Is there a favored idiom for mimicing Java's try/finally in C++?

Asked 2009-02-01T05:04:29.560

Been doing Java for number of years so haven't been tracking C++. Has finally clause been added to C++ exception handling in the language definition?

Is there a favored idiom that mimics Java's try/finally?

Am also bothered that C++ doesn't have an ultimate super type for all possible exceptions that could be thrown - like Java's Throwable class.

I can write:

try {
  // do something
} catch(...) {
  // alas, can't examine the exception
  // can only do cleanup code and perhaps rethrow, ala:
  throw;
}

ADDENDUM EDIT:

I ended up accepting the answer that had the most up votes, i.e., use destructors to do cleanup. Of course, from my own comments, it is clear I don't entirely agree with that. However, C++ is what it is and so in the application endeavor I have in mind, I'm going to more or less strive to adhere to common community practice. I'll use template classes to wrap resources that don't already have a class destructor (i.e., C library resources), thus bestowing on them destructor semantics.

NEW ADDENDUM EDIT:

Hmm, instead of finally then a closure feature perhaps? A closure combined with ScopeGuard approach (see one of the answers below) would be a way to accomplish cleanup with arbitrary actions and access to the cleanup code's outer scope context. Cleanup could be done in the idiom fashion that is seen in Ruby programming where they supply cleanup blocks when a resource is being opened. Isn't a closure feature being considered for C++?

Edit Report

2 Answers

Cleanup functions, themselves, are thoroughly lame. They have low cohesion, in that they are expected to perform a series of activities only related in when they happen. They have high coupling, in that they need to have their internals modified when the functions that actually do something are changed. Because of this, they're error-prone.

The try...finally construct is a framework for cleanup functions. It is a language-encouraged way to write lousy code. Moreover, since it encourages writing the same cleanup code over and over, it undermines the DRY principle.

The C++ way is far preferable for these purposes. The cleanup code for a resource is written precisely once, in the destructor. It's in the same place as the rest of the code for that resource, and therefore has good cohesiveness. The cleanup code doesn't have to be put into unrelated modules, and therefore this cuts down on coupling. It is written precisely once, when well designed.

Moreover, the C++ way is much more uniform. C++, with the smart pointer additions, handles all sorts of resources in the same way, while Java handles memory well and provides inadequate constructs to release other resources.

There are plenty of problems with C++, but this isn't one of them. There are ways in which Java is better than C++, but this isn't one of them.

Java would be much better off with a way to implement RAII instead of try...finally.

answered 2009-02-02T17:44:22.040

Using C++11 with its lambda expressions, I've recently started using the following code to mimic finally:

class FinallyGuard {
private:
  std::function<void()> f_;
public:
  FinallyGuard(std::function<void()> f) : f_(f) { }
  ~FinallyGuard() { f_(); }
};

void foo() {
  // Code before the try/finally goes here
  { // Open a new scope for the try/finally
    FinallyGuard signalEndGuard([&]{
      // Code for the finally block goes here
    });
    // Code for the try block goes here
  } // End scope, will call destructor of FinallyGuard
  // Code after the try/finally goes here
}

The FinallyGuard is an object which is constructed with a callable function-like argument, preferrably a lambda expression. It will simply remember that function until its destructor is called, which is the case when the object goes out of scope, either due to normal control flow or due to stack unwinding during exception handling. In both cases, the destructor will call the function, thus executing the code in question.

It is a bit strange that you have to write the code for the finally before the code for the try block, but apart from that it actually feels a lot like a genuine try/finally from Java. I guess one should not abuse this for situations where an object with its own proper destructor would be more appropriate, but there are cases where I consider this approach above more suitable. I discussed one such scenario in this question.

As far as I understand things, std::function<void()> will use some pointer indirection and at least one virtual function call to perform its type erasure, so there will be a

answered 2013-03-05T13:21:36.277

Is there a favored idiom for mimicing Java's try/finally in C++?

2 Answers

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