Is there a way to merge / concatenate precompiler lists with C++ macros?

Question

I have two lists:

#define LIST1 {1, 2, 3}
#define LIST2 {4, 5, 6}

and using C++ macros I would like to write something like this:

// Obviously doesn't work
#define MERGE LIST1 ## LIST2
int my_array[] = MERGE;

to yield:

int my_array[] = {1, 2, 3, 4, 5, 6};

during compile-time.

Is something like this possible? There are other questions concerning this with strings, however I can't figure out for the life of me how to do this with non-string array declarations.

Edit: I certainly would prefer to not use macros, and would prefer that the list format be different as well. Unfortunately, the header file that contains these list definitions isn’t a file that I can edit.

Answer 1

Don't use macros unless there is no other option, prefer templates. They are typesafe.

For example you can make a compile time evaluated function (constexpr) that merges two lists (arrays) and returns an array.

#include <array>

// type_t is the type held by the array (an int in this example)
// N = size of first array
// M = size of second array
// const type_t(&arr)[N] is the syntax for passing an array by const reference

template<typename type_t, std::size_t N, std::size_t M>
constexpr auto merge(const type_t(&arr1)[N], const type_t(&arr2)[M])
{
    std::array<type_t, N + M> arr{}; // this initialization is needed in constexpr
    std::size_t index{ 0 };

    for (const auto& value : arr1) arr[index++] = value;
    for (const auto& value : arr2) arr[index++] = value;

    return arr;
}

int main()
{
    constexpr auto arr = merge({ 1,2,3 }, { 4,5,6 });
    constexpr auto strings = merge( {"abc", "def" }, {"ijk", "lmn"} );

    // static_assert is like assert, but evaluated at compile time.
    static_assert(arr.size() == 6);
    static_assert(arr[4] == 5);

    return 0;
}

Answer 2

It is easily achievable by slightly restating your code:

#define LIST1 1, 2, 3
#define LIST2 4, 5, 6
#define MERGE LIST1, LIST2
int my_array[] = { MERGE };

#include <iostream>
int main() {
  for(auto const& x : my_array) {
    std::cout << x << "\n";
  }
}

Demo