C++: const array instance generation in header file

Question

I could use a define macro to do what I want to achieve. But I'm sure there is a better way with C++14.

Basically I would like to instantiate a std::array of objets in the header of a class without defining by hand each instance.

    const std::array<property<uint16_t>, 16> cmd_eng1 =
    {{
            {PROG_MEM_ENG1, 0},
            {PROG_MEM_ENG1+2, 0},
            ...
            {PROG_MEM_ENG1+30, 0}
    }};

and if you wonder, below is the property class constructor (template not shown).

property(uint8_t addr, uint8_t bit_offset)

So is there a way (other than a macro) to define this array without writing everything by hand?

score 1 · Accepted Answer · 2020-05-05 16:40:05Z

1

@Micheal's answered can be slightly improved by using a lambda instead of a function.

This is somewhat similar to the trick used to initialize a local const variable using complex logic without having to write a helper method.

const auto cmd_eng1 = []{
    std::array<property<std::uint16_t>, 16> result = {};

    for (std::size_t i = 0; i < 16; ++i) {
        result[i] = property<std::uint16_t>(PROG_MEM_ENG1 + 2 * i, 0);
    }

    return result;
}();

Edit: This assumes that property can be default-constructed.

If you cannot go through the create-then-assign sequence and have to initialize the array on construction, you have to be a little more crafty and use a variadic template, in which case, I don't think you can avoid having to write a helper function:

template<std::size_t... Is>
auto make_array(std::index_sequence<Is...>) {
   return std::array<property<std::uint16_t>, sizeof...(Is)> {property<std::uint16_t>{PROG_MEM_ENG1 + 2 * Is, 0}...};
}

const auto cmd_eng1 = make_array(std::make_index_sequence<16>());

edited May 5, 2020 at 16:40

answered May 5, 2020 at 16:17

user4442671

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6 Comments

Michael Over a year ago

I think constexpr lambda are C++17 feature

Victor Over a year ago

@Frank, thanks, this seems quite good. However, I'm getting

error: could not convert '<brace-enclosed initializer list>()' from '<brace-enclosed initializer list>' to 'driver_LP5562::property<short unsigned int>'

I don't understand the problem, yet.

user4442671 Over a year ago

@Victor That's probably because property has no default constructor. See my ammended answer for an alternative in that case.

Victor Over a year ago

@Frank, indeed changing the constructor to property(uint8_t addr=0, uint8_t bit_offset=0) works like a charm :)

user4442671 Over a year ago

@Victor You do you in your code base, but adding a default constructor by using default values is pretty bad and error-prone. I would just add a separate default constructor instead. What if someone wrote property(12)?

|

Michael · Accepted Answer · 2020-05-05 16:15:12Z

1

Is this a solution to your problem?

template<std::size_t Size>
constexpr std::array<property<std::uint16_t>, Size> makeArray()
{
    std::array<property<std::uint16_t>, Size> array = {};
    for (std::size_t i = 0; i < Size; ++i) {
        array[i] = property<std::uint16_t>(PROG_MEM_ENG1 + 2*i, 0);
    }

    return array;
}

const auto cmd_eng1 = makeArray<16>();

edited May 5, 2020 at 16:15

answered May 5, 2020 at 16:10

Michael

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1 Comment

Victor Over a year ago

Yes it is, thanks! However I think Frank answer is more readable (for example for another programmer reading the code), but as you pointed out, I'm not sure yet his solution can work with C++14

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