Initializing Array of Arrays - Trouble

Question

OK, I know that in C++ a - let's say 2-dimensional - array can be initialized this way :

int theArray[5][3] = { 
    {1,2,3},
    {4,5,6},
    {7,8,9},
    {10,11,12},
    {13,14,15} 
};

Now, what if I want to use pre-existing arrays as theArray's elements?

E.g.

// A, B, C, D,... have already been declared as :
// `const U64 A[] = { 1,2,3,4 };` etc...

const U64 multiDimArray[12][64] = { 
     A, B, C, D, E, F,  
     G, H, I, J, K, L
};

This one, throws an error though :

cannot initialize an array element of type 'const U64' 
(aka 'const unsigned long long') with an lvalue of type 'const U64 [64]'

I see the point, but hopefully you can see mine.

Is there a workaround so that I can easily achieve the same thing? (Any suggestion - perhaps something using Boost? - is welcome)

Answer 1

If you use C++11, the initializer list for an array is flexible:

std::array< array<U64, 64>, 12> multiDimArray = {
     A, B, C, D, E, F,  
     G, H, I, J, K, L
};

will work fine, assuming A..L are std::array<64, U64>.

The array does have no overhead to the c-style array. Click here for official reference.

"The size and efficiency of array for some number of elements is equivalent to size and efficiency of the corresponding C-style array T[N]." (From the reference)

---

I said "flexible", since you can use a mixed initializer-list like this:

std::array<int, 3> first_row = {1,2,3};
std::array<array<int, 3>, 2> a={
  first_row,
  {2, 2, 2}
};

You can use this as a fixed-size array, with the same operations:

a[1][2]=2; a[0][1]=1; a[0][2]=3;