Undefined reference to function from a different header file c++ [duplicate] - c++

Question

This question already has answers here:
Linking files in g++
(4 answers)
Why can templates only be implemented in the header file?
(17 answers)
Closed 8 years ago.
I am simply trying to use a function whose prototype is declared in a separate header file (ML_hash.h) and whose declaration is made in separate cpp file. I am trying to call this function in a different header file (HashNode.h). Here is the relevant code:
HashNode.h:
#ifndef HASHNODE_H
#define HASHNODE_H
#include "ML_hash.h"
template < typename T >
class HashNode
{
... // function prototype declarations
};
template< typename T >
void HashNode< T >::insert(int key, T* object)
{
...
int retVal = ML_hash(1, 3);
...
}
...
#endif
ML_hash.h:
#ifndef INC_ML_HASH
#define INC_ML_HASH
int ML_hash(int level, int key );
#endif
The error I am getting is:
g++ -o hash Hashtest.o:
Hashtest.o: In function `HashNode<int>::insert(int, int*)':
/home/adalal1/programs/class/final_project/HashNode.h:72: undefined reference to ' ML_hash(int, int)'
/home/adalal1/programs/class/final_project/HashNode.h:88: undefined reference to ` ML_hash(int, int)'
Hashtest.o: In function `HashNode<int>::explode()':
/home/adalal1/programs/class/final_project/HashNode.h:117: undefined reference to ` ML_hash(int, int)'
collect2: error: ld returned 1 exit status
What I don't understand is why the C++ compiler doesn't recognize the ML_hash function defined in ML_hash.cpp. I did include ML_hash.h in that cpp file. Could anyone provide insight into why this is happening?
EDIT:
I compile the code with a Makefile shown below:
C++ = g++
CFLAGS = -c -g
all: hash
hash: Hashtest.o
$(C++) -o hash Hashtest.o
clean:
rm -f *.o
%.o: %.cpp
$(C++) $(CFLAGS) $*.cpp

Answer 1

'ld returned 1 exit status' -> this is a linker error, not a compiler error.
Looks like you are running into this issue:
Linking files in g++
EDIT:
Either
g++ -o hash ML_hash.cpp HashNode.cpp HashTest.cpp
or
g++ -c ML_hash.cpp
g++ -c HashNode.cpp
g++ -c HashTest.cpp
g++ -o hash ML_hash.o HashNode.o HashTest.o
EDIT2 with OP edit:
I'm no makefile expert, but it looks like the 'hash:' target is just missing ML_hash.o and HashNode.cpp
hash: HashNode.o
ML_hash.o
Hashtest.o
$(C++) -o hash Hashtest.o ML_hash.o HashNode.o