C++ vector of uint8_t to vector of uint32_t

Question

std::vector<uint8_t> vector1 = { 1, 2, 3, 4 };

I would like to transform the vector above in its uint32_t version. I tried doing:

std::vector<uint32_t> vector2(vector1.begin(), vector2.end());

but this code returns the same array in 32 bit version, so the content is still { 1, 2, 3, 4 }.

What I expect to get is an array of a single value (in this case) of 0x01020304.

Is there any way to achieve that preferably without using for loop?

Thank you.

EDIT:

My solution, don't know if it's a good practice, but from what I learned about vectors it should be completely valid:

std::vector<uint32_t> vector2((uint32_t*)vector1.data(), (uint32_t*)(vector1.data() + vector1.size()*sizeof(uint8_t)));

Answer 1

I recommend using a loop to do this for simplicity

#include <cstdint>
#include <exception>
#include <vector>

std::uint32_t 
combine(std::uint8_t a, 
        std::uint8_t b, 
        std::uint8_t c, 
        std::uint8_t d)
{
    return (std::uint32_t(a) << 24) | 
           (std::uint32_t(b) << 16) | 
           (std::uint32_t(c) << 8) | 
            std::uint32_t(d);
}

std::vector<std::uint32_t> 
combine_vector(const std::vector<std::uint8_t>& items)
{
    if (items.size() % 4 != 0)
        throw std::exception();

    std::vector<std::uint32_t> ret;
    ret.reserve(items.size() / 4);
    for (std::size_t i = 0; i < items.size(); i += 4) {
        ret.push_back(
            combine(items[i + 0], 
                    items[i + 1],
                    items[i + 2],
                    items[i + 3]));
    }

    return ret;
}

I suspect you would need to implement a custom iterator type to do this without using a raw loop.

I made such an iterator

#include <iterator>

class combine_iterator
{
public:
    using value_type = std::uint32_t;
    using reference = const value_type&;
    using pointer = const value_type*;
    using difference_type = std::ptrdiff_t;
    using iterator_category = std::input_iterator_tag;

    combine_iterator(const std::uint8_t* data, std::size_t count) :
        m_data(data)
    {
        if (count % 4 != 0)
            throw std::exception();

        if (count > 0)
            ++*this;
    }

    reference
    operator*() const
    {
        return m_cur_val;
    }

    pointer
    operator->() const
    {
        return &m_cur_val;
    }

    friend combine_iterator&
    operator++(combine_iterator& rhs)
    {
        std::uint8_t a = *(rhs.m_data++);
        std::uint8_t b = *(rhs.m_data++);
        std::uint8_t c = *(rhs.m_data++);
        std::uint8_t d = *(rhs.m_data++);
        rhs.m_cur_val = combine(a, b, c, d);
        return rhs;
    }

    friend combine_iterator
    operator++(combine_iterator& lhs, int)
    {
        auto cp = lhs;
        ++lhs;
        return cp;
    }

    friend bool
    operator!=(const combine_iterator& lhs, const combine_iterator& rhs)
    {
        return (lhs.m_data != rhs.m_data);
    }

private:
    const std::uint8_t* m_data;
    std::uint32_t m_cur_val;
};

int main()
{
    std::vector<std::uint8_t> data = { 1, 2, 3, 4, 5, 6, 7, 8 };

    std::vector<std::uint32_t> res(
        combine_iterator(data.data(), data.size()),
        combine_iterator(data.data() + data.size(), 0));
}

The iterator contains at least one bug. I'm leaving the bugs in as an educational lesson why using a loop if often easier to get correct than implementing custom iterators. Custom iterators should ideally only be created if we create a container which needs it, or the mental overhead of creating a custom iterator can be justified.

Answer 2

You can use ranges, either by using the range-v3 library, or the C++20 std::ranges.

std::vector<uint8_t> vector1 = { 1, 2, 3, 4, 5, 6, 7, 8 };

std::vector<uint32_t> vec2 = 
            vector1 
            | view::chunk(4) 
            | views::transform(
                [](auto&& range){
                    return accumulate(
                        range, 
                        static_cast<uint32_t>(0), 
                        [](auto acc, auto i)
                        {
                            return acc << 8 | i;
                        }
                    );
                })
            | to<std::vector>();

vec2 will contain {0x1020304, 0x5060708}.

See it on godbolt: https://godbolt.org/z/6z6ehfKbq