linker error: undefined symbol

Question

What I'm trying to do

I am attempting to create 2 c++ classes.

• One, named Agent that will be implemented as a member of class 2
• Two, named Env that will be exposed to Python through boost.python (though I suspect this detail to be inconsequential to my problem)

The problem

After successful compilation with my make file, I attempt to run my python script and I receive an import error on my extension module (the c++ code) that reads "undefined symbol: _ZN5AgentC1Effff". All the boost-python stuff aside, I believe this to be a simple c++ linker error.

Here are my files:

Agent.h

class Agent {
public:
    float xy_pos[2];
    float xy_vel[2];
    float yaw;
    float z_pos;
    Agent(float x_pos, float y_pos, float yaw, float z_pos);

};

Agent.cpp

#include "Agent.h"
Agent::Agent(float x_pos, float y_pos, float yaw, float z_pos)
{
   xy_vel[0] = 0;
   xy_vel[1] = 0;
   xy_pos[0] = x_pos;
   xy_pos[1] = y_pos;
   z_pos = z_pos;
   yaw = yaw;
};

test_ext.cpp (where my Env class lives)

#include "Agent.h"
#include <boost/python.hpp>
class Env{
    public: 
        Agent * agent;
        //some other members
        Env() {
            agent = new Agent(13, 10, 0, 2);
        }
        np::ndarray get_agent_vel() {
        return np::from_data(agent->xy_vel, np::dtype::get_builtin<float>(),
                                 p::make_tuple(2),
                                 p::make_tuple(sizeof(float)),
                                 p::object());
         }
         void set_agent_vel(np::ndarray vel) {
             agent->xy_vel[0] = p::extract<float>(vel[0]);
             agent->xy_vel[1] = p::extract<float>(vel[1]);
         }
}

BOOST_PYTHON_MODULE(test_ext) {
    using namespace boost::python;
    class_<Env>("Env")
    .def("set_agent_vel", &Env::set_agent_vel)
    .def("get_agent_vel", &Env::get_agent_vel)
}

Makefile

PYTHON_VERSION = 3.5

PYTHON_INCLUDE = /usr/include/python$(PYTHON_VERSION)

# location of the Boost Python include files and library

BOOST_INC = /usr/local/include/boost_1_66_0

BOOST_LIB = /usr/local/include/boost_1_66_0/stage/lib/

# compile mesh classes

TARGET = test_ext

CFLAGS = --std=c++11

$(TARGET).so: $(TARGET).o

    g++ -shared -Wl,--export-dynamic $(TARGET).o -L$(BOOST_LIB) -lboost_python3 -lboost_numpy3 -L/usr/lib/python3.5/config-3.5m-x86_64-linux-gnu -lpython3.5 -o $(TARGET).so


$(TARGET).o: $(TARGET).cpp Agent.o

    g++ -I$(PYTHON_INCLUDE) -I$(BOOST_INC) -fPIC -c $(TARGET).cpp $(CFLAGS)

Agent.o: Agent.cpp Agent.h

    g++ -c -Wall Agent.cpp $(CFLAGS)

Answer 1

You never link with Agent.o anywhere.

First of all you need to build it like you build test_ext.o with the same flags. Then you need to actually link with Agent.o when creating the shared library.