How to properly aggregate-initialize nested std::array?

Asked Feb 07 '22 at 13:12

Active Feb 07 '22 at 14:19

Viewed 160 times

I have a very simple structure that looks like this:

struct Arr
{
    static constexpr auto width = 2;
    static constexpr auto height = 2;
    std::array<std::array<int, width>, height> values;
};

How do I properly initialize this structure without getting compiler errors or warnings? I tried the following (see example on Compiler Explorer):

auto arr = Arr{1, 2, 3, 4}; // clang: warning: suggest braces around initialization of subobject [-Wmissing-braces]
auto arr = Arr{{1, 2}, {3, 4}}; // clang: error: excess elements in struct initializer
auto arr = Arr{{{1, 2}, {3, 4}}}; // clang: error: excess elements in struct initializer

I know there have been similar questions asked (e.g. Nested aggregate initialization of std::array), but it seems all of them focus on C++11/14.

edited Feb 07 '22 at 14:19

Nicol Bolas

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asked Feb 07 '22 at 13:12

Krzysiek Karbowiak

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Not sure this is the same issue, but Clang lags behind on aggregate initialization – JHBonarius Feb 07 '22 at 13:16
gcc accepts 1), but warns in 2) and errors in 3) – Krzysiek Karbowiak Feb 07 '22 at 13:22
both gcc and clang seem to accept `Arr { {{ {{1,2}}, {{3,4}} }} };` with no warnings. (Each std::array level uses a double set of braces). – n. m. could be an AI Feb 07 '22 at 13:27
1

I think I fell one brace-pair short of that when I was trying things out, so thanks for the tip. But anyway, are these just quirks of the compilers, or is there anything in the standard that mandates this brace proliferation? – Krzysiek Karbowiak Feb 07 '22 at 13:33
There seems to be nothing in the standard that mandates this. If you need a bulletproof method, say `{ std::array{std::array{1,2}, std::array{3,4}} };`. – n. m. could be an AI Feb 07 '22 at 22:09

How to properly aggregate-initialize nested std::array?

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