Question

I’ve just started making libraries in Arduino. I’ve made a library named inSerialCmd. I want to call a function named delegate() that is defined in the main program file, stackedcontrol.ino, after the inSerialCmd library is included.

When I try to compile, one error is thrown:

…\Arduino\libraries\inSerialCmd\inSerialCmd.cpp: In member function
‘void inSerialCmd::serialListen()’:
…\Arduino\libraries\inSerialCmd\inSerialCmd.cpp:32: error:
‘delegate’ has not been declared

After doing a bit of searching, it seemed that adding the scope resolution operator might do the trick. So I added the “::” before delegate(), now “::delegate()”, but the same error is thrown.

Now I’m stumped.

Answer 1

You cannot and should not directly call a function in a program from a library. Keep in mind a key aspect that makes a library into a library:

A library does not depend on the specific application. A library can be fully compiled and packaged into the .a file without the existence of a program.

So there is a one way dependency, a program depends on a library. This at first glance may seem to prevent you from achieving what you want. You can achieve the functionality you are asking about through what is sometimes referred to as a callback. The main program would provide to the library at runtime a pointer to the function to execute.

// in program somwehere
int myDelegate(int a, int b);

// you set this to the library
setDelegate( myDelegate );

You see this in the arduino if you look at how interrupt handlers are installed. This same concept exists in many environments – event listeners, action adapters – all with the same goal of allowing a program to define the specific action that a library cannot know.

The library would store and call the function via the function pointer. Here is a rough sketch of what this looks like:

// in the main program
int someAction(int t1, int t2) {
  return 1;
}

/* in library
this is the delegate function pointer
a function that takes two int's and returns an int */
int (*fpAction)(int, int) = 0;   

/* in library
this is how an application registers its action */
void setDelegate( int (*fp)(int,int) ) {
  fpAction = fp; 
}

/* in libary
this is how the library can safely execute the action */
int doAction(int t1, int t2) {
  int r;
  if( 0 != fpAction ) {
    r = (*fpAction)(t1,t2);
  }
  else {
    // some error or default action here
    r = 0;
  }
  return r;
}

/* in program
The main program installs its delegate, likely in setup() */
void setup () {
  ...      
  setDelegate(someAction);
  ...