What's wrong with my Makefile or C++ Program?

Question

I write a Makefile to compile my C++ program, the Makefile code is as follow:

  1 TARGET = main
  2 
  3 CXX = g++
  4 CXXFLAGS = -Wall -g
  5 HEADER = $(wildcard ./*.h)
  6 SRCS = $(wildcard *.cpp)
  7 OBJS = $(patsubst %.cpp, %.o, $(SRCS))
  8 RM = rm -f
  9 
 10 $(TARGET):$(OBJS)
 11     $(CXX) -o $@ $^
 12 
 13 $(OBJS):$(SRCS)
 14     $(CXX) $(CXXFLAGS) $< -o $@
 15 
 16 
 17 .PHONY:clean
 18     clean:
 19     $(RM) $(OBJS) $(TARGET)

The file structure of the current directory is as follow:

.
├── main.cpp
├── Makefile
├── MyQueue.cpp
└── MyQueue.h
0 directories, 4 files

I use g++ main.cpp MyQueue.cpp -o queue to compile the program, the result is right; BUT I use make to compile the program the results are as follows:

g++ -Wall -g  main.cpp -o main.o
/tmp/ccA6CmCE.o: In function `main':
/home/tzk/DSA/main.cpp:7: undefined reference to `MyQueue::MyQueue(int)'
collect2: error: ld returned 1 exit status
make: *** [main.o] Error 1

I have defined the MyQueue(int)!The code of MyQueue.h,MyQueue.cpp and main.cpp are as follows:

MyQueue.h
  1 #ifndef MYQUEUE_H_
  2 #define MYQUEUE_H_
  3 class MyQueue
  4 {
  5     public:
  6         MyQueue(int queueCapacity);
  7         virtual ~MyQueue();
  8         void ClearQueue();
  9         bool QueueEmpty() const;
 10         bool QueueFull() const;
 11         int QueueLength() const;
 12         bool EnQueue(int element);
 13         bool DeQueue(int& element);
 14         void QueueTraverse();
 15 
 16     private:
 17         int *m_pQueue;
 18         int m_iQueueLen;
 19         int m_iQueueCapacity;
 20         int m_iHead;
 21         int m_iTail;
 22 };
 23 
 24 #endif

MyQueue.cpp
  1 #include "MyQueue.h"
  2 #include <iostream>
  3 using namespace std;
  4 
  5 MyQueue::MyQueue(int queueCapacity)
  6 {
  7     m_iQueueCapacity = queueCapacity;
  8     m_iHead = 0;
  9     m_iTail = 0;
 10     m_iQueueLen = 0;
 11     m_pQueue = new int[m_iQueueCapacity];
 12 }

main.cpp
  1 #include <iostream>
  2 #include <cstdlib>
  3 #include "MyQueue.h"
  4 
  5 int main(void)
  6 {
  7     MyQueue *p = new MyQueue(4);
  8 
  9     delete p;
 10     p = NULL;
 11 
 12     return 0;
 13 }

So What's wrong with my program or Makefile ?

==========================================================

I try to add -c to the command as follows:

 13 $(OBJS):$(SRCS)
 14     $(CXX) $(CXXFLAGS) -c  $< -o $@

But, the result still wrong:

g++ -Wall -g  -c  main.cpp -o main.o
g++ -Wall -g  -c  main.cpp -o MyQueue.o
g++ -o main main.o MyQueue.o
MyQueue.o: In function `main':
/home/tzk/DSA/main.cpp:6: multiple definition of `main'
main.o:/home/tzk/DSA/main.cpp:6: first defined here
main.o: In function `main':
main.cpp:(.text+0x21): undefined reference to `MyQueue::MyQueue(int)'
MyQueue.o: In function `main':
main.cpp:(.text+0x21): undefined reference to `MyQueue::MyQueue(int)'
collect2: error: ld returned 1 exit status
make: *** [main] Error 1

Answer 1

When you create object files you need the -c option. You don't have it, which means g++ tries to create and link the executable file main.o.

---

Your target to build object files is also wrong. It should be e.g.

%.o: %.cpp
    $(CXX) $(CXXFLAGS) -c $< -o $@

Answer 2

You have to use the -c flag to turn source files in object files.

Like in

$(OBJS):$(SRCS)
    $(CXX) $(CXXFLAGS) -c $< -o $@