Accelerated C++: compiling the programs in chapter 4 - c++

Question

I wrote a Makefile to compile the files and output the file "main", which I will eventually execute by "./main". Below is my Makefile:
CXX=clang++
CXXFLAGS=-g -std=c++11 -Wall -pedantic
BIN=prog
SRC=$(wildcard *.cpp)
OBJ=$(SRC:%.cpp=%.o)
all: Student_info median grade main
main: main.cpp
$(CXX) $(CXXFLAGS) main.cpp -o main grade.o median.o Student_info.o
grade: grade.h grade.cpp
$(CXX) $(CXXFLAGS) grade.cpp -o grade.o
median: median.h median.cpp
$(CXX) $(CXXFLAGS) median.cpp -o median.o
Student_info: Student_info.h Student_info.cpp
$(CXX) $(CXXFLAGS) Student_info.cpp -o Student_info.o
clean:
rm -f *.o
rm -f main
However, it says
clang++ -g -std=c++11 -Wall -pedantic Student_info.cpp -o Student_info.o
Undefined symbols for architecture x86_64:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [Student_info] Error 1
What is wrong with my Makefile?

Answer 1

Student_info: Student_info.h Student_info.cpp
$(CXX) $(CXXFLAGS) Student_info.cpp -o Student_info.o
Looks like you want this to compile to an object file - without linking. To do this, you need to pass -c. Also, the name of the target should be Student_info.o:
Student_info.o: Student_info.h Student_info.cpp
$(CXX) $(CXXFLAGS) -c Student_info.cpp -o Student_info.o
Ditto for grade and median.
main should depend on not only main.cpp but also the object files you are linking it with:
main: main.cpp Student_info.o grade.o median.o
$(CXX) $(CXXFLAGS) main.cpp -o main grade.o median.o Student_info.o