Month: June 2010

The Simplest Possible Introduction to Traits


This is the simplest introduction to traits I could think of:

A trait is information about a type stored outside of the type.

Imagine you are making a templated algorithm, and want to use different policies for different types. If you wrote those types, you could implement the policy selection in your types directly, but this is generally not possible for built in types and types you haven’t created yourself. Traits to the rescue!

Given

template <class T>
void f(T var) {
    bool advanced = policy_trait<T>::advanced;
    if ( advanced )
        cout << "Do it fancy!" << endl;
    else
        cout << "KISS." << endl;
}

you can define policies for whatever type you want by creating template specializations of policy_trait

template <>
struct policy_trait<char> {
    static const bool advanced = false;
};

That’s it! :)

(The original paper on traits is Traits: a new and useful template technique by Nathan C. Myers, but An introduction to C++ Traits by Thaddaeus Frogley is probably a more intuitive introduction.)

Why You Shouldn’t Throw in Destructors


In my post two weeks ago, What’s the Point of Function Try Blocks, I mentioned that you shouldn’t throw in destructors. Why exactly is this?

Imagine the following:

void f() {
  MyClass c;
  functionThatMightThrow();
}

If functionThatMightThrow throws, stack unwinding occurs. What this means is basically that all the objects on the stack get destructed, from the innermost block and out. In this case, c is destroyed before the exception is thrown out of f.

But what happens if ~MyClass() throws? There would now be two simultaneously active exceptions, something that luckily is forbidden by the standard. Instead, terminate() is called, and the whole program stops.

So what can you do? Design around it. Log and swallow. Ignore. Abort. If your class is managing some resource that should be cleaned up, but failing to do so is not critical, you could also provide a close() method that throws on error. A client who feels it is necessary to be 100% sure the resource is cleaned up will then have the option of calling close() and handling the exception himself, before your destructor is called.

Note that relying on the client to remember to call close(), setUp(), init() etc. is generally a bad idea, but might be acceptable in some cases. What makes it acceptable in this case is if cleaning up the resource is optional.

Why Algorithms Need two Iterators


When using algorithms in C++, such as find(), sort(), etc., you might wonder why you need to specify both the beginning and the end of the container, and not just a reference to the container itself. See for instance:

vector<int> v; //Assume it also has some elements
sort(v.begin(), v.end());

Wouldn’t it be better if we could just write

sort(v);

and let sort call begin() and end()?

If this was the case, algorithms would only work on containers that define begin() and end(). Algorithms in C++ do however work on any type that has an iterator. And an iterator is not a type, it is pure abstraction. Anything that behaves like an iterator is an iterator. If a type has a concept of the element currently pointed to, pointing to the next element and equality, it can be an iterator.

One important example is pointers. C++ algorithms work with standard arrays, because pointers behave as iterators. It would be impossible to do this:

int a[42]; //
sort(a);

How would sort get a hold of iterators (pointers) to the beginning and end of the array? The beginning is easy, but since the length of the array is lost when passed to a function, the end is not in sight. For this to work, one would have to have special versions of all algorithms, taking the size of the array as an extra argument. This would lead to a loss of generality, and double the amount of algorithms.

Passing two iterators however preserves generality, and can be used with anything that behaves like an iterator.

Note that algorithms are not defined to take a specific iterator type, like this:

void sort(iterator begin, iterator end); //All iterators inherit from iterator

Instead, they are defined like this:

template <class It>
void sort(It begin, It end); //The type of It can be whatever

First, this lets us use pointers as iterators. Pointers of course do not inherit from an iterator type. Second, this relieves us of runtime polymorphism (which could negatively impact both speed and size). If you try use a type that does not provide the necessary iterator operators, the compiler will catch it when sort() tries to use them.

This is typical for templates; with compile time polymorphism, anything that provides the interface used by the templated function is acceptable, regardless of what (if anything) it inherits from. This is often referred to as duck typing: “when I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.” This is also what allows you to make a vector of any type. Note that this is different from for instance Java, which uses inheritance for collections instead of duck typing. In Java, all classes inherit from Object, and you can only have collections of objects, not primitive types.

What’s the Point of Function Try Blocks


This post is a follow up to The Returning Function that Never Returned, which I wrote a couple of months ago. You can read it first if you want, it is only around a hundred words, but this post can be read on its own as well.

In my previous post, I presented a function with try outside of the braces. This is called a “function try block”. The example went like this: 

std::string foo() try {
    bar();
    return "foo";
} catch (...) {
    log("Unable to foo!")
}

This is completely useless, even dangerous. The try should be moved inside the function itself, like this:

std::string foo() {
    try {
        bar();
        return "foo";
    } catch (...) {
        log("Unable to foo!")
        //either rethrow or throw something else
    }
    //or make sure something is returned
}

So why were function try blocks introduced to the language in the first place? Have a look at this:

class A : public B{
    A(int x) : c(x){}
    C c;
}

How do you catch an exception thrown by Cs constructor? Having try inside the function block is too late. Initializing c inside the function block is not a solution either, since c will always be initialized before the body of the constructor, even if it is not mentioned in the initializer list. The only way to catch such an exception is to use a function try block on the constructor, and that is also why they were introduced. (Note that this argument is also valid if Bs constructor might throw.)

This is also the only sane case in which to use them. The other to candidates are regular functions and destructors, both of which I will get back to in a moment.

First, let’s have a look at what you can do with a function try block on a constructor. When the catch block reaches its end, it will rethrow the exception. It is impossible to swallow it. If you could swallow the exception, the code that tried to construct an object of this type would have no way of knowing that construction failed. A failed construction means the object doesn’t exist, and it doesn’t make sense to continue pretending nothing has happened. The only thing you can do is to throw an exception of another type, or cause a side effect (such as logging) and then retrhow. In particular, you cannot try to recover from the problem.

The same goes for destructors, you cannot swallow the exception. And since you really should avoid having destructors throw, using function try blocks on destructors is a bad idea.

Try blocks on regular functions behave a bit differently; if the end of the catch block is reached, the function will automatically return. But if you have a non-void function, this doesn’t make sense at all, as I mentioned in my previous post.

So in conclusion:

  1. Only use function try blocks for constructors.
  2. Don’t try to do anything else than rethrowing (possibly another type) or cause side effects like logging.

For a more in-depth discussion of this, have a look at Sutter’s Mill: Constructor Failures (or, The Objects That Never Were) by Herb Sutter. If you would like a recap of exception handling and constructor initializers thrown in, I recommend to start with Introduction to Function Try Blocks by Alan Nash.