Background: I am tasked with a work project of creating interoperability between an existing large Fortran code basis and a modern C++ GUI using Qt. I am using Qt Creator 6.0.2 based on Qt 6.2.2 (MSVC 2019, 64 bit) and VS 2019 Pro with the Intel Fortran Compiler.
I have been able to successfully pass basic data types and simple structures/UDT between Fortran and Qt, but as I try to get into more complex data structures I am running into lots of issues and confusion. I've spent many hours googling this, but everything I can find is limited to basic data type examples.
So my question ultimately is if you have a data structure in Fortran that looks like this:
module example
type top_struct
type(sub_struct), allocatable :: sStruct(:)
complex , allocatable :: complex1(:)
real , allocatable :: real1(:, :)
integer , allocatable :: ints1(:, :)
character , allocatable :: label1(:)
end type top_struct
type sub_struct
complex , allocatable :: complex2(:)
real , allocatable :: real2(:, :)
integer , allocatable :: ints2(:, :)
character , allocatable :: label2(:)
end type sub_struct
end module
how would you implement code in both C++ and Fortran that would allow you to pass this structure back and forth between them?
I found other questions that discussed using pointers to workaround there not being a direct correlation between allocatable in Fortran and something like std::vector in C++, but they didn't give any examples as to how to handle this if the data is inside of a structure/UDT.
Any help would be greatly appreciated!
Unless your structure is bind(C), no exact correspondence between C(++) and Fortran can be guaranteed. The compilers can choose to use different paddings or similar. But you cannot make a bind(C) structure with allocatable components. All that is left are hacks.
As a workaround you could make a proxy structure with type(c_ptr) pointers that point to those allocatable arrays and pass to C(++) this proxy.