Can C Metaprogramming Be Used for Dynamically Creating Compile-Time Static Arrays?

Programmatically Creating Static Arrays at Compile Time in C

Problem Introduction

Traditionally, static arrays in C can be defined at compile time using fixed-size arrays. However, for certain scenarios, it may be desirable to assign values programmatically at compile time. This article explores metaprogramming techniques to achieve such dynamic creation of static arrays.

Question 1: Assigning Values Programmatically

Using C 0x features, it is possible to initialize local or member arrays of templates from a variadic template argument list. This workaround has limitations due to maximum template instantiation depth.

Question 2: Selective Value Assignment

To selectively assign values at compile time, a combination of variadic templates and metafunctions can be employed. The MetaFunc template serves as a parameter pack that generates a sequence of values based on its index. A generate_array template can then create an array of the desired size using the generated values.

Example Implementation

template<size_t index> struct MetaFunc { enum { value = index + 1 }; };

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

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

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

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

Usage Example

void test() {
    const size_t count = 5;
    typedef generate_array<count, MetaFunc>::result A;

    for (size_t i = 0; i < count; ++i) 
        std::cout << A::data[i] << "\n";
}

This example defines a static array of size 5, with values {1, 2, 3, 4, 5} assigned at compile time using the MetaFunc metafunction.

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