Le 15/01/13 18:44, Jeffrey Lee Hellrung, Jr. a écrit :
On Sun, Jan 13, 2013 at 4:30 AM, TONGARI <tongari95@gmail.com> wrote:
Hi all,
One of the few things I appreciate in Java is the labeled-break feature. I think C++ would provide it as well in future standard but it seems that there's no even such a proposal.
In lack of the language feature, a simple emulation can be used:
--------------------------------------------------- #define BOOST_SCOPE(name) \ if (const bool LABELED_SCOPE_##name = false){break(name); name:;} else
#define break(name) \ (void)LABELED_SCOPE_##name; goto name; --------------------------------------------------- Now we can write:
BOOST_SCOPE(a) { break(a); cout << "123\n"; }
The real world usage would reside in nested loop&switch where labeled-break really shines.
Thoughts? Sorry if this idea is too simple and somewhat rejected before...
It's a nice thought but: (a) I think the situation where you want to break and/or continue more than one loop-level out at a time is rare; and Agree and its use /could/ be a sign that a refactoring is needed, but not always. (b) in those rare situations, explicitly using gotos and labels (with a descriptive name) is reasonably brief and readable; so I would be hesitant to adopt this.
When we use label/goto directly there is a risk that the user goto to the label without constructing all the scoped variables, v in this case. if (cnd3) goto L; // bad as v constructor will not be called. { T v; for(;;) { while (cnd) { if (cnd2) goto L; // go to [1] } } L: // [1] std::cout << v <<std::endl; } With named blocks this code will just not compile. if (cnd3) BOOST_BREAK(L); // compile fail: only allowed from inside the block. { T v; BOOST_NAMED_BLOCK(L) for(;;) { while (cnd) { if (cnd2) BOOST_BREAK(L); // go to [1] } } // [1] std::cout << v <<std::endl; } Best, Vicente