c++ - Any RAII template in boost or C++0x

Question

Is there any template available in boost for RAII. There are classes like scoped_ptr, shared_ptr which basically work on pointer. Can those classes be used for any other resources other than pointers. Is there any template which works with a general resources.

Take for example some resource which is acquired in the beginning of a scope and has to be somehow released at the end of scope. Both acquire and release take some steps. We could write a template which takes two(or maybe one object) functors which do this task. I havent thought it through how this can be achieved, i was just wondering are there any existing methods to do it

Edit: How about one in C++0x with support for lambda functions

Answer 1

shared_ptr provides the possibility to specify a custom deleter. When the pointer needs to be destroyed, the deleter will be invoked and can do whatever cleanup actions are necessary. This way more complicated resources than simple pointers can be managed with this smart pointer class.

Answer 2

The most generic approach is the ScopeGuard one (basic idea in this ddj article, implemented e.g. with convenience macros in Boost.ScopeExit), and lets you execute functions or clean up resources at scope exit.

But to be honest, i don't see why you'd want that. While i understand that its a bit annoying to write a class every time for a one-step-aquire and one-step-release pattern, you are talking about multi-step-aquire and -release.
If its taken multiple steps, it, in my opinion, belongs in an appropiately named utility class so that the details are hidden and the code in place (thus reducing error probability).
If you weigh it against the gains, those few additional lines are not really something to worry about.

Answer 3

A more generic and more efficient (no call through function pointer) version is as follows:

#include <boost/type_traits.hpp>

template<typename FuncType, FuncType * Func>
class RAIIFunc
{
public:
   typedef typename boost::function_traits<FuncType>::arg1_type arg_type;
   RAIIFunc(arg_type p) : p_(p) {}
   ~RAIIFunc() { Func(p_); }
   arg_type & getValue() { return p_; }
   arg_type const & getValue() const { return p_; }
private:
   arg_type p_;
};

Example use:

RAIIFunc<int (int), ::close> f = ::open("...");

Answer 4

I have to admit I don't really see the point. Writing a RAII wrapper from scratch is ridiculously simple already. There's just not much work to be saved by using some kind of predefined wrapper:

struct scoped_foo : private boost::noncopyable {
  scoped_foo() : f(...) {}
  ~scoped_foo() {...}

  foo& get_foo() { return f; }

private:
  foo f;
};

Now, the ...'s are essentially the bits that'd have to be filled out manually if you used some kind of general RAII template: creation and destruction of our foo resource. And without them there's really not much left. A few lines of boilerplate code, but it's so little it just doesn't seem worth it to extract it into a reusable template, at least not at the moment. With the addition of lambdas in C++0x, we could write the functors for creation and destruction so concisely that it might be worth it to write those and plug them into a reusable template. But until then, it seems like it'd be more trouble than worth. If you were to define two functors to plug into a RAII template, you'd have already written most of this boilerplate code twice.

Answer 5

I was thinking about something similar:

template <typename T>
class RAII {
    private:
        T (*constructor)();
        void (*destructor)(T);
    public:
        T value;
        RAII(T (*constructor)(), void (*destructor)(T)) : 
                    constructor(constructor), 
                    destructor(destructor) {
            value = constructor();
        }
        ~RAII() {
            destructor(value);
        }
};

and to be used like this (using OpenGL's GLUquadric as an example):

RAII<GLUquadric*> quad = RAII<GLUquadric*>(gluNewQuadric, gluDeleteQuadric);
gluSphere(quad.value, 3, 20, 20)

Answer 6

Here's yet another C++11 RAII helper: https://github.com/ArtemGr/libglim/blob/master/raii.hpp

It runs a C++ functor at destruction:

auto unmap = raiiFun ([&]() {munmap (fd, size);});