Can Static Arrays in C Be Programmatically Initialized at Compile Time?

Programmatic Creation of Static Arrays at Compile Time in C

Creating static arrays at compile time in C is a valuable technique for optimizing performance and ensuring data integrity. One way to define a static array is through the following syntax:

const std::size_t size = 5;
unsigned int list[size] = { 1, 2, 3, 4, 5 };

While this approach is straightforward, it requires explicitly specifying the array values. To gain more flexibility, it may be desirable to assign these values "programmatically" at compile time using various metaprogramming techniques.

Question 1: Programmatic Value Assignment

Can we use metaprogramming techniques to assign array values "programmatically" at compile time?

Answer:

Yes, using C 0x features, we can initialize local or member arrays of templates from a variadic template argument list. While this approach may be limited by template instantiation depth, it can provide a significant improvement in flexibility.

Question 2: Selective Value Assignment

Assuming that all the values in an array are the same except for a few, can we selectively assign values at compile time in a programmatic manner?

Answer:

Yes, we can use a combination of variadic templates and function templates to generate arrays with specific values. For example, consider the following code:

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

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

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

This code allows us to create arrays with specific values by specifying a template function that determines the value at each index.

Example:

To create an array of size 5 with values 1 through 5:

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

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";
}

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