Static arrays at compile time in C++

Question

I was wondering if its possible to make the following answer more generic, in the sense that the type of the array be templated instead of just unsigned:

I've enclosed the whole thing in a struct like so:

template<typename ArrayType>
struct Array
{
template<ArrayType... args> struct ArrayHolder {
    static const ArrayType data[sizeof...(args)];
};

template<ArrayType... args> 
const ArrayType ArrayHolder<args...>::data[sizeof...(args)] = { args... };

template<size_t N, template<size_t> class F, ArrayType... args> 
struct generate_array_impl {
    typedef typename generate_array_impl<N-1, F, F<N>::value, args...>::result result;
};

template<template<size_t> class F, ArrayType... args> 
struct generate_array_impl<0, F, args...> {
    typedef ArrayHolder<F<0>::value, args...> result;
};

template<size_t N, template<size_t> class F> 
struct generate_array {
    typedef typename generate_array_impl<N-1, F>::result result;
};
};

but I get the following errors:

c++-4.6 -std=c++0x -o test test.cpp
test.cpp:49:17: error: specializing member ‘Array<ArrayType>::ArrayHolder<args>::data’ requires ‘template<>’ syntax

Answer 1

I don't think the expression like 'template ' is a valid usage of C++11 variadic templates.

It can be of two forms as I understand:

1. template <typename... args> : variable number of generic type parameters
2. template <int... args> : variable number of integer(non-type) parameters

Refer to this wikipedia article for variadic templates.

Answer 2

Erm... Why don't you just use std::vector? Or if you want a non-resiable array, then use the equivalent type in boost?

Answer 3

It helps if you indent the struct. The problem is that you are defining the data static member variable inside the Array struct. But it should be at namespace scope:

template<typename ArrayType>
struct Array
{
    template<ArrayType... args> struct ArrayHolder {
        static const ArrayType data[sizeof...(args)];
    };

    template<size_t N, template<size_t> class F, ArrayType... args> 
        struct generate_array_impl {
            typedef typename generate_array_impl<N-1, F, F<N>::value, args...>::result result;
        };

    template<template<size_t> class F, ArrayType... args> 
        struct generate_array_impl<0, F, args...> {
            typedef ArrayHolder<F<0>::value, args...> result;
        };

    template<size_t N, template<size_t> class F> 
        struct generate_array {
            typedef typename generate_array_impl<N-1, F>::result result;
        };
};

template<typename ArrayType> template<ArrayType... args> 
        const ArrayType Array<ArrayType>::ArrayHolder<args...>::data[sizeof...(args)] = { args... };