从其他 C++ 源调用 function 不起作用

Question

I've looked at several posts of the same questions on here and as far as I've figured I've done what they said to do. However, I still get a "undefined reference to `cmb::functionA()'" warning.

I have the header:

//combine.h
#ifndef COMBINE_H
#define COMBINE_H

namespace cmb 
{
    void functionA();
}

#endif

Function source file:

// combine.cc
#include <iostream>
#include "combine.h"
using namespace std;

namespace cmb
{
    void functionA()
    {
        cout << "print something\n"; 
    }
}

And main:

//main.cc
#include "combine.h"
using namespace std;
using namespace cmd;

int main(int argc, char *argv[])            
{   
    functionA();
}

It is now working when compiling manually (g++ -o Test *.cc -Wall --std=c++17) but using make still gives me the same error. I really don't understand make files so any help would be appreciated.

makefile:

CXX := g++
CXXFLAGS += -Wall -std=c++17

LIBSRCS = $(filter-out main.cc,$(shell find -name \*.cc))
LIBOBJS = $(patsubst %.cc,%.o,$(LIBSRCS))

main: main.o combine.o libproject.a
    $(CXX) $(CPPFLAGS) $(CXXFLAGS) $(LDFLAGS) -o $@ $<

$(LIBOBJS): %.o: %.cc
    $(CXX) $(CPPFLAGS) $(CXXFLAGS) -c -o $@ $<

libproject.a: $(LIBOBJS)
    ar rcs $@ $^
clean:
    rm -f libproject.a $(LIBOBJS)
.PHONY: clean

I just use make main in terminal.

Answer 1

You must add combine.o after main: in the makefile.

Answer 2

Since you use a library, you need to tell the linker to use it (LDFLAGS), and it should be after the main in g++ command. As in previous comments, the using namespace cmd needed to be changed to cmb

This one worked for me:

CXX := g++
CXXFLAGS += -Wall -std=c++17

LIBSRCS = $(filter-out ./main.cc,$(shell find -name \*.cc))
LIBOBJS = $(patsubst %.cc,%.o,$(LIBSRCS))
LDFLAGS += -L. -lproject

main: main.o libproject.a
    $(CXX) $(CPPFLAGS) $(CXXFLAGS) -o $@ $< $(LDFLAGS) 

libproject.a: $(LIBOBJS)
    ar rcs $@ $^

.PHONY: clean
clean:
    rm -f libproject.a $(LIBOBJS) main main.o

I also needed to add./ in filtering out main.cc

Example run:

jontte@jontte-Latitude-E5420:~/Temp/maketest$ make
g++ -Wall -std=c++17   -c -o main.o main.cc
g++ -Wall -std=c++17   -c -o combine.o combine.cc
ar rcs libproject.a combine.o
g++  -Wall -std=c++17 -o main main.o -L. -lproject 
jontte@jontte-Latitude-E5420:~/Temp/maketest$ ./main 
print something
jontte@jontte-Latitude-E5420:~/Temp/maketest$ make clean
rm -f libproject.a ./combine.o main main.o
jontte@jontte-Latitude-E5420:~/Temp/maketest$