Create array/vector of items with different constructor arguments per item

Question

I want to create an array/vector (indifferent to which one) of class instances. I have the following constraints:

Each instance is constructed with a different argument (e.g., Item(0), Item(1), Item(2), ...).
The Item class has both its copy and move constructors deleted.

Constraint #2 above seems to rule out using an std::vector since the vector would need to either copy or move the enqueued instances when it resizes its backing store.

This leaves me with an array. For both a C-style array and std::array, there does not seem to be any way to specify custom constructors for items in the array, so I can't custom construct items in-place in array indices. The only other option is to create an array of pointers and do array[0] = new Object(0);, array[1] = new Object(1);, array[2] = new Object(2);, but this is messy since it allocates memory on the heap rather than just the stack and requires me to explicitly free memory.

Is there a way to do this?

Just to clarify: what you want is to pass the index of the object in the array to its constructor? — Kein
– Kein, Commented May 20, 2020 at 3:55
@Kein That happens to be true, but I would also want to know how to do this when that is not the case. — Jack Humphries
– Jack Humphries, Commented May 20, 2020 at 4:57

bolov · Accepted Answer · 2020-05-20 04:22:56Z

3

Given

struct Item
{
    Item(int);
    Item(const Item&) = delete;
    Item(Item&&) = delete;
};

You have some options:

smart pointers

You can use smart pointers, in this case std::unique_ptr:

auto test()
{
    std::vector<std::unique_ptr<Item>> v;

    v.emplace_back(std::make_unique<Item>(1));
    v.emplace_back(std::make_unique<Item>(2));
    v.emplace_back(std::make_unique<Item>(3));
}

`std::array`

If you do not need to resize or move the container you can use std::array (since C++17):

auto test2()
{
    std::array<Item, 3> arr{1, 2, 3};
}

This works since C++17 because of the new temporary materialization rules.

edited May 20, 2020 at 4:22

answered May 20, 2020 at 3:54

bolov

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4 Comments

Jack Humphries Over a year ago

Wow, using temporary materialization to do this looks great! For my own knowledge, can we do this with a C array?

Jack Humphries Over a year ago

Additionally, is there a way to do this when I don't know the argument values (1, 2, 3 above) at compile time?

Jarod42 Over a year ago

@JackHumphries: auto test3(int a, int b, int c) { std::array<Item, 3> arr{a, b, c}; } is fine.

Jack Humphries Over a year ago

It also looks like stackoverflow.com/questions/3763846/… could be useful here.

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