bytedeco/javacpp

Cppbuild Control Callback Error

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#771 opened on Jul 12, 2024

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Description

We are trying to wrap Franka Emika's libfranka library, but are getting this issue when defining a class called ControlCallback in our libfrankaConfig.java file. We have the following code:

`public static class ControlCallback extends FunctionPointer {
        static { Loader.load(); }
        public    ControlCallback(Pointer p) { super(p); }
        protected ControlCallback() { allocate(); }
        private native void allocate();
        public native Torques call(@ByRef @Const RobotState robotState, Duration duration);
    }`

In libfranka, ControlCallback represents an argument in the Robot class's control method. It should be of type std::function<franka::Torques(const franka::RobotState&, franka::Duration)>. We get the following error during runs of cppbuild.bash: In function ‘void Java_us_ihmc_libfranka_Robot_control__Lus_ihmc_libfranka_libfrankaConfig_00024ControlCallback_2(JNIEnv*, jobject, jobject)’: error: no matching function for call to ‘franka::Robot::control(JavaCPP_us_ihmc_libfranka_libfrankaConfig_00024ControlCallback&)’ 5293 | ptr->control(*ptr0); | ^ In file included from /home/franka/IdeaProjects/repository-group/libfranka-java/cppbuild/include/franka/model.h:8, note: candidate: ‘void franka::Robot::control(std::function<franka::Torques(const franka::RobotState&, franka::Duration)>, bool, double)’ 172 | void control(std::function<Torques(const RobotState&, franka::Duration)> control_callback, | ^~~~~~~ no known conversion for argument 1 from ‘JavaCPP_us_ihmc_libfranka_libfrankaConfig_00024ControlCallback’ to ‘std::function<franka::Torques(const franka::RobotState&, franka::Duration)>’ 172 | void control(std::function<Torques(const RobotState&, franka::Duration)> control_callback,

Contributor guide

Tech stack
javacpp
Domain
backendtooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
JavaC++JNIJavaCPP basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
The issue is a type mismatch when wrapping a C++ std::function callback with JavaCPP. The JavaCPP generated code attempts to pass a JavaCPP FunctionPointer to the franka::Robot::control method, which expects a std::function. Investigate how JavaCPP handles std::function and whether a custom adapter or a different mapping approach is needed. Look at existing JavaCPP examples for callback handling and the generated C++ code in the build output. Consider if the ControlCallback class should implement a different interface or use a Typedef to match the expected signature.