Why undefiner reference? - c++

Question

In my test.cpp I have:
#include <iostream>
#include "first.h"
using namespace std;
int main ()
{
auto dliste = d::data_preparation::prepare_d(100);
cout << "Hello World!\n";
return 0;
}
In my first.h I have:
namespace d {
namespace data_preparation {
something;
std::vector<row<mdata::dliste>> prepare_d(int f);
something;
}
}
In my first.cpp I have:
#include "first.h"
something;
namespace d {
namespace data_preparation {
vector<row<mdata::dliste>> prepare_d(int f) {
vector<row<mdata::dliste>> dliste;
cout << f << '\n';
return dliste;
}
}
}
When I compile this I get:
undefined reference to
`d::data_preparation::prepare_d(int)'
EDITED
In my Makefile I have:
test: test.o
$(CXX) -o $# $(LDFLAGS) $^ $(LDLIBS)
Should I modify it somehow?

Answer 1

You have most likely forgot to link first.cpp to your executable.
Just run this commands (if you are using gcc):
g++ -c first.cpp -o first.o
g++ -c test.cpp -o test.o
g++ test.o first.o
Or just use the compact version:
g++ first.cpp test.cpp -o app
You should edit your Makefile along the lines of:
app: test.o first.o
$(CXX) $^ -o $# $(LDFLAGS) $(LDLIBS)
test.o: test.cpp
$(CXX) -c test.cpp -o first.o
first.o: first.cpp
$(CXX) -c first.cpp -o first.o
Notice: I'm forced to use 4 spaces for indentation but Makefile may require tabs instead.