Function arguments by reference

[ianhbell]

I know dealing with references is never easy when wrapping low-level languages in high-level languages (I've fought with this in SWIG and Cython), but how would I get a dummy function like this working:

void plus_one_inplace(int &i) { i++; }

I tried:

PYBIND11_PLUGIN(ex) {
    py::module m("ex", "example");
    m.def("plus_one_inplace", &plus_one_inplace, "add one inplace");
    return m.ptr();
}

but when I do

i=1
ex.plus_one_inplace(i)

i is still equal to 1

[wjakob]

You can't. Try writing a function like that in Python and see if you can get it to work ;)

[ianhbell]

But passing arguments by reference is a very common idiom in C/C++. So I think it would be nice if the docs at least said:
a) You can't do it
b) Provided a small example showing how to work around this limitation

[wjakob]

Yeah, adding it to the FAQ now. I fear that there is no workaround in this case -- whatever you do, it has to make sense to the language itself and ints are immutable in Python.

[ianhbell]

Of course the obvious solution is to wrap everything in a class, which is then stateful, but I am trying to avoid that if I can

[wjakob]

faq added

[ianhbell]

Thanks for the FAQ. The "solution" for SWIG if you are using references as a means of returning multiple values is to use a typemap which returns a tuple rather than a single double. How would one go about that doing something like that in pybind11 without changing the C++ code?

[wjakob]

write a lambda function which calls your function and then bind that lambda function -- that kind of stuff is shown in the advanced section of the docs. There, simply return a tuple or pair, etc.

[ianhbell]

Us folks coming from the pre-C++11 world might need a little bit of assistance getting up to speed with this concept. A simple example in the advanced section would be appreciated.

[wjakob]

If you read through the advanced section, I think that this stuff should be pretty clear. If not, I can add something.

[ianhbell]

Yes, a simple example would be illustrative. For instance, let's suppose I have a trivial function (for illustrative purposes) like this:

void set_values(double &v1, double &v2){
    v1 = 42;
    v2 = 43;
}

how would I wrap that with a lambda to return the output values?

[wjakob]

updated -- please note that I generally don't have time for this kind of C++11 tutoring ;)

[ianhbell]

Fair enough.

I'm very, very appreciative of what you have done with pybind11. As a heavy user of both C++ and python, this is the nicest solution that I have seen thus far for bridging the gap. I've previously used SWIG and cython extensively, and I am considering moving all my interfaces to pybind11

The only gripe that I have is that C++11 is required, which means no visual studio compilation for the standard python distributions, and for windows you need to fight a bit with MINGW to get it to play nicely, especially when you link against static libraries compiled with MINGW. Getting the compilation flags right is a real pain.

[wjakob]

Compilation should "just" work (even with Visual Studio/MINGW) but obviously you'll need to have a C++11 capable version. (Visual studio 2015, not sure for MinGW but there are people who successfully used it)

[ianhbell]

Right, but the issues are on windows (as usual). Python 2.7 is build with Visual Studio 9 in most distributions, and some 3.x versions are built with Visual Studio 10, neither of which support C++11. Therefore, for these python versions, you MUST use mingw since Visual Studio 15 doesn't generate binaries compatible with either VS9 or VS10.