i2c_slave.cpp

i2c_master.cpp

void SDAcallback(void) __attribute__((weak));
void SDAcallback(void)
{
  //weak functiom, used only whwn pure-master case is used. When slave is linked in, this function 
  //gets replaced by a strong-attributed one that is empty.
  //Master collision detection logic goes here
  //...
}

i2c_slave.cpp

void SDAcallback(void)
{
  //strong function, empty of code
  //replaces weak function in master to remove it for slave case.
}