In C one can allocate dynamic arrays using malloc(sizeof(T) * N) and then use pointer arithmetic to get elements at i offset in this dynamic array.
In C++ one can do similar using operator new() in the same way as malloc() and then placement new (for an example one can see solution for item 13 in a book "Exceptional C++: 47 engineering puzzles, programming problems, and solutions" by Herb Sutter). If you don't have one, the summary of the solution for this question would be:
T* storage = operator new(sizeof(T)*size);
// insert element
T* p = storage + i;
new (p) T(element);
// get element
T* element = storage[i];
For me this looked legit since I'm asking for a chunk of memory with enough memory to hold N aligned elements of size = sizeof(T). Since sizeof(T) should return a size of element which is aligned, and they are laid one after another in a chunk of memory, using pointer arithmetic is OK here.
However I was then pointed to links like: http://eel.is/c++draft/expr.add#4 or http://eel.is/c++draft/intro.object#def:object and claiming that in C++ operator new() does not return an array object, so pointer arithmetic over what it has returned and using it as an array is undefined behavior as opposed to ANSI C.
I'm not this good at such low level stuff and I'm really trying to understand by reading this: https://www.ibm.com/developerworks/library/pa-dalign/ or this: http://jrruethe.github.io/blog/2015/08/23/placement-new/ but I still fail to understand if Sutter was just plain wrong?
I do understand that alignas make sense in constructions such as:
alignas(double) char array[sizeof(double)];
(c) http://georgeflanagin.com/alignas.php
If array appears to be not in a boundary of double (perhaps following char in a structure ran at 2-byte reading processor).
But this is different - I've requested memory from the heap/free storage especially requested operator new to return memory which will hold elements aligned to sizeof(T).
To summarize in case this was TL;DR:
malloc() for dynamic arrays in C++?operator new() and placement new for dynamic arrays in older C++ which has no alignas keyword?operator new()?Sorry if this is dumb.
The C++ standards contain an open issue that underlying representation of objects is not an "array" but a "sequence" of unsigned char objects. Still, everyone treats it as an array (which is intended), so it is safe to write the code like:
char* storage = static_cast<char*>(operator new(sizeof(T)*size));
// ...
char* p = storage + sizeof(T)*i; // precondition: 0 <= i < size
new (p) T(element);
as long as void* operator new(size_t) returns a properly aligned value. Using sizeof-multiplied offsets to keep the alignment is safe.
In C++17, there is a macro STDCPP_DEFAULT_NEW_ALIGNMENT, which specifies the maximum safe alignment for "normal" void* operator new(size_t), and void* operator new(std::size_t size, std::align_val_t alignment) should be used if a larger alignment is required.
In earlier versions of C++, there is no such distinction, which means that void* operator new(size_t) needs to be implemented in a way that is compatible with the alignment of any object.
As to being able to do pointer arithmetic directly on T*, I am not sure it needs to be required by the standard. However, it is hard to implement the C++ memory model in such a way that it would not work.