Makefile for linking an extern global - c++

Question

I'm trying to use the g_struct variable that's defined in struct.cpp and declared in struct.hpp inside the test.cpp, but the linking fails. Why is that?
// test.cpp
#include "struct.hpp"
int main(void)
{
g_struct.a = 1;
return 0;
}
// struct.cpp
#include "struct.hpp"
Struct g_struct;
// struct.hpp
#pragma once
struct Struct {
int a;
};
extern Struct g_struct;
# Makefile
CC = g++
all: struct.o test
test: test.cpp
$(CC) -o test test.cpp
struct.o: struct.cpp
$(CC) -c -o struct.o struct.cpp
.PHONY: clean
clean:
rm -f *.o test
Linking error:
$ make
g++ -c -o struct.o struct.cpp
g++ -o test test.cpp
/tmp/ccgSoJhd.o: In function `main':
test.cpp:(.text+0xd): undefined reference to `g_struct'
collect2: error: ld returned 1 exit status
Makefile:6: recipe for target 'test' failed
make: *** [test] Error 1
If I link without the -c flag, I get:
$ make
g++ -o struct.o struct.cpp
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
Makefile:9: recipe for target 'struct.o' failed
make: *** [struct.o] Error 1

Answer 1

You have to link struct.o, which has Struct g_struct;.
test: test.cpp
$(CC) -o test test.cpp struct.o