[boost] try... catch... finally?

30 Mar 2007

      Hi,

You've probably seen "modern" languages that incorporate the keyword  
finally, but nothing such exists in C++.  I've come with a simple  
class that could perform close to that keyword. The main exception is  
that you have to define what finally does BEFORE the try catch  
clause, which is kind of counter-intuitive but not hurtful.

The obvious advantages of such a class over a smart_ptr<X> (X could  
be void*) is that you can use Boost.Bind (cf my code sample), and the  
order of the instructions in the finally "clause" is predictable  
(which is IIRC not guaranteed on destruction of variables).   
Obviously, though, one could not throw inside a finally...

Is such a concept existent currently in Boost (or any library)?  If  
not, would it be useful?  I think so, but eh could be wrong :)

Hans Larsen

----------------
#include <boost/lambda/lambda.hpp>
#include <boost/bind.hpp>
#include <iostream>
#include <vector>

class finally {
protected:
	struct helper_base_t { virtual ~helper_base_t() {} };
	template< class T > struct helper_t : public helper_base_t {
		const T& _;
		helper_t( const T& t ) : _(t) { }
		~helper_t() { _(); }
	};
	std::vector< helper_base_t* > vec;

public:
	finally() {}
	~finally() {
		for( std::vector<helper_base_t*>::iterator it = vec.begin(); it !=  
vec.end(); it++ )
			delete (*it);
	}

	template< class T > void operator()( const T& t ) {
		vec.push_back( new helper_t<T>(t) );
	}
};

struct func1 {
	void operator()() const { std::cout << 1 << std::endl; }
};

void func2( int& x ) {
	std::cout << x << std::endl;
	x++;
}

void function()
{
	using namespace boost;
	using boost::lambda::var;

	finally f;
	int x = 0;

	f( func1()               ); // Simply print 1
	// This way we can pass parameters between calls...
	f( bind( func2, ref(x) ) ); // using Boost.Bind, modify x - will  
print 0.
	f( bind( func2, ref(x) ) ); // using Boost.Bind, modify x - will  
print 1
	f( var(x) = 5            ); // using Boost.Bind && Boost.Lambda
	f( bind( func2, ref(x) ) ); // using Boost.Bind, modify x - will  
print 5

	try {
		throw int(5);
	} catch( short x ) {
		std::cout << (x+1) << std::endl;
	}
}

int main()
{
	try {
		function();
	}
	catch( int x )
	{
		std::cout << x << std::endl;
	}
	return 0;
}