struct heap {
    concurrent_queue<void*> to_free;
    void free(void* ptr);
    void* alloc(size) {
        void* ptr;
        while(to_free.try_pop(ptr)) { free(ptr); }
    }
};

concurrent_queue<unique_ptr<heap>> heaps;

void* alloc(size) {
    unique_ptr<heap> h;
    if (!heaps.try_pop(h))
        h = unique_ptr<heap>(new heap); // locked global structure
    auto mem = h->alloc(size + sizeof(void*));
    // place heap* at the beginning
    return mem;
}
void free(void* ptr) {
    heap* h = // get heap from mem
    h->to_free.push_back(ptr);
}