Re: [boost] Interest in breakable labeled scope emulation?

Subject: Re: [boost] Interest in breakable labeled scope emulation?
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-01-14 02:42:39

Le 14/01/13 02:50, TONGARI a écrit :
> 2013/1/14 Brian Ravnsgaard Riis <brian_at_[hidden]>
>
>> Den Sun, 13 Jan 2013 20:30:14 +0100 skrev Vicente J. Botet Escriba <
>> vicente.botet_at_[hidden]>:
>>
>> [snip]
>>
>>   Of course in order to be really structired the jumping to the underlying
>>> label should be forbiden (by the compiler), but I don't think that a
>>> preprocessor solution could take care of this. So the following not desired
>>> and not structured code could compile even if we would like an compile
>>> error.
>>>
>>>
>>> BOOST_NAMED_BLOCK(L)
>>> {
>>> // ...
>>> }
>>> // [1]
>>> //...
>>> BOOST_BREAK(L); // go to [1]
>>>
>> So, the underlying idea is to get something like "break" and "continue" to
>> work inside any "named" block, not just loops? If so (as you demonstrate
>> above) I'm fairly certain a correct solution cannot be implemented using
>> just the preprocessor.
>>
> Hey guys,
>
> Please try the code attached and see the error message that your compiler
> gives.
> It DOES avoid the ill-formed code like that to compile.
>
>
Yeah. I was your macro forbid the access from outside the block. I 
though that the trick was used only to be able to add a label at the end 
of the block. But the use of the variable on the BREAK macro make it 
fail. Really clever.

#define BOOST_SCOPE(name) \
if (const bool LABELED_SCOPE_##name = false){BOOST_BREAK(name); 
name##_end_scope:;} else

#define BOOST_BREAK(name) \
(void)LABELED_SCOPE_##name; goto name##_end_scope

I have tried

int main()
{
BOOST_SCOPE(a2)
{
BOOST_BREAK(a2);
cout << "123\n";
}
BOOST_SCOPE(a1)
{
BOOST_BREAK(a2);
cout << "123\n";
}
return 0;
}

and here are the error for gcc

darwin.compile.c++ 
../../../bin.v2/libs/thread/test/test_so.test/darwin-4.7.2x/debug/threading-multi/test_so.o
../example/test_so.cpp: In function ‘int main()’:
../example/test_so.cpp:22:1: erreur: ‘LABELED_SCOPE_a2’ was not declared 
in this scope
../example/test_so.cpp:15:1: attention : jump to label ‘a2_end_scope’ 
[-fpermissive]
../example/test_so.cpp:22:1: attention : à partir d'ici [-fpermissive]
../example/test_so.cpp:15:1: erreur: skips initialization of ‘const bool 
LABELED_SCOPE_a2’

and clang

clang-darwin.compile.c++ 
../../../bin.v2/libs/thread/test/test_so.test/clang-darwin-3.2xl/debug/threading-multi/test_so.o
../example/test_so.cpp:22:7: error: use of undeclared identifier 
'LABELED_SCOPE_a2'
BOOST_BREAK(a2);
^
../example/test_so.cpp:10:7: note: expanded from macro 'BOOST_BREAK'
(void)LABELED_SCOPE_##name; goto name##_end_scope
^
<scratch space>:232:1: note: expanded from macro 'LABELED_SCOPE_'
LABELED_SCOPE_a2
^

Best,
Vicente