Feasibility of std::array with Set

[gyuro]

I'm trying to call the print_values function in Codon, but the value of a is printed as zero, while b is 3 and c is 4.

In C:

extern "C" void print_values(const std::array<double, 2>& a, double b, double c)
{
    // TODO
}

In Codon:

from C import LIBRARY.print_values(Set[float], float, float)

print_values({1, 2}, 3, 4)

This happened with const double[2] a. Could you help me find a solution?

[inumanag]

Hi @gyuro

Set is completely different structure. To make it compatible C, you would have to cast it to list and then pass it as a pointer, e.g.:

from C import LIBRARY.print_values(ptr[float], i32, float, float)
foo = {1, 2}
foo_l = list(foo)
print_values(foo_l.arr.ptr, len(foo_l), 3, 4)

In C, you would have to use

void print_values(double *a, int a_sz, double b, double c) {
...}

I don't know how is std::array<> represented; in general, all extern "C" functions should only deal with C-compatible types (Codon has no C++ interop).

[gyuro]

@inumanag, thanks for the comments.

Based on my observations, declaring a tuple value and passing it by pointer to a C function works fine, as shown below, even though Codon lacks C++ interop:

foo = (1, 2)
print_values(__ptr__(foo), 3, 4)

However, my question is about how to pass an array or any container by value to a C function on the Codon side, like this:

foo = (1, 2)
print_values(foo, 3, 4)

[inumanag]

In a nutshell, you cannot---you would need to mock up Codon's type internal structure in C (can be done but is not supported nor recommended). The best way is to cast everything to lists and then pass list pointers with sizes.