Efficient Data Structures

Context

• Create data structures for controling the data layout (AoS vs SoA)
• These data structures should hide the underlying data layout.
• We want to change the underlying data layout without affecting the code using it.

Using std::span

We can now create and manage instances of S with F == std::span.

struct Point2D { double x, y; };

template <template <class> class F>
struct S {
    F<int> x;
    F<int> y;
    F<Point2D> point;
    F<std::string> identifier;

    int abs2() const { return x * x + y * y; }
    int& getX() { return x; }
    const int& getX() const { return x; }
    void setX(auto x_new) { x = x_new; }
};

Using Memory of an Existing Buffer

Problem: Given an existing buffer, we want to allocate our data structures within that buffer.

Solution: We use std::pmr::memory_resource that takes a pointer to the buffer and its size in bytes.

struct BufferResource : public std::pmr::memory_resource {
    char* buffer_begin;
    char* buffer_end;
    char* current;

    BufferResource(char* buffer, std::size_t bytes)
        : buffer_begin(buffer), buffer_end(buffer + bytes), current(buffer) {}

protected:

    void* do_allocate(std::size_t bytes, std::size_t alignment) override {
        std::size_t space = buffer_end - current;
        void* aligned = std::align(
            alignment,
            bytes,
            reinterpret_cast<void*&>(current),
            space
        );
        if (!aligned) throw std::bad_alloc();
        current += bytes;
        return aligned;
    }

    void do_deallocate(void*, std::size_t, std::size_t) override {}

    bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {
        return this == &other;
    }
};