;
std::size_t sz;
MyAllocator() : p(data), sz(N) {}
template<typename T>
T* aligned_alloc(std::size_t a = alignof(T))
{
if (std::align(a, sizeof(T), p, sz))
{
T* result = reinterpret_cast<T*>(p);
p = (char*)p + sizeof(T);
sz -= sizeof(T);
return result;
}
return nullptr;
}
};

int main()
{
MyAllocator<64> a;
std::cout << "allocated a.data at " << (void*)a.data
<< " (" << sizeof a.data << " bytes)\n";

// allocate a char
if (char* p = a.aligned_alloc<char>())
{
*p = 'a';
std::cout << "allocated a char at " << (void*)p << '\n';
}

// allocate an int
if (int* p = a.aligned_alloc<int>())
{
*p = 1;
std::cout << "allocated an int at " << (void*)p << '\n';
}

// allocate an int, aligned at 32-byte boundary
if (int* p = a.aligned_alloc<int>(32))
{
*p = 2;
std::cout << "allocated an int at " << (void*)p << " (32 byte alignment)\n";
}
}

Possible output:¶

allocated a.data at 0x7ffd0b331f80 (64 bytes)
allocated a char at 0x7ffd0b331f80
allocated an int at 0x7ffd0b331f84
allocated an int at 0x7ffd0b331fa0 (32 byte alignment)

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2377 C++11 alignment required to be a fundamental only need to be a power
or supported extended alignment value of two

See also¶

alignof operator(C++11) queries alignment requirements of a type
alignas specifier(C++11) specifies that the storage for the variable should be
aligned by specific amount
aligned_storage defines the type suitable for use as uninitialized
(C++11)(deprecated in C++23) storage for types of given size
(class template)
assume_aligned informs the compiler that a pointer is aligned
(C++20) (function template)