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Defined in header <memory> | ||
void* align(std::size_t alignment, std::size_t size, | (since C++11) | |
Given a pointerptr to a buffer of sizespace, returns a pointer aligned by the specifiedalignment forsize number of bytes and decreasesspace 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 returnsnullptr.
The behavior is undefined ifalignment is not a power of two.
Contents |
| alignment | - | the desired alignment |
| size | - | the size of the storage to be aligned |
| ptr | - | pointer to contiguous storage (a buffer) of at leastspace bytes |
| space | - | the size of the buffer in which to operate |
The adjusted value ofptr, or null pointer value if the space provided is too small.
Demonstrates the use ofstd::align to place objects of different type in memory.
#include <iostream>#include <memory>#include <new> template<std::size_t N>struct MyAllocator{std::byte data[N];std::size_t sz{N};void* p{data}; MyAllocator()=default; // Note: only well-defined for implicit-lifetime typestemplate<typename T> T* implicit_aligned_alloc(std::size_t a= alignof(T)){if(std::align(a, sizeof(T), p, sz)){ T* result=std::launder(reinterpret_cast<T*>(p)); p=static_cast<std::byte*>(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 charif(char* p= a.implicit_aligned_alloc<char>()){*p='a';std::cout<<"allocated a char at "<<(void*)p<<'\n';} // Allocate an intif(int* p= a.implicit_aligned_alloc<int>()){*p=1;std::cout<<"allocated an int at "<<(void*)p<<'\n';} // Allocate an int, aligned at a 32-byte boundaryif(int* p= a.implicit_aligned_alloc<int>(32)){*p=2;std::cout<<"allocated an int at "<<(void*)p<<" (32-byte alignment)\n";}}
Possible output:
allocated a.data at 0x7ffc654e8530 (64 bytes)allocated a char at 0x7ffc654e8530allocated an int at 0x7ffc654e8534allocated an int at 0x7ffc654e8540 (32-byte alignment)
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 or supported extended alignment value | only need to be a power of two |
alignof(C++11) | queries alignment requirements of a type (operator)[edit] |
alignas(C++11) | specifies that the storage for the variable should be aligned by specific amount (specifier)[edit] |
(since C++11)(deprecated in C++23) | defines the type suitable for use as uninitialized storage for types of given size (class template)[edit] |
(C++20) | informs the compiler that a pointer is aligned (function template)[edit] |
