[/ (c) 2014-2015 Glen Joseph Fernandes Distributed under the Boost Software License, Version 1.0. http://boost.org/LICENSE_1_0.txt ] [library Boost.Align [quickbook 1.6] [id align] [copyright 2014-2016 Glen Joseph Fernandes] [authors [Fernandes, Glen]] [dirname align] [license Distributed under the Boost Software License, Version 1.0.] ] [section Introduction] This library provides an alignment function, aligned allocation and deallocation functions, an aligned allocator, an aligned allocator adaptor, an aligned deleter, a type trait to query alignment requirements, a macro to hint pointer alignment, and a function to verify pointer value alignment. [table The Boost.Align Library [ [Component] [Description] ] [ [`align`] [Pointer alignment function] ] [ [`align_up`, `align_down`] [Pointer and integral alignment functions] ] [ [`aligned_alloc`, `aligned_free`] [Aligned allocation and deallocation functions] ] [ [`aligned_allocator`] [Alignment aware allocator] ] [ [`aligned_allocator_adaptor`] [Alignment aware allocator adaptor] ] [ [`aligned_delete`] [Deleter for deallocation of aligned allocations] ] [ [`alignment_of`] [Trait to query alignment requirement of a type] ] [ [`assume_aligned`] [Macro for static pointer alignment hint] ] [ [`is_aligned`] [Pointer and integral alignment checking] ] ] [endsect] [section Rationale] [heading Dynamic allocation] C++11 added the ability to specify increased alignment (over-alignment) for class types. Unfortunately, `::operator new` allocation functions, `new` expressions, and the default allocator, `std::allocator`, do not support dynamic memory allocation of over-aligned data. This library provides allocation functions, allocators, allocator adaptors, and deleters, that are alignment aware. [table Boost.Align solutions [ [Problem] [Solution] ] [ [`::operator new(std::size_t, const std::no_throw_t&)`] [`aligned_alloc(std::size_t, std::size_t)`] ] [ [`::operator delete(void*)`] [`aligned_free(void*)`] ] [ [`std::allocator`] [`aligned_allocator`] ] [ [`Allocator`] [`aligned_allocator_adaptor`] ] [ [`std::default_delete`] [`aligned_delete`] ] ] [heading Alignment functions] C++11 provided `std::align` in the standard library to align a pointer value. Unfortunately some C++ standard library implementations do not support it yet (libstdc++ as far as gcc 4.8.0) and other standard library implementations implement it incorrectly (dinkumware in msvc 11.0). This library provides it for those implementations and also for C++03 compilers where it is equally useful. [heading Alignment traits] C++11 provided the `std::alignment_of` trait in the standard library to query the alignment requirement of a type. Unfortunately some C++ standard library vendors do not implement it in an entirely standard conforming manner, such as for array types (libc++ as far as clang 3.4). Other vendor implementations report incorrect values for certain types, such as pointer to members (msvc 14.0). This library provides it for those implementations and also for C++03 compilers where it is equally useful. [heading Alignment hints] Allocating aligned memory is sometimes not enough to ensure that optimal code is generated. Developers use specific compiler intrinsics to notify the compiler of a given alignment property of a memory block. This library provides a macro to abstract that functionality for compilers with the appropriate intrinsics. [heading Alignment testing] This library provides a function to test the alignment of a pointer value. It is generally useful in assertions to validate that memory is correctly aligned. [endsect] [section Tutorial] [section align] The alignment function can be used to find the first address of a given alignment value within a given buffer of a given size. It adjusts the pointer provided, returns that value, and decreases space by the amount advanced, if the alignment succeeds, provided sufficient space in the buffer. Otherwise it yields a null pointer to indicate failure due to insufficient space. `` #include #include #include struct alignas(16) type { float data[4]; }; void use(void* ptr, std::size_t size) { auto p = boost::alignment::align(16, sizeof(type), ptr, size); if (p) { auto q = ::new(p) type; q->~type(); } } int main() { char c[64]; use(c, sizeof c); } `` [endsect] [section:aligned_alloc aligned_alloc and aligned_free] Consider these functions alignment enabled versions of `std::malloc`, `std::free` or `::operator new(std::size_t, const std::no_throw_t&)`, `::operator delete(void*)`. The aligned allocation function allocates space with the specified size and alignment. The aligned deallocation function can then deallocate this space. `` #include int main() { void* p = boost::alignment::aligned_alloc(16, 100); if (p) { boost::alignment::aligned_free(p); } } `` [endsect] [section aligned_allocator] Consider this class template a superior version of the default allocator, `std::allocator`, because it can be used with types that are over-aligned. `` #include #include struct alignas(16) type { float data[4]; }; int main() { std::vector > v; v.emplace_back(); } `` The optional template parameter of this class allows specifying a minimum alignment to use for allocations. The default minimum alignment is 1. `` #include #include int main() { std::vector > v; v.emplace_back(); } `` [endsect] [section aligned_allocator_adaptor] This class template can turn any existing allocator type, C++11 or C++03, stateful or stateless, into one that supports types which are over-aligned. `` #include #include template class utility { public: void add() { v.emplace_back(); } private: std::vector > v; }; struct alignas(16) type { float data[4]; }; int main() { utility > u; u.add(); } `` The optional template parameter of this class allows specifying a minimum alignment to use for allocations. The default minimum alignment is 1. `` #include #include template class utility { public: void add() { v.v.emplace_back(); } private: std::vector > v; }; int main() { utility, 16> u; u.add(); } `` [endsect] [section aligned_delete] Consider this class an alignment aware version of the class template `std::default_delete`. It is a deleter that destroys the object and then deallocates space using our aligned deallocation function. It should be used with constructed objects that were allocated with our aligned allocation function and is useful with deleter enabled types like `std::unique_ptr`. `` #include #include struct alignas(16) type { float data[4]; }; int main() { void* p = boost::alignment::aligned_alloc(16, sizeof(type)); if (p) { type* q = ::new(p) type; std::unique_ptr(q); } } `` [endsect] [section alignment_of] This type trait can be used to query the alignment requirement of a type at compile time. `` #include #include template class utility { public: void construct() { void* p = &v; ::new(p) T(); } private: std::aligned_storage_t::value> v; }; struct alignas(16) type { float data[4]; }; int main() { utility u; u.construct(); } `` [endsect] [section assume_aligned] This macro is used to notify the compiler of a given pointer variable's alignment. It is useful for guide optimizing compilers into vectorizing or applying other compiler specific, alignment related optimizations. `` #include void use(double* array, std::size_t size) { BOOST_ALIGN_ASSUME_ALIGNED(array, 16); for (std::size_t i = 0; i < size; i++) { array[i]++; } } int main() { alignas(16) double d[4] { }; use(d, 4); } `` [endsect] [section is_aligned] This function is used to compare the alignment of a pointer. It is useful in assertions that validate a pointer value is aligned on a given boundary. `` #include #include void use(void* ptr) { assert(boost::alignment::is_aligned(ptr, 16)); } int main() { alignas(16) char c[64]; use(c); } `` [endsect] [endsect] [section Examples] [section:aligned_ptr aligned_ptr and make_aligned] This example presents an alternative to `std::unique_ptr` for objects allocated with the aligned allocation function. It is defined simply by providing an alias template which uses `std::unique_ptr` with our aligned deleter in place of the default `std::default_delete` deleter. It also presents an alternative to `std::make_unique` for the creation of these aligned unique pointer objects. It is implemented using our aligned allocation function. [heading Implementation] `` #include #include #include #include template using aligned_ptr = std::unique_ptr; template inline aligned_ptr make_aligned(Args&&... args) { auto p = boost::alignment::aligned_alloc(boost:: alignment::alignment_of::value, sizeof(T)); if (!p) { throw std::bad_alloc(); } try { auto q = ::new(p) T(std::forward(args)...); return aligned_ptr(q); } catch (...) { boost::alignment::aligned_free(p); throw; } } `` [heading Usage] `` struct alignas(16) type { float data[4]; }; int main() { auto p = make_aligned(); p.reset(); } `` [endsect] [section aligned_vector] This example presents an alternative to `std::vector` that can be used with over-aligned types, and allows specifying a minimum alignment. It is defined simply by providing an alias template which uses `std::vector` with our aligned allocator. [heading Implementation] `` #include #include template using aligned_vector = std::vector >; `` [heading Usage] `` enum : std::size_t { cache_line = 64 }; int main() { aligned_vector v; v.emplace_back(); } `` [endsect] [endsect] [section Reference] [section align] The alignment function is used to obtain a pointer to the first address within the specified buffer that is a multiple of the specified alignment value. This function exists in the C++11 standard library but is provided in this library for those C++11 and C++03 library implementations which do not yet support it. [heading:synopsis Header ] `` namespace boost { namespace alignment { void* align(std::size_t alignment, std::size_t size, void*& ptr, std::size_t& space); } } `` [heading:align Function align] `` void* align(std::size_t alignment, std::size_t size, void*& ptr, std::size_t& space); `` [*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 point to the first possible address of such storage and decreases `space` by the number of bytes used for alignment. Otherwise, the function does nothing. [*Requires:] [itemized_list [`alignment` shall be a fundamental alignment value or an extended alignment value, and shall be a power of two] [`ptr` shall point to contiguous storage of at least `space` bytes] ] [*Returns:] A null pointer if the requested aligned buffer would not fit into the available space, otherwise the adjusted value of `ptr`. [*Note:] 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. [endsect] [section:align_down align_down] The directional alignment functions can be used with pointers or integral values to align down. This functionality is not yet provided by the C++ standard. [heading:synopsis Header ] `` namespace boost { namespace alignment { constexpr std::size_t align_down(std::size_t value, std::size_t alignment) noexcept; void* align_down(void* ptr, std::size_t alignment) noexcept; } } `` [heading:align_down Function align_down] `` void* align_down(void* ptr, std::size_t alignment) noexcept; `` [*Requires:] [itemized_list [`ptr` shall point to contiguous storage with sufficient space for alignment] [`alignment` shall be a power of two] ] [*Returns:] A pointer value at or before `ptr` that is at least `alignment` bytes aligned. [endsect] [section:align_up align_up] The directional alignment functions can be used with pointers or integral values to align up. This functionality is not yet provided by the C++ standard. [heading:synopsis Header ] `` namespace boost { namespace alignment { constexpr std::size_t align_up(std::size_t value, std::size_t alignment) noexcept; void* align_up(void* ptr, std::size_t alignment) noexcept; } } `` [heading:align_up Function align_up] `` void* align_up(void* ptr, std::size_t alignment) noexcept; `` [*Requires:] [itemized_list [`ptr` shall point to contiguous storage with sufficient space for alignment] [`alignment` shall be a power of two] ] [*Returns:] A pointer value at or after `ptr` that is at least `alignment` bytes aligned. [endsect] [section:aligned_alloc aligned_alloc and aligned_free] The aligned allocation function is a replacement for `::operator new(std::size_t, const std::no_throw_t&)` that allows requesting aligned memory. The deallocation function replaces the corresponding `::operator delete(void*)` function. This functionality is not yet provided by the C++ standard. [heading:synopsis Header ] `` namespace boost { namespace alignment { void* aligned_alloc(std::size_t alignment, std::size_t size); void aligned_free(void* ptr); } } `` [heading:aligned_alloc Function aligned_alloc] `` void* aligned_alloc(std::size_t alignment, std::size_t size); `` [*Effects:] Allocates space for an object whose alignment is specified by `alignment`, whose size is specified by `size`, and whose value is indeterminate. The value of `alignment` shall be a power of two. [*Requires:] `alignment` shall be a power of two. [*Returns:] A null pointer or a pointer to the allocated space. [*Note:] On certain platforms, the alignment may be rounded up to `alignof(void*)` and the space allocated may be slightly larger than `size` bytes, by an additional `sizeof(void*)` and `alignment - 1` bytes. [heading:aligned_free Function aligned_free] `` void aligned_free(void* ptr); `` [*Effects:] Causes the space pointed to by `ptr` to be deallocated, that is, made available for further allocation. If `ptr` is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by the `aligned_alloc` function, or if the space has been deallocated by a call to `aligned_free`, the behavior is undefined. [*Requires:] `ptr` is a null pointer or a pointer earlier returned by the `aligned_alloc` function that has not been deallocated by a call to `aligned_free`. [*Returns:] The `aligned_free` function returns no value. [endsect] [section aligned_allocator] The aligned allocator is a replacement for the default allocator, `std::allocator`, that supports value types which are over-aligned. It also allows specifying a minimum alignment value used for all allocations, via the optional template parameter. An alignment aware allocator is not yet provided by the C++ standard. [tip Using the aligned allocator with a minimum alignment value is generally only suitable with containers that are not node-based such as `std::vector`. With node-based containers, such as list, the node object would have the minimum alignment instead of the value type object.] [heading:synopsis Header ] `` namespace boost { namespace alignment { template class aligned_allocator; template class aligned_allocator; template bool operator==(const aligned_allocator&, const aligned_allocator&) noexcept; template bool operator!=(const aligned_allocator&, const aligned_allocator&) noexcept; } } `` [heading:aligned_allocator Class template aligned_allocator] `` template class aligned_allocator { public: typedef T value_type; typedef T* pointer; typedef const T* const_pointer; typedef void* void_pointer; typedef const void* const_void_pointer; typedef std::size_t size_type; typedef std::ptrdiff_t difference_type; typedef T& reference; typedef const T& const_reference; template struct rebind { typedef aligned_allocator other; }; aligned_allocator() = default; template aligned_allocator(const aligned_allocator&) noexcept; pointer address(reference value) const noexcept; const_pointer address(const_reference value) const noexcept; pointer allocate(size_type size, const_void_pointer = 0); void deallocate(pointer ptr, size_type); size_type max_size() const noexcept; template void construct(U* ptr, Args&&... args); template void destroy(U* ptr); }; template class aligned_allocator { public: typedef void value_type; typedef void* pointer; typedef const void* const_pointer; template struct rebind { typedef aligned_allocator other; }; }; `` [heading Members] Except for the destructor, member functions of the aligned allocator shall not introduce data races as a result of concurrent calls to those member functions from different threads. Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order. `` pointer address(reference value) const noexcept; `` [*Returns:] The actual address of the object referenced by `value`, even in the presence of an overloaded `operator&`. `` const_pointer address(const_reference value) const noexcept; `` [*Returns:] The actual address of the object referenced by `value`, even in the presence of an overloaded `operator&`. `` pointer allocate(size_type size, const_void_pointer = 0); `` [*Returns:] A pointer to the initial element of an array of storage of size `n * sizeof(T)`, aligned on the maximum of the minimum alignment specified and the alignment of objects of type `T`. [*Remark:] The storage is obtained by calling `aligned_alloc(std::size_t, std::size_t)`. [*Throws:] `std::bad_alloc` if the storage cannot be obtained. `` void deallocate(pointer ptr, size_type); `` [*Requires:] `ptr` shall be a pointer value obtained from `allocate()`. [*Effects:] Deallocates the storage referenced by `ptr`. [*Remark:] Uses `alignment::aligned_free(void*)`. `` size_type max_size() const noexcept; `` [*Returns:] The largest value `N` for which the call `allocate(N)` might succeed. `` template void construct(U* ptr, Args&&... args); `` [*Effects:] `::new((void*)ptr) U(std::forward(args)...)` `` template void destroy(U* ptr); `` [*Effects:] `ptr->~U()` [heading Globals] `` template bool operator==(const aligned_allocator&, const aligned_allocator&) noexcept; `` [*Returns:] `true`. `` template bool operator!=(const aligned_allocator&, const aligned_allocator&) noexcept; `` [*Returns:] `false`. [endsect] [section aligned_allocator_adaptor] The aligned allocator adaptor can turn any existing C++03 or C++11 allocator into one that supports value types that are over-aligned. It also allows specifying a minimum alignment value used for all allocations, via the optional template parameter. An alignment aware allocator adaptor is not yet provided by the C++ standard. [tip This adaptor can be used with a C++11 allocator whose pointer type is a smart pointer but the adaptor can choose to expose only raw pointer types.] [heading:synopsis Header ] `` namespace boost { namespace alignment { template class aligned_allocator_adaptor; template bool operator==(const aligned_allocator_adaptor& a1, const aligned_allocator_adaptor& a2) noexcept; template bool operator!=(const aligned_allocator_adaptor& a1, const aligned_allocator_adaptor& a2) noexcept; } } `` [heading:aligned_allocator_adaptor Class template aligned_allocator_adaptor] `` template class aligned_allocator_adaptor : public Allocator { typedef std::allocator_traits Traits; // exposition public: typedef typename Traits::value_type value_type; typedef typename Traits::size_type size_type; typedef value_type* pointer; typedef const value_type* const_pointer; typedef void* void_pointer; typedef const void* const_void_pointer; typedef std::ptrdiff_t difference_type; template struct rebind { typedef aligned_allocator_adaptor, Alignment> other; }; aligned_allocator_adaptor() = default; template explicit aligned_allocator_adaptor(A&& alloc) noexcept; template aligned_allocator_adaptor(const aligned_allocator_adaptor& other) noexcept; Allocator& base() noexcept; const Allocator& base() const noexcept; pointer allocate(size_type size); pointer allocate(size_type size, const_void_pointer hint); void deallocate(pointer ptr, size_type size); }; `` [heading Constructors] `` aligned_allocator_adaptor() = default; `` [*Effects:] Value-initializes the `Allocator` base class. `` template explicit aligned_allocator_adaptor(A&& alloc) noexcept; `` [*Requires:] `Allocator` shall be constructible from `A`. [*Effects:] Initializes the `Allocator` base class with `std::forward(alloc)`. `` template aligned_allocator_adaptor(const aligned_allocator_adaptor& other) noexcept; `` [*Requires:] `Allocator` shall be constructible from `A`. [*Effects:] Initializes the `Allocator` base class with `other.base()`. [heading Members] `` Allocator& base() noexcept; `` [*Returns:] `static_cast(*this)` `` const Allocator& base() const noexcept; `` [*Returns:] `static_cast(*this)` `` pointer allocate(size_type size); `` [*Returns:] A pointer to the initial element of an array of storage of size `n * sizeof(value_type)`, aligned on the maximum of the minimum alignment specified and the alignment of objects of type `value_type`. [*Remark:] The storage is obtained by calling `A2::allocate` on an object `a2`, where `a2` of type `A2` is a rebound copy of `base()` where its `value_type` is unspecified. [*Throws:] Throws an exception thrown from `A2::allocate` if the storage cannot be obtained. `` pointer allocate(size_type size, const_void_pointer hint); `` [*Requires:] `hint` is a value obtained by calling `allocate()` on any equivalent aligned allocator adaptor object, or else `nullptr`. [*Returns:] A pointer to the initial element of an array of storage of size `n * sizeof(value_type)`, aligned on the maximum of the minimum alignment specified and the alignment of objects of type `value_type`. [*Remark:] The storage is obtained by calling `A2::allocate` on an object `a2`, where `a2` of type `A2` is a rebound copy of `base()` where its `value_type` is unspecified. [*Throws:] Throws an exception thrown from `A2::allocate` if the storage cannot be obtained. `` void deallocate(pointer ptr, size_type size); `` [*Requires:] [itemized_list [`ptr` shall be a pointer value obtained from `allocate()`] [`size` shall equal the value passed as the first argument to the invocation of `allocate()` which returned `ptr`] ] [*Effects:] Deallocates the storage referenced by `ptr`. [*Note:] Uses `A2::deallocate` on an object `a2`, where `a2` of type `A2` is a rebound copy of `base()` where its `value_type` is unspecified. [heading Globals] `` template bool operator==(const aligned_allocator_adaptor& a1, const aligned_allocator_adaptor& a2) noexcept; `` [*Returns:] `a1.base() == a2.base()` `` template bool operator!=(const aligned_allocator_adaptor& a1, const aligned_allocator_adaptor& a2) noexcept; `` [*Returns:] `!(a1 == a2)` [endsect] [section aligned_delete] The aligned deleter class is convenient utility for destroying and then deallocating the constructed objects that were allocated using aligned allocation function provided in this library. It serves as a replacement for the `std::default_delete` class for this case. [heading:synopsis Header ] `` namespace boost { namespace alignment { class aligned_delete; } } `` [heading:aligned_delete Class aligned_delete] `` class aligned_delete { public: template void operator()(T* ptr) const noexcept(noexcept(ptr->~T())); }; `` [heading Members] `` template void operator()(T* ptr) const noexcept(noexcept(ptr->~T())); `` [*Effects:] Calls `~T()` on `ptr` to destroy the object and then calls `alignment::aligned_free` on `ptr` to free the allocated memory. [*Note:] If `T` is an incomplete type, the program is ill-formed. [endsect] [section alignment_of] The alignment type trait is used to query the alignment requirement of a type at compile time. It is provided by the C++11 standard library but is provided in this library for C++11 and C++03 implementations that do not provide this functionality. [heading:synopsis Header ] `` namespace boost { namespace alignment { template struct alignment_of; template constexpr std::size_t alignment_of_v = alignment_of::value; } } `` [heading:alignment_of Type trait alignment_of] `` template struct alignment_of; `` [*Value:] The alignment requirement of the type `T` as an integral constant of type `std::size_t`. When `T` is a reference array type, the value shall be the alignment of the referenced type. When `T` is an array type, the value shall be the alignment of the element type. [*Requires:] `T` shall be a complete object type, or an array thereof, or a reference to one of those types. [endsect] [section assume_aligned] The alignment hint macro can be used to inform the compiler of the alignment of a memory block, to enable vectorizing or other compiler specific alignment related optimizations. [heading:synopsis Header ] `` BOOST_ALIGN_ASSUME_ALIGNED(ptr, alignment) `` [heading:assume_aligned Macro BOOST_ALIGN_ASSUME_ALIGNED] `` BOOST_ALIGN_ASSUME_ALIGNED(ptr, alignment) `` [*Requires:] [itemized_list [`alignment` shall be a power of two] [`ptr` shall be mutable] ] [*Effect:] `ptr` may be modified in an implementation specific way to inform the compiler of its alignment. [endsect] [section is_aligned] The alignment validation function indicates whether or not an address is a multiple of the specified alignment value. It is generally useful in assertions to verify memory is correctly aligned. This functionality is not yet provided by the C++ standard. [heading:synopsis Header ] `` namespace boost { namespace alignment { constexpr bool is_aligned(std::size_t value, std::size_t alignment) noexcept; bool is_aligned(const void* ptr, std::size_t alignment) noexcept; } } `` [heading:is_aligned Function is_aligned] `` bool is_aligned(const void* ptr, std::size_t alignment) noexcept; `` [*Requires:] `alignment` shall be a power of two. [*Returns:] `true` if the value of `ptr` is aligned on the boundary specified by `alignment`, otherwise `false`. [endsect] [endsect] [section Vocabulary] [heading:basic_align \[basic.align\]] Object types have *alignment requirements* which place restrictions on the addresses at which an object of that type may be allocated. An *alignment* is an implementation-defined integer value representing the number of bytes between successive addresses at which a given object can be allocated. An object type imposes an alignment requirement on every object of that type; stricter alignment can be requested using the alignment specifier. A *fundamental alignment* is represented by an alignment less than or equal to the greatest alignment supported by the implementation in all contexts, which is equal to `alignof(std::max_align_t)`. The alignment required for a type might be different when it is used as the type of a complete object and when it is used as the type of a subobject. [tip `` struct B { long double d; }; struct D : virtual B { char c; }; `` When `D` is the type of a complete object, it will have a subobject of type `B`, so it must be aligned appropriately for a `long double`. If `D` appears as a subobject of another object that also has `B` as a virtual base class, the `B` subobject might be part of a different subobject, reducing the alignment requirements on the `D` subobject.] The result of the `alignof` operator reflects the alignment requirement of the type in the complete-object case. An *extended alignment* is represented by an alignment greater than `alignof(std::max_align_t)`. It is implementation-defined whether any extended alignments are supported and the contexts in which they are supported. A type having an extended alignment requirement is an *over-aligned type*. [note Every over-aligned type is or contains a class type to which extended alignment applies (possibly through a non-static data member).] Alignments are represented as values of the type `std::size_t`. Valid alignments include only those values returned by an `alignof` expression for the fundamental types plus an additional implementation-defined set of values, which may be empty. Every alignment value shall be a non-negative integral power of two. Alignments have an order from *weaker* to *stronger* or *stricter* alignments. Stricter alignments have larger alignment values. An address that satisfies an alignment requirement also satisfies any weaker valid alignment requirement. The alignment requirement of a complete type can be queried using an `alignof` expression. Furthermore, the types `char`, `signed char`, and `unsigned char` shall have the weakest alignment requirement. [note This enables the character types to be used as the underlying type for an aligned memory area.] Comparing alignments is meaningful and provides the obvious results: * Two alignments are equal when their numeric values are equal. * Two alignments are different when their numeric values are not equal. * When an alignment is larger than another it represents a stricter alignment. [note The runtime pointer alignment function can be used to obtain an aligned pointer within a buffer; the aligned-storage templates in the library can be used to obtain aligned storage.] If a request for a specific extended alignment in a specific context is not supported by an implementation, the program is ill-formed. Additionally, a request for runtime allocation of dynamic storage for which the requested alignment cannot be honored shall be treated as an allocation failure. [endsect] [section Compatibility] Boost.Align has been tested with GCC, Clang, Visual C++, and Intel C++, on Linux, Windows, and OS X. It supports any conforming C++11 or C++03 compilers. [table Compilers Tested [ [Compiler] [Version] [Library] [Platform] [Hardware] [Standard] ] [ [clang] [3.0 - 3.6] [libstdc++, libc++] [linux, darwin] [x86, x64] [c++11, c++03] ] [ [gcc] [4.4 - 5.1] [libstdc++] [linux, windows] [x86, x64, arm] [c++11, c++03] ] [ [intel] [11.1 - 15.0] [libstdc++] [linux] [x86, x64] [c++11, c++03]] [ [msvc] [8.0 - 14.0] [dinkumware] [windows] [x86, x64] [default] ] ] [endsect] [section Acknowledgments] Glen Fernandes authored the library and contributed it to the Boost C++ library collection. [heading Library Reviews] Thank you to the following developers who reviewed the design, code, examples, tests, or documentation. * Peter Dimov * Andrey Semashev * Bjorn Reese * Steven Watanabe * Antony Polukhin * Lars Viklund * Michael Spencer * Paul A. Bristow Thank you to Ahmed Charles who served as review manager for the formal review of the library. [heading Contributions] Thank you to Joel Falcou and Charly Chevalier for both your contribution and your patience. [endsect] [section History] [heading Boost 1.61] * Added functions for aligning up and aligning down of pointer and integral values. [heading Boost 1.59] * Joel Falcou and Charly Chevalier together contributed the alignment hint macros. [heading Boost 1.56] * Glen Fernandes implemented the Boost.Align library. [endsect]