Aligned heap allocation for unknown types

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Given alignment and size of an otherwise unknown type, how can one safely construct free storage (heap) that is properly aligned?

For context: The automatic storage version is well documented. See the example at https://en.cppreference.com/w/cpp/types/aligned_storage: An opaque uninitialized storage on stack gets properly aligned, which can later be placement new'ed once type is known.

Is it possible to similarly do alignment-safe uninitialized allocation on the heap? This storage would later get placement-new initialized once the type is known (and deleted in type-aware context with proper destruction etc.)

The following naïve attempt fails: https://godbolt.org/z/PM1xWoYox

What is the mental model here with heap storage?

c++17 memory-alignment memory c++
1 Answers

Given the problems you are having with new and delete, you might consider using aligned_alloc and free instead. For example:

#include <cstdlib>
#include <new>

struct Foo
{
    Foo (double d) : m_d (d) {}
    double m_d;
};

int main ()
{
    void *storage = aligned_alloc (alignof (Foo), sizeof (Foo));
    Foo* foo = new (storage) Foo (3.14);
    ...
    foo->~Foo ();
    free (storage);
}

Note that, to destroy an object created by placement new, you must call the destructor explicitly. You don't call delete. Then you must free the underlying storage.

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