invoke functions defined in other files in c ++ [duplicate]

I have the need to include a function defined in an r.cpp file in another called a.cpp, I tried the following:

In the r.cpp file:

    #include "r.h"

    void lee(){
     cout<<" Hola "<<endl;
    }

In the r.h file:

    #ifndef R_H
    #define R_H

    void leer();

    #endif

In the a.cpp file:

    #include <iostream>
    #include "r.h"
    using namespace std;
    int main(){
      lee();
      return 0;
    }

Compiled in the console as follows:

g++ a.cpp

But I threw the following error:

  

/tmp/ccFRVxGt.o: In function main': a.cpp:(.text+0x5f): undefined reference to reads () 'collect2: error: ld returned 1 exit status

How could I solve it?