[maccard]

If you disable exceptions hoe do you handle failures in constructors?

[dgfgfdagasdfgfa]

I mean it's kind of a smartass answer, but—don't fail during constructors. Move all possible code that can fail into an initialize method; check explicitly for allocation failure and/or put things on the stack instead of heap when possible; consider failing hard with a stack trace or core dump over catching and processing exceptions before (likely) failing anyway.

[dan00]

Instead of using a constructor you can use a constructor method e.g.:

    class Foo {
       public:
          static std::optional<Foo> create();

       private:
          Foo();
    };

[maccard]

In general if you're not using exceptions, you're not going to be using features that haven't actually been published in a formal standard (optional).

This now means that you can't use any constructors, so how do you have Containers of foo?

[imron]

> you're not going to be using features that haven't actually been published in a formal standard (optional).

So you then have things like:

  class Foo
  {
  public:
    static Foo* create();
    ...
  };
  ...
  Foo* foo = Foo::create()
  if ( foo != nullptr ) ...

> so how do you have Containers of foo?

std::vector<Foo*>

Not saying either of those are better than the alternative (I prefer using exceptions and RAII), just pointing out what I've seen in real world projects.

[mikepurvis]

I don't think this is the proposal. The proposal is that the object contains a genuine constructor that only does the bare-bones "safe" stuff, and then it has a separate non-static method that does the might-fail initialization. So:

  class Foo
  {
  public:
    Foo();
    bool initialize();  // returns success
    ...
  };
  ...
  Foo foo;
  if ( !foo->initialize() ) { // handle error }

This also means you can break up your initialization so that you drive the risky pieces from outside the object, rather than monolithically from within.

This has a further benefit for testing, since you can use your major objects without fully initializing the entire world that they depend on.

[imron]

It was almost the exact proposal specified by my grandparent post except using a pointer rather than an optional.

It's also a technique that is widely used. See for example the cocos2d-x game library.

The benefit of such a technique is that you can then make the constructor private, making it impossible to create an object and not also call the initialize() method.

[gpderetta]

It is very different from having a static method returning an optional, which guarantees that you can access the optional (with at least an assert in debug mode) only if the object is actually successfully constructed.

Using a separate initialize member function means that you may have objects in a zombie state laying around after a failed construction which lead to all kind of initialization order issues (you might get a pointer to the object, but is it initialized?). Also you need to remember to check the return type, which also need to be meaningful (does it return false on failure? or it returns 0 on success?).

Two phase initialization is a known antipattern which is, unfortunately, widely used and lead to all kind of pains.

Friends do not let friends use 2PI.

edit: sorry, I misread your comment, you were referring to the static function returning a pointer, which as you note is almost the same as the optional version. It forces heap allocation though, which is bad.

[dan00]

My bad, I thought std::optional is part of C++14, it seems to be part of the next standard C++17, but there's still boost::optional.

About the container issue: if you have objects that might fail during the creation it seems like a bad idea to allow things like:

    std::vector<Foo> foos(10);

Having a separate initialization method which might fail - like proposed by others - is another option, but this means your objects need some kind of internal initialization state, and whenever you're handling such an object you never can be absolute sure that it's in a valid state.

I'm quite a big fan of making invalid state not representable in an object and handling failure cases as early as possible.

What the create method returns depends heavily on your use case. If the returned objects can always be allocated on the heap, then a pointer or unique_ptr can be returned.

[ryandrake]

How would you handle failure in copy constructors?

[dan00]

Most likely having an explicit copy method instead of a copy constructor.