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

From: Alexander Nasonov (alnsn_at_[hidden])
Date: 2007-04-01 17:38:46

Hans Larsen wrote:
> Hello,
> 
> Better is such a big word.  You still have to say in which way it is  
> better; readability, efficiency, simplicity, something else?  ;-) 
Readability and simplicity. Compare

std::FILE* f = NULL;

try
{
    f = std::fopen("/etc/passwd", "r");
    // ...
}
finally
{
    if(f)
        std::fclose(f);
}

with

std::FILE* f = std::fopen("/etc/passwd", "r");
scope(exit)
{
    if(f)
        std::fclose(f);
}

// ...

Even without D's scope(exit) syntax it looks appealing

std::FILE* f = std::fopen("/etc/passwd", "r");
BOOST_SCOPE_EXIT(f, (f))
{
    if(f)
        std::fclose(f);
} BOOST_SCOPE_EXIT_END(f)

> I admit my solution is not perfect, but it does have certain advantages...
> 
> First, you seem to use boost/typeof/typeof.hpp, which I failed to
> find in the boost version I currently use (stable 1.33.1).
Check it out from cvs or wait for 1.34.

> I didn't  find the documentation in the full doc, only the BoostBook.
It's not in a distribution but you can generate it or read online at
http://194.6.223.221/~nasonov/scope_exit-0.02/libs/scope_exit/doc/html/

> Why do  you need that library and is it absolutely necessary?
Because I saw so much exception unaware code.

> I don't think it should from the concepts necessary to put the thing together.
> All  you need, after all, is a destructor to call the code, and some  
> variable passing (Boost.Bind is very wonderful).

Bost.Bind and Boost.Lambda are very useful but it's hard to compete with
a plain old C++ code. Try to express the 'if(f) std::fclose(f)', for
example.

> Also, I'm not a big fan of macros, even more when using IDs in them,  
> I think they reduce readability.
Me too. But this macro can significantly increase a quality of code.

> I prefer compilers to pre- compilers, if you know what i mean.
> In your case, adding a scope inside the SCOPE_XXX makes for more
> indentation, which is not  necessarily A Good Thingie.
SCOPE_EXIT, like _if_ or _while_, controls an execution of a code block.

> I suggest at least putting those brackets inside the SCOPE_XXX defines.
> This is all, of course, a  matter of taste.
I personally like brackets because they make a small chuck of code
surrounded by BIG MACROS more visible. It's also easier to jump
between brackets (% in vim) and go to the opening bracket([{ in vim).
But if many people don't like brackets, it's a trivial change ...

> BTW, what happens if I typo the ID or just use the  
> same in two different places (either in the same scope or inlined  
> scopes).

Typo:

hello.cpp: In function `int main()':
hello.cpp:18: error: `boost_scope_exit_struct_typo' was not declared in
this sco
pe
hello.cpp:18: error: expected `;' before "const"
hello.cpp:18: error: `boost_scope_exit_typo' was not declared in this
scope

Duplicate id:

real_world.cpp:134: error: redefinition of `struct World::addPerson(const Person
&)::rollback_boost_scope_exit_nested_typeofold_id'
real_world.cpp:121: error: previous definition of `struct World::addPerson(const
 Person&)::rollback_boost_scope_exit_nested_typeofold_id'

Believe me, I don't like id argument either but I couldn't come up
with a better solution.

> My solution is CopyConstructable.  Many things become interesting  
> when you can copy a chain of commands.  It is also bidirectionnal.   
> With a flag at construction, we could invert the order of execution.   
> We could also reorder it in the middle (cannot think of a real use  
> for this, though).
> 
> BTW, did you check my code?  You never told me what you thought of it...
I looked at it quickly and noticed dynamic allocations.

This code
>        template< class T > void operator()( const T& t ) {
>                vec.push_back( new helper_t<T>(t) );
>        }
is not exception aware.

You can rewrite it like this:
std::auto_ptr< helper_t<T> > p(new helper_t<T>(t) );
vec.push_back( p.get() );
p.release();

-- 
Alexander Nasonov
http://nasonov.blogspot.com
To be pleased with one`s limits is a wretched state. -- Johann
Wolfgang von Goethe --
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