Is there a way to index an array in c++, like in python?

Question

In order to write a packet sniffer I need to be able to index through the bytes of the packet.

I have come from python, in python you can index through a list very easily eg;

variable = data[2:5]. In c++ you can't do it like that.

Here is a snippet of my code

void ethernet_frame(char raw_data[19]){

    struct ethernet{ 
        char dest_mac[6] = get_mac_addr(raw_data[:7]);
        char src_mac[6] = get_mac_addr(raw_data[7:14]);
        unsigned short proto  = proto[14:17]; 
    };
}

I expect that there is an alternative or perhaps a library I can use to index through the packet.

Possible duplicate of Is there a way in C++ to get a sub array from an array? — Andrew Fan
– Andrew Fan, Commented Dec 23, 2018 at 14:48

Vittorio Romeo · Accepted Answer · 2018-12-23 15:14:57Z

2

There's no built-in way of doing this, but C++ allows you to create abstractions to achieve your goals. E.g.

void ethernet_frame(std::array<char, 19> raw_data)
{
    struct ethernet
    {
        std::array<char, 6> dest_mac;
        std::array<char, 6> src_mac;
    };

    ethernet e{slice<0, 7>(raw_data), slice<7, 14>(raw_data)};
}

Where slice is something along the lines of:

template <std::size_t Begin, std::size_t End, typename T>
auto slice(const T& range)
{
    constexpr auto count = End - Begin - 1;
    std::array<std::decay_t<decltype(*range.begin())>, count> result;
    std::copy(range.begin() + Begin, range.begin() + End, result.begin());
    return result;
}

answered Dec 23, 2018 at 15:14

Vittorio Romeo

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Comments

Basile Starynkevitch · Accepted Answer · 2018-12-23 14:47:47Z

0

C++ is not Python. So you cannot code like this.

You might want to use pointer arithmetic (e.g. raw_data+7 if raw_data is some char*)

You could use iterators and range for. Then you want to use containers like std::array or std::vector

Please take several days or weeks to read a good C++ programming book. We don't have time or space for that here.

answered Dec 23, 2018 at 14:47

Basile Starynkevitch

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