It is not clear from N1919 if the size of an initializer list is a
constexpr

>From 4.5:

"""
Clearly, there is enough information to deduce that s.size() is 3.
Equally clearly, making
s.size() a constant expression requires a special rule. The proposal
for generalizing
constant expressions (N1920=05-0180) suggests that this can be made to
work. We are
not sure whether this is really important or just something people
thought interesting. We
need a solid use case.
"""

Has there been any update on this? I would personally find static
sizing very useful. Also, what about nested lists? This would be
useful (for instance) for initializing a matrix, eg:

Matrix<3,3> m = {{1,2,3}, {4,5,6}, 7,8,9}};

template<int Rows, int Cols> struct Matrix{
   Matrix(initializer_list<initializer_list<double>> l)
   {
     ...
   }
};

Is there any way of statically determining the size of the sub-
initializer lists? ie, in the constructor, would the result of  m
[0].size() also be a constexpr? If initializer lists could be passed
around, then static verification  of sizes could be done using code
such as:

...    //In Matrix constructor
      Verify<l[0].size, l[0].size()-1>::verify(l);
...

template<int Size, int N> struct Verify {
 static void verify(initializer_list<initializer_list<double>> l)
 {
    static_assert(l[N].size() == Size);
    Verify(Size, N-1)::verify(l);
 }
};

template<int Size> struct Verify<Size, -1>{
   static void verify(initializer_list<initializer_list<double>> l)
   {}
};

-Ed
--
(You can't go wrong with psycho-rats.)(http://mi.eng.cam.ac.uk/~er258)

/d{def}def/f{/Times s selectfont}d/s{11}d/r{roll}d f 2/m{moveto}d -1
r 230 350 m 0 1 179{ 1 index show 88 rotate 4 mul 0 rmoveto}for/s 12
   d f pop 235 420 translate 0 0 moveto 1 2 scale show showpage

--
[ comp.std.c++ is moderated.  To submit articles, try just posting with ]
[ your news-reader.  If that fails, use mailto:std-c++@netlab.cs.rpi.edu]
[              --- Please see the FAQ before posting. ---               ]
[ FAQ: http://www.comeaucomputing.com/csc/faq.html                      ]