Actual Examples of RAII in embedded C++

Posted on 2 mins read

I have been using C++ in embedded microcontroller applications for over 10 years. Recently, I re-wrote all the frameworks I use to better leverage modern C++. One of the big changes was using RAII wherever possible. I have a compilation of actual use cases where RAII has been awesome.

But first:

Using RAII in the Real World

RAII lets you undo something you do without thinking about it. This comes in really handy when working on embedded systems: close something you opened, turn off something you turned on, free something you allocated, unlock something you locked and so on.

Critical Code Contexts

What is your approach for critical code sections that can’t be interrupted? Check out this CriticalContext class and example. RAII will automatically turn interrupts back on when critical_context goes out of scope. This means you don’t need to worry about re-enabling interrupts in multiple places.

class CriticalContext {
public:
  CriticalContext(){ disable_interrupts(); }
  ~CriticalContext(){ enable_interrupts(); }
}

void my_critical_function(int value){
  CriticalContext critical_context;

  switch(value){
    case 0:
    // do something then abort
    return;
    case 1:
    // do something else
    break;
    case 2:
    // another thing
    break;
    default:
    break;
  }

  if( value > 1000 ){
    return;
  }

  //do the last thing
  return;
}

Power Mangement

You can also use RAII to guarantee you turn off a device when you are done accessing it. Say you have a GPIO line that controls a power source that you only want enabled when you are accessing a device. RAII can automatically turn off the power when the context goes out of scope.

class PowerContext {
public:
  PowerContext(){ enable_power(); }
  ~PowerContext(){ disable_power(); }
}

int my_function(){
  PowerContext pc;
  return read_sensor();
}

Performance Profiling

Have you ever needed to track how long snippets of code take to execute. You can use RAII.

class PerformanceTimerContext {
public:
  PerformanceTimerContext(const char * name) : m_name(name){ 
    printf("start %s context --->\n", name);
    m_timer.start(); 
  }
  ~PerformanceTimerContext(){ 
    timer.stop(); 
    printf("stop %s context: %ld us--->\n", , m_timer.microseconds());
  }

private:
  const char * m_name;
  Timer m_timer;
}

void my_function(){
  PerformanceTimerContext ptc("file");
  int fd;
  {
    PerformanceTimerContext ptc("openFile");
    fd = open("/home/my_file.txt", O_RDWR);
  }

  {
    char buffer[64];
    PerformanceTimerContext ptc("readFile");
    read(fd, buffer, 64);
  }

    {
    PerformanceTimerContext ptc("closeFile");
    close(fd);
  }
}

And More

By now, I think you have gotten the idea. RAII is used extensively in the Stratify API. Other examples include: