Re: [Boost-users] Scoped Pairs of functions?

19 Aug 2011

      On Thu, Aug 18, 2011 at 1:04 PM, Nat Linden <nat@lindenlab.com> wrote:
...
On Wed, Aug 17, 2011 at 10:49 AM, Brian Allison
<brian.w.allison@gmail.com> wrote:
...
I'm on a project which uses a C++ compiler, but with a lot of C-isms.
One of those is that there are objects with pairs of functions which
allocate/deallocate resources. I don't have time to fix all of the design
issues with the classes that do this (yet), but within one compilation unit
I'd like to remove the chance for leaks.
So - if I wanted to use boost types to wrap around 2 functions f() and
g(), where each function
has a void or primitive return type
takes zero or 1 argument
where f() is called at some point in a scope, and then g() is called on
exiting the scope from which f() is called and only if f() doesn't throw..
has its return value ignored. [Gahh....]
Is there a boost::bind (or lambda) expression that I can wrap in a type?
How do I generalize to include the possibility of a single argument?
The following works for me with gcc 4.0.1 (yeah, I know, old compiler):
#include <iostream>
#include <boost/function.hpp>
#include <boost/bind.hpp>
void acquire_fixed()
{
   std::cout << "Acquiring fixed resource\n";
}
void release_fixed()
{
   std::cout << "Releasing fixed resource\n";
}
void acquire(const std::string& resource)
{
   std::cout << "Acquiring " << resource << '\n';
}
void release(const std::string& resource)
{
   std::cout << "Releasing " << resource << '\n';
}
class ScopedResource
{
public:
   typedef boost::function<void()> ResourceFunc;
   ScopedResource(const ResourceFunc& acq, const ResourceFunc& rel):
       mRelease(rel)
   {
       acq();
   }
virtual ~ScopedResource()
   {
       mRelease();
   }
private:
   ResourceFunc mRelease;
};
int main(int argc, char *argv[])
{
   {
       ScopedResource fixed(acquire_fixed, release_fixed);
       std::cout << "during lifespan of 'fixed'\n";
   }
   {
       ScopedResource something(boost::bind(acquire, "something"),
                                boost::bind(release, "something"));
       std::cout << "during lifespan of 'something'\n";
   }
   return 0;
}
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Is there a way to generalize the return type so that ScopedResource could be
in a library and the user could use it for pairing two functions that have
int and void return types respectively?

I'm not familiar with boost::function, so I don't know if I could mod the
class and typedef to:

template *<type R1 = void, type R2 = void>*
class ScopedResource
{
public:*
   typedef boost::function<R1()> ResourceFunc1;  *  // will this work?*
   typedef boost::function<R2()> ResourceFunc2; *   // will this work?*
*

and the object declarations as

   {
       ScopedResource fixed(acquire_fixed, release_fixed);
       std::cout << "during lifespan of 'fixed'\n";
   }
   {
       ScopedResource*<void, int>* something(*f_that_returns_int,
g_with_void_return*);
       std::cout << "during lifespan of 'something'\n";
   }