c++ std::array warning

Question

On the following code

std::array<int,3> myarray = {10,20,30};

I am receiving the following compiler warning

warning: missing braces around initializer for ‘std::array<int, 3u>::value_type [3] {aka int [3]}’ [-Wmissing-braces]

Why ?

toolchain: (edit)

g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Should we guess what toolchain you're using?

WhozCraig
– WhozCraig

2013-02-10 03:48:19 +00:00
Commented Feb 10, 2013 at 3:48 — WhozCraig
– WhozCraig, Commented Feb 10, 2013 at 3:48

Tyler Jandreau · Accepted Answer · 2013-02-10 03:49:59Z

6

Try this:

std::array<int,3> = {{10, 20, 30}}

I think this was a bug they fixed in versions > 4.6

answered Feb 10, 2013 at 3:49

Tyler Jandreau

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Comments

Seth Carnegie · Accepted Answer · 2013-02-10 03:58:47Z

6

As Tyler indicated, std::array is a POD, so it has no constructors, and it contains an array. To initialise it with brace syntax, you initialise the variable, and then the array inside the variable, with the nested braces.

{ { 10, 20, 30 } }
  ^ For the array member variable inside the std::array object
^ For the std::array object

Actually this is a bug in your compiler, because aggregate initialisation allows you to remove a layer of braces after an =. So these two are legal:

std::array<int,3> x = {10, 20, 30};
std::array<int,3> y  {{10, 20, 30}};

But not

std::array<int,3> z {10, 20, 30};

The last one compiles on GCC but it's a nonstandard extension and you should get a warning.

edited Feb 10, 2013 at 3:58

answered Feb 10, 2013 at 3:51

Seth Carnegie

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9 Comments

us2012 Over a year ago

Works with only a single pair of braces on g++ 4.7, though.

WhozCraig Over a year ago

Works single-bracket as well on my mac, LLVM 4.2. Thus the reason I asked what chain he had.

Seth Carnegie Over a year ago

@WhozCraig apparently aggregate initialisation allows you to remove a layer after an =.

WhozCraig Over a year ago

@SethCarnegie yeah, I see that now. Interesting how you're need to be container-aware without it. At least you can still do it though, which is more than many of things in older implementations.

us2012 Over a year ago

@Seth "But not std::array<int,3> z {10, 20, 30}; " <- that works on g++ 4.7.

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