How do I pass a 2D array in C++ to a Fortran subroutine?

Question

I am writing a small C++ program which passes a 2-D array (of complex numbers) to a Fortran subroutine and receives it back filled with values. I wrote a version which passe

Answer 1

New advice

A two-dimensional array in c++ is not--I repeat not--the same as a pointer to a pointer{*}!

When you do

struct cpx** A;

you are setting up to construct a so called "ragged array" for which there is not a fortran equivalent. You want something like

struct cpx *A[2][2] = new struct cpx[2][2];

which is a pointer to a two-dimensional array with rows 2 long.

{*} Yes, you can access a pointer-to-pointer structure using the two-dimensional array notation, but they are laid out differently in memory. ::grumble:: People who tell other people that arrays and pointers are the same thing in c need to meet the Big Foam Clue Bat.

• A two dimensional array is a single allocation of **sizeof(). It's name decays to a pointer-to-type.
• A ragged array is 1+ allocation. The one is *) and the other are *sizeof().

Old, correct, but non-applicable advice

The thing to be aware of here is that c++ and fortran believe arrays are stored differently in memory.

C++ thinks that the memory position after a[1][1] is a[1][2], while fortran believes that is is a[2][1]. The distinction is between row-major (c, c++, etc) and column major (fortran, matlab, a few others).

Note that this is separate from fortran indexing arrays from 1 by default.