std::align

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

Pairs and tuples

pair

tuple (C++11)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

Swap, forward and move

swap

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

declval (C++11)

 

Dynamic memory management

Smart pointers
unique_ptr (C++11)
shared_ptr (C++11)
weak_ptr (C++11)
auto_ptr (deprecated)
owner_less (C++11)
enable_shared_from_this (C++11)
bad_weak_ptr (C++11)
default_delete (C++11)
Allocators
allocator
allocator_traits (C++11)
allocator_arg_t (C++11)
allocator_arg (C++11)
uses_allocator (C++11)
scoped_allocator_adaptor (C++11)
Uninitialized storage
uninitialized_copy
uninitialized_copy_n (C++11)
uninitialized_fill
uninitialized_fill_n
raw_storage_iterator
get_temporary_buffer
return_temporary_buffer
Garbage collection support
declare_reachable (C++11)
undeclare_reachable (C++11)
declare_no_pointers (C++11)
undeclare_no_pointers (C++11)
pointer_safety (C++11)
get_pointer_safety (C++11)
Miscellaneous
pointer_traits (C++11)
addressof (C++11)
align (C++11)
C Library
malloc calloc	realloc free
Low level memory management

 

Defined in header <memory>
void* align( std::size_t alignment,              std::size_t size,              void*& ptr,              std::size_t& space );		(since C++11)

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 modifies ptr to point to the first possible address of such aligned storage and decreases space by the number of bytes used for alignment. If it is impossible (the buffer is too small), align does nothing.

Parameters

alignment	-	the desired alignment
size	-	the size of the storage to be aligned
ptr	-	pointer to contiguous storage 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;
 
    // allocate a char
    char* p1 = a.aligned_alloc<char>();
    if (p1)
        *p1 = 'a';
    std::cout << "allocated a char at " << (void*)p1 << '\n';
 
    // allocate an int
    int* p2 = a.aligned_alloc<int>();
    if (p2)
        *p2 = 1;
    std::cout << "allocated an int at " << (void*)p2 << '\n';
 
    // allocate an int, aligned at 32-byte boundary
    int* p3 = a.aligned_alloc<int>(32);
    if (p3)
        *p3 = 2;
    std::cout << "allocated an int at " << (void*)p3 << " (32 byte alignment)\n";
}

Possible output:

allocated a char at 0x2ff21a08
allocated an int at 0x2ff21a0c
allocated an int at 0x2ff21a20 (32 byte alignment)

See also

alignof operator	queries alignment requirements of a type (since C++11)
alignas specifier	specifies that the storage for the variable should be aligned by specific amount (C++11)
aligned_storage (C++11)	defines the type suitable for use as uninitialized storage for types of given size (class template)