The C++11 standard introduced alignment support as one of the many features of the C++ programming language that can be used with the newest C++ compilers today. One of the new features of this support was a new keyword align std::aligned_storage that is used to provide the nested type which can be used as an uninitialized storage for any object whose size is at most given object size by the alignment size requirement. In this post, we explain how we can use aligned_storage in Modern C++.
What is alignment support in modern C++?
When we talk about ‘alignment’ in C++ it means a set of hints or instructions that tell the compiler to place the physical representation of data structures and variables in memory so that they line up at specific intervals of bytes – the underlaying digital representation of all data items. Alignment tells the compiler and linker to add additional ‘space’ to a value in memory so that the next object begins ‘nicely’ on a particular memory boundary.
Due to the way the CPU and other logic chips in a computer work, alignment can help create more computationally efficient programs. Think of it in the way you cut a cake – if you use an even number of slices for your cake it’s much easier and quicker to divide it up because you can cut the cake into nice even chunks of two. If you get an odd number of cake slices, it’s a lot harder to work out how big those slices need to be to make sure you don’t have a huge slice of cake left over – or someone gets a tiny slice (or none) and stays hungry! By cutting the cake in half each time you are ‘aligning’ your cake slices evenly and optimally so all cake is efficiently used with no wasted cake.
The C++11 standard intends to extend the standard language and library with alignment-related features, known as alignment support. These alignment features include:
- The alignment specifier
alignasfor declarations. - The
alignofexpression to retrieve alignment requirements of a type. - Alignment arithmetic by library support (
aligned_storage,aligned_union). - std::align standard function for pointer alignment at run time.
Alignment support in C++ can be found in more detail here in the C++ standards [Note: PDF link]. We also discuss the alignof keyword in this blog post: https://learncplusplus.org/what-is-the-alignof-expression-in-modern-c/.
In this post, we will explain the aligned_storage.
What is aligned_storage in modern C++?
The aligned_storage (std::aligned_storage) in C++ is used to provide a nested type which can be used as an uninitialized storage for any object whose size is at most a given object size by the alignment size requirement. Here are the syntax and helper type for the aligned_storage.
Syntax (Since C++11, deprecated in C++23):
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template< std::size_t object_size, std::size_t align_size > struct aligned_storage; |
Helper type (Since C++14, deprecated in C++23):
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template < std::size_t object_size, std::size_t align_size = alignment_of<T>::value > using aligned_storage_t = typename aligned_storage< object_size, align_size >::type; |
The main advantage of the std::aligned_storage is that it manages the alignment of given object size and it can be copied with memcpy(), also it can be used with POD and non-POD types.
std::aligned_storage can be used to decouple memory allocation from an object creation.
Note that aligned_storage and aligned_union are deprecated in C++23. There is also aligned_alloc which is released in C++17.
Is there an example of aligned_storage in modern C++?
Here is a full C++ example of how to use aligned_storage.
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#include <type_traits> #include <iostream> struct my_struct { int X,Y,Z; // 3x4bytes = 12 bytes }; #define block_size 25 typedef std::aligned_storage< block_size, alignof(my_struct)>::type aligned_block; int main() { std::cout<< "size of my_struct:" << sizeof(my_struct) << std::endl; std::cout<< "size of alignment:" << std::alignment_of<aligned_block>::value << std::endl; std::cout<< "requested object size:" << block_size << std::endl; std::cout<< "size of aligned block:" << sizeof(aligned_block) << std::endl; system("pause"); return (0); } |
and the output will be as follows:
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size of my_struct:12 size of alignment:4 requested object size:25 size of aligned block:28 Press any key to continue . . . |
as you see 28 is the minimum size close to 25, which is 2×12+4.
If you need more in-depth details about this feature, see the original Alignment Support Proposal document.
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