Apple M1 silicon error: ld symbol(s) not found for arch arm64

Question

I just wanna run a very simple c++ code on my macbook(M1). It shows the error as follows

>> g++ main.cpp -o main
Undefined symbols for architecture arm64:
  "sayhi()", referenced from:
      _main in main-636bbc.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

There are 3 files:

# main.copp
#include <iostream>
#include "method.h"
using namespace std;

int main(){
    sayhi();
    return 0;
}

# method.cpp
#include <iostream>
#include "method.h"
using namespace std;

void sayhi(){
    //cout << "hi" << endl;
}

# method.h
void sayhi();

Could anyone provide some suggestions?

If I delete the line "sayhi();" in main.cpp, it works. So, I'm quite sure the problem happens at the line "sayhi();".