Initializing an std::array of non-default-constructible elements? [duplicate]

Question

Suppose type foo_t with a named constructor idiom, make_foo(). Now, I want to have exactly 123 foo's - no more, no less. So, I'm thinking about an std::array<foo_t, 123>. Now, if foo_t were default-constructible, I would write:

std::array<foo_t, 123> pity_the_foos;
std::generate(
    std::begin(pity_the_foos), std::end(pity_the_foos),
    []() { return make_foo(); }
);

and Bob's my uncle, right? Unfortunately... foo_t has no default ctor.

How should I initialize my array, then? Do I need to use some variadic template expansion voodoo perhaps?

Note: Answers may use anything in C++11, C++14 or C++17 if that helps at all.

@DevNull: Yes, but - how idiomatic is that array generator snippet you have there? — einpoklum
– einpoklum, Commented Oct 23, 2017 at 23:05
@DevNull: There's a tag for that... the idiom that the most people use; the "common, appropriate thing to do". Also - that looks like it needs some fixup to use C++14 integer sequences, am I right? — einpoklum
– einpoklum, Commented Oct 23, 2017 at 23:07

T.C. · Accepted Answer · 2017-10-23 23:06:58Z

19

The usual.

template<size_t...Is>
std::array<foo_t, sizeof...(Is)> make_foos(std::index_sequence<Is...>) {
    return { ((void)Is, make_foo())... };
}

template<size_t N>
std::array<foo_t, N> make_foos() {
    return make_foos(std::make_index_sequence<N>());
}

answered Oct 23, 2017 at 23:06

T.C.

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2 Comments

einpoklum Over a year ago

That index sequence does keep popping up, doesn't it... although, really, I shouldn't even need the integer sequence in my case, just any tuple of length N.

qbolec Over a year ago

one issue I have with that, is that even with -O2 gcc is unrolling the loop godbolt.org/z/34TKjfEKb which is not cool for 512 elements :/

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