1

Let's say I have a simple class in hello.h

#ifndef LIB_HELLO_GREET_H_
#define LIB_HELLO_GREET_H_

class A{
public:
    int a = 0;
    int b = 0;
    int add(){
        return a+b;
    }
};

#endif

with bazel build file:

load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library")

cc_library(
    name = "hello",
    hdrs = ["hello.h"],
)

cc_binary(
    name = "hello.so",
    deps = [
        ":hello",
    ],
    linkshared=True,
    linkstatic=False
)

After I run bazel build hello.so, there is a shared object file generated in bazel-bin/main and bazel-bin/main/hello.so.runfiles/__main__/main/hello.so. Using those files, I want call class A with a python script. Ideally, I'd want to use cppyy or something similar.

I've tried with simple python scripts

import cppyy
cppyy.load_reflection_info('hello')
print(dir(cppyy.gbl))

or

import cppyy
cppyy.load_library('hello')
print(dir(cppyy.gbl))

with both .so files, but cppyy can't seem to detect class A - it is never inside cppyy.gbl

I'm wondering the best way to solve this problem, any help appreciated!

checkmark1234321
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1 Answers1

0

With cppyy, you also need to give it the header file, that is:

cppyy.include("hello.h")

and optionally use cppyy.add_include_path() to add the directory where hello.h resides, so that it can be found.

An alternative is to use so-called "dictionaries." These package (customizable) directory locations, names of necessary headers files and (through linking) the shared library with the implementation into a single, new, shared library (named "dictionary" because of history). There can be a so-called "rootmap" file on the side that allows a class loader to find A automatically on first use. See the example in the documentation.

Wim Lavrijsen
  • 3,453
  • 1
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  • Thanks - I'm running into trouble with bazel dependencies when I try `cppyy.include("hello.h")` though. For example, if my build target had some external dependency such as `deps=["@some_lib/some_class"]`, while I include via `#include `, then `cppyy.include("hello.h")` will error out with `some_lib/some_class.h not found` – checkmark1234321 Mar 29 '22 at 06:40
  • The absolute directory may not be known as the installer can customize it, but the relative layout of the package after installation should be as you control that. The bindings code is going to live in some python module, which will have a `__file__` attribute. From there you can create the absolute path knowing the relative one from the python module to the header, given the package layout. Hand the result to `cppyy.add_include_path()`. – Wim Lavrijsen Mar 29 '22 at 16:52