Re: [Boost-users] [boost][array] Multi-dimensional initialization

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-08-09 08:34:27

Michael Marcin wrote:
> At least under MSVC 8 I can do:
>
> boost::array< boost::array<int,2>, 4 > foo = { -1,1, 0,0, 1,-1, 2,-2
> };
> or
>
> int foo[4][2] = { {-1,1}, {0,0}, {1,-1}, {2,-2} };
>
> or
>
> boost::array<int,2> foo[4] = { {-1,1}, {0,0}, {1,-1}, {2,-2} };
>
> but not
>
> boost::array< boost::array<int,2>, 4 > foo = { {-1,1}, {0,0}, {1,-1},
> {2,-2} };
>
> just wondering why this is the case.

To initialize a boost::array "by the book" you need double braces:

boost::array<int,2> foo = {{ 1, 2 }};

because a boost::array<int, 2> is essentially

struct array
{
    int data[2];
};

and, according to the initializer rules, the outer braces are for the array 
struct, and the inner braces correspond to the data[] array.

In most cases you can drop the outer braces. In your last example, however, 
dropping them creates an ambiguity, and you need:

boost::array< boost::array<int,2>, 4 > foo = {{ {-1,1}, {0,0}, {1,-1}, 
{2,-2} }};