c++ : Initialize constexpr std-2DArray programmatically

Question

I am new to C++ and in the process of learning. I have a matrix deftype:

using Matrix = std::array<std::array<T, COL>, ROW>;

I can initialize a constexpr matrix "manually" as such

constexpr Matrix<int, 2,2> mat{ {
        {{ 1,2 }},
        {{ 3, 4 } }
}};

This is not feasible for large examples of ROW,COL. Instead i would like to initialize it "programmatically"(i.e. through a loop or similar) with a function fill_entry so that mat[i][j] takes value fill_entry(i,j) and fill_entry is something like

 template<T>
constexpr T fill_entry(T This_object_has_type_T,int i , int j).

What is a correct way to do this? I use C++17

IlCapitano · Accepted Answer · 2020-11-13 10:55:15Z

4

You can use an immediately invoked lambda:

constexpr auto mat = []() {
    Matrix<int, 2, 2> result{};

    int value = 1;
    for (std::size_t i = 0; i < result.size(); ++i) {
        for (std::size_t j = 0; j < result[0].size(); ++j) {
            result[i][j] = value;
            ++value;
        }
    }

    return result;
}();

If you need to initialize multiple variables like this, you can make this into a function:

template<typename T, std::size_t Rows, std::size_t Cols>
constexpr Matrix<T, Rows, Cols> create_mat() {
    Matrix<T, Rows, Cols> result{};

    int value = 1;
    for (std::size_t i = 0; i < result.size(); ++i) {
        for (std::size_t j = 0; j < result[0].size(); ++j) {
            result[i][j] = static_cast<T>(value);
            ++value;
        }
    }

    return result;
}

and call it as create_mat<int, 2, 2>().

edited Nov 13, 2020 at 10:55

answered Nov 12, 2020 at 23:47

IlCapitano

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5 Comments

Conformal Over a year ago

Thanks. Your solution does not work with my compiler (MS visual studio), it gives me the error when caling create_mat that "constexpr function create_mat cannot result in a constant expression". I found out that your solution works if i change from C++17 to the option "latest" which i guess means c++ 20. Which compiler/OS did you test yours on?

IlCapitano Over a year ago

@Conformal You're right, it doesn't compile with C++17 (but does with C++20), the reason is result needs to be value initialized, so changing result to result{} should work (it works on godbolt). I also fixed a few typos in my answer.

Alex D Over a year ago

@IlCapitano I have a very similar issue that I can't get working. Instead of using Matrix; I'm trying to use tuple. I'm using the same approach but it won't compile: godbolt.org/z/c7bxa8zhP . Any ideas?

IlCapitano Over a year ago

@AlexD Looks like std::tuple::operator= is constexpr only since C++20, so it won't work in C++17.

Alex D Over a year ago

@IlCapitano your wisdom is much appreciated

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