linker error: undefined symbol - c++

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.