Gotchas: C + classes

2018/11/06

Or writing in C using C++

I was recently introduced to C++ in my Internet of Things module where we were writing some firmware for our micro controller.

While we were using C++, the libraries we were using used lots of char *s, nothing from std at their interface level, and most of the time when they did define classes it was for a singleton instance only. So it was more like coding in C with a few classes for scoping than anything object orientated. We were also not taught any C or C++ so it was very easy to fall back on the familiar control structures of imperative programming from other languages and hope for the best. Unfortunately embedded systems don’t seem so good at telling you when you’re messing up your memory. This post will hopefully be the crash course in not reading garbled strings that I never had, and perhaps a reference going forward in my IoT module.

There was one place where objects came up a lot and that was Arduino’s String class.

The default ‘Sketch’ template in Arduino’s IDE is

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  // put your main code here, to run repeatedly:
}

You might notice that if you’re creating things in the setup() function and want them in the loop() function you’re going to have to make sure they’re still alive, and in scope. Arduino’s String class, much like you might expect of a C++ object if you learnt C++ and then started writing programs, owns its characters.

The const keyword

This will fail to compile.

int BAUD_RATE = 115200;
char *hello;

void setup() {
  Serial.begin(BAUD_RATE);
  // put your setup code here, to run once:
  hello = String(random(10)).c_str(); // const char *
}

void loop() {
  // put your main code here, to run repeatedly:
  String(random(10));
  delay(1000);
  Serial.println(hello);
}

Unlike every other programming language I’ve used const in C and C++ is part of the type, not a restriction on a variable.

My String went out of scope and I held a reference to it

int BAUD_RATE = 115200;
const char *hello;

void setup() {
  Serial.begin(BAUD_RATE);
  // put your setup code here, to run once:
  hello = String(random(10)).c_str();
}

void loop() {
  // put your main code here, to run repeatedly:
  String(random(10));
  delay(1000);
  Serial.println(hello);
}

This compiles, but if you read the documentation for c_str() this is a terrible idea. On my micro controller rather than generating a random number once and then printing it repeatedly we get a random number every loop!

In the setup function hello does indeed point to the char * that c_str() gives us, but the String object goes out of scope by the time we reach the loop and in fact points to the new String object we create just before in the loop every time. There is also no reason hello should be pointing to the String in the loop function :)

My struct’s struct is gone

int BAUD_RATE = 115200;

struct Foo {
  int bar;
};

struct FooBar {
  Foo *bar;
};

FooBar fooBar;

void setup() {
  Serial.begin(BAUD_RATE);
  // put your setup code here, to run once:
  Foo foo;
  foo.bar = 1;
  fooBar.bar = &foo;
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(1000);
  Serial.println(fooBar.bar->bar);
}

What does this print? 0. foo went out of scope.

I allocated my struct’s struct on the heap so it didn’t die

int BAUD_RATE = 115200;

struct Foo {
  int bar;
};

struct FooBar {
  Foo *bar;
};

FooBar fooBar;

void setup() {
  Serial.begin(BAUD_RATE);
  // put your setup code here, to run once:
  Foo *foo = new Foo();
  foo->bar = 1;
  fooBar.bar = foo;
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(1000);
  Serial.println(fooBar.bar->bar);
}

What does this print? 1. And now there will be a memory leak if you forget to delete foo when you’re done with fooBar. Memory leaks on embedded systems with very scarce memory will cause problems for you.

At this point I know enough to point you to RAII but not enough to explain it.