Makefile to compile a class with main opencv - c++

Question

I hope you're all fine.
I'm learning opencv (c++) currently and I'm trying to compile a "main.cpp" with a class called "Algorithms"(.h and .cpp) and for some reason I get an error like this one when trying :
g++ `pkg-config --cflags opencv4` `pkg-config --libs opencv4` src/Algorithms.cpp -o obj/Algorihtms.o -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_imgcodecs
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o: in function `_start':
(.text+0x24): undefined reference to `main'
collect2: error: ld returned 1 exit status
make: *** [makefile:15: obj/Algorithms.o] Error 1
Here is my makefile:
CFLAGS = `pkg-config --cflags opencv4`
LIBS = `pkg-config --libs opencv4`
B = bin
O = obj
S = src
FLAGS = -c -Wall
all: $(O) $(B) $(B)/main
$(B)/main: $(O)/Algorithms.o
g++ $(FLAGS) -ggdb $(S)/main.cpp -o $(O)/main.o
$(O)/Algorithms.o:
g++ $(CFLAGS) $(LIBS) $(S)/Algorithms.cpp -o $(O)/Algorihtms.o -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_imgcodecs
$(O):
mkdir $(O)
$(B):
mkdir $(B)
clean: $(O) $(B)
rm -rf $(O)
rm -rf $(B)
(Opencv is installed and I know that if I compile "main.cpp" alone it works, but I can't get "main.cpp" and the class "Algorithm" to compile simultaneously.)
Thanks a lot.

Answer 1

If you do not want to link to an executable, you need to make GCC aware. The default is to compile and link. Use option -c:
$(O)/Algorithms.o:
g++ -c $(CFLAGS) $(LIBS) $(S)/Algorithms.cpp -o $(O)/Algorihtms.o -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_imgcodecs
From the manpage:
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The
"overall options" allow you to stop this process at an intermediate stage. For example, the -c
option says not to run the linker. Then the output consists of object files output by the
assembler.
However in your case, as mentioned in my comment, the question is why you need an intermediate object file at all.

Answer 2

Answering to my own question.
I don't know why I didn't figure that out immediately in my makefile.
Whatever, here is a working makefile that does the job :
CFLAGS = `pkg-config --cflags opencv4`
LIBS = `pkg-config --libs opencv4`
B = bin
O = obj
S = src
FLAGS = -c -Wall
all: $(O) $(B) $(B)/main
$(B)/main: $(O)/Algorithms.o
g++ $(CFLAGS) $(LIBS) $(S)/main.cpp -o $(B)/main $(O)/*.o -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_imgcodecs
$(O)/Algorithms.o:
g++ $(FLAGS) $(CFLAGS) $(LIBS) $(S)/Algorithms.cpp -o $(O)/Algorithms.o
$(O):
mkdir $(O)
$(B):
mkdir $(B)
clean: $(O) $(B)
rm -rf $(O)
rm -rf $(B)
I'm sure this makefile is not the most efficient and it can be improved in many ways.