alignas keyword not respected

Question

I want to overalign my type on a cache boundary, so I used alignas:

struct alignas(64) W { };

This compiles fine. But then, to my surprise, when I try to allocate a bunch of Ws, they're not 64-byte aligned but actually 16-byte aligned:

#include <iostream>
#include <iomanip>
#include <unordered_map>

struct alignas(64) W { };

int main() {
    std::unordered_map<int, int> offset;

    for (int i = 0; i < 1000; ++i) {
        auto w = new W;
        offset[(uintptr_t)w % 64]++;
    }   

    for (const auto& p : offset) {
        std::cout << p.first << ' ' << p.second << '\n';
    }   
}

Yields:

0 250
16 250
32 250
48 250

on several compiles (gcc 4.8.2, gcc 5.2.0, clang 3.7.1). What's up? I told it to align, why isn't it aligning?

Answer 1

This is answered nicely here: https://stackoverflow.com/a/16510895

Basically: new (at least in its normal usage) only guarantees a constant maximum alignment (alignof(std::max_align_t)) for every call to new.