SWIG: Access Array of Structs in Python

Say I have the following static constexpr array of c struct:

#include <cstdint>

namespace ns1::ns2 {
    struct Person {
        char name[32];
        uint8_t age;    
    };
    static constexpr Person PERSONS[] = {
        {"Ken", 8},
        {"Cat", 27}
    };
}

How can I access elements in ns1::ns2::PERSONS in python by using swig?

One way I can think of is to create a accessor like const Person& get(uint32_t index) in the swig interface file. Tho, I wonder whether there is a more elegant way that I don’t have to create an accessor function for each array of c struct.

Thanks!

Answer

One way I can think of is to create a accessor like const Person& get(uint32_t index) in the swig interface file.

According to 5.4.5 Arrays in the SWIG documentation, that’s the way to do it:

%module test

%include <stdint.i>

%inline %{
#include <cstdint>

namespace ns1::ns2 {
    struct Person {
        char name[32];
        uint8_t age;
    };
    static constexpr Person PERSONS[] = {
        {"Ken", 8},
        {"Cat", 27}
    };
}

// helper function
const ns1::ns2::Person* Person_get(size_t index) {
    if(index < sizeof(ns1::ns2::PERSONS) / sizeof(ns1::ns2::Person))  // protection
        return ns1::ns2::PERSONS + index;
    else
        return nullptr;
}
%}

Demo:

>>> import test
>>> test.PERSONS.name # can only access the first element
'Ken'
>>> test.Person_get(1).name
'Cat'
>>> test.Person_get(2).name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'name'

SWIG can also generate array wrappers for you with 11.2.2 carrays.i, but there is no bounds checking:

%module test

%include <stdint.i>

%inline %{
#include <cstdint>

namespace ns1::ns2 {
    struct Person {
        char name[32];
        uint8_t age;
    };
    static constexpr Person PERSONS[] = {
        {"Ken", 8},
        {"Cat", 27}
    };
}
%}

%include <carrays.i>
%array_functions(ns1::ns2::Person,arrayPerson);

Demo:

>>> import test
>>> test.arrayPerson_getitem(test.PERSONS,1).name
'Cat'