C++11 Fun with initializer lists, arrays, and enumerations

Question

Background

C++11 initializer lists can be used to initialize vectors and arrays with argument passing to constructors.

I have a piece of code below

Answer 1

I like your problem. Long ago, this kind of thing used to be handled with X macros http://www.drdobbs.com/the-new-c-x-macros/184401387

I'm a c++11 newb, but after some fiddling around I've got some kind of solution (g++ 4.8.4):

enum class Symbols { FOO, BAR, BAZ, First=FOO, Last=BAZ };

I kept your Size() but added some other boilerplate to make the initialization, lower down, easier to read.

template< typename E > constexpr size_t Size() { return (size_t)(E::Last) - (size_t)(E::First) + 1; }
template< typename E > constexpr size_t as_sizet( E s ) { return (size_t)s; }
template< typename E > constexpr E operator++( E& s, int ) { return (E)(1 + (size_t)s); }
template< typename E > constexpr bool operator<=( E& a, E& b ) { return (size_t)a < (size_t)b; }

There are two bits of magic here:

• We return a reference to the initialized array (itself another template parameter)
• We initialize a static array during the recursive call using a junk argument

Like so:

template< typename E, typename EARR > 
constexpr EARR& init_array( EARR& zArr, E sym = E::First, E junk = E::Last )
{
    return sym <= E::Last ? init_array( zArr, sym++, zArr[ as_sizet( sym ) ] = sym ) : zArr;
}

In the end, it comes together with:

• The typedef
• The static declaration of the array
• The reference to the array that gets initialized

Like so:

typedef Symbols SymbolArr[ Size() ];
static SymbolArr symbolArr;
SymbolArr& symbolArrRef = init_array(symbolArr);

Edit:

The junk parameter in the recursive initialization function can be removed using:

template< typename E > constexpr E next( E& s ) { return (E)(1 + (size_t)s); }

template< typename E, typename EARR > 
constexpr EARR& init_array( EARR& zArr, E sym = E::First )
{
    return sym <= E::Last ? init_array( zArr, next( zArr[ as_sizet( sym ) ] = sym ) ) : zArr;
}