Code:
std::vector<int> x{1,2,3,4};
std::array<int, 4> y{{1,2,3,4}};
Why do I need double curly braces for std::array?
2 Answers
std::array<T, N> is an aggregate: it doesn't have any user-declared constructors, not even one taking a std::initializer_list. Initialization using braces is performed using aggregate initialization, a feature of C++ that was inherited from C.
The "old style" of aggregate initialization uses the =:
std::array<int, 4> y = { { 1, 2, 3, 4 } };
With this old style of aggregate initialization, extra braces may be elided, so this is equivalent to:
std::array<int, 4> y = { 1, 2, 3, 4 };
However, these extra braces may only be elided "in a declaration of the form T x = { a };" (C++11 §8.5.1/11), that is, when the old style = is used . This rule allowing brace elision does not apply for direct list initialization. A footnote here reads: "Braces cannot be elided in other uses of list-initialization."
There is a defect report concerning this restriction: CWG defect #1270. If the proposed resolution is adopted, brace elision will be allowed for other forms of list initialization, and the following will be well-formed:
std::array<int, 4> y{ 1, 2, 3, 4 };
(Hat tip to Ville Voutilainen for finding the defect report.)
7 Comments
Mark Ransom
So it's a failure of the abstraction model being presented for
array?
Nicol Bolas
@Mehrdad: It is uniform. What isn't uniform is the fact that you're initializing two completely different types.
user541686
@NicolBolas: I thought the whole point of uniformity was to use the same initialization syntax for different types? (Yes, I do understand what's happening... I'm just saying it's not "uniform" to the user, regardless of whether there's an explanation for it.)
Xeo
@MarkRansom: Well, it's rather a quirk of the language, because
std::array<int> y = {1,2,3,4}; works with a warning from Clang suggesting braces, instead of a hard error about not being allowed to "omit braces around initialization of subobject when using direct list-initialization".
James McNellis
@Xeo: Actually, brace elision is permitted in aggregate initialization, but (apparently) not when the direct list initialization syntax is used.
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Because std::vector offers a constructor that takes in a std::initializer_list<T>, while std::array has no constructors and the {1, 2, 3, 4} braced init-list is in fact not interpreted as a std::initializer_list, but aggregate initialization for the inner C-style array of std::array (that's where the second set of braces comes from: One for std::array, one for the inner C-style member array).
std::array, or are you just getting a warning?std::array<int,4> y{1,2,3,4};works for me.-Werroranyway for code I've written, so it doesn't harm my portability any. Others' mileage may vary, if they're lightweights or need to include header files written by lightweights :-)