Re: [boost] [conversion] try_lexical_cast and 200$

On Thu, Dec 12, 2013 at 9:41 AM, Peter Dimov wrote:

Antony Polukhin wrote:

...

* `try_lexical_cast` must be added or not?

The motivation given is convincing.

"At Facebook we've seen many instances of programs that call lexical_cast in a try/catch. In cases where the parsing is expected to fail (eg, an argument could be an int or a date) we frequently see extremely poor performance in multithreading. GCC is known to have poor multithreaded exception handling gcc.gnu.org/ml/gcc/2013-05/msg00253.html"

Even without the performance argument, seeing repeated occurrences of calling a function in a try block is a strong hint that the function perhaps ought to not report the error via an exception.

...

And if yes, what signature is preffered:

bool try_lexical_cast(Out&, const In&) bool try_lexical_cast(const In&, Out&) boost::optional<Out> try_lexical_cast(const In&)

bool try_lexical_cast(const In&, Out&), I'd say. optional<> is cute because you can use if( auto x = try_lexical_cast<Out>(in) ), but not very convenient for the common case of try_lexical_cast(in1, out1) && try_lexical_cast(in2, out2).

I have no strong preference, but with optional it's a little easier to provide a default if the cast fails: int n = try_lexical_cast< int >(str).get_value_or(-1); But it adds a dependency on Boost.Optional. This syntax is also closer to casts, while the two-argument version looks like an ordinary function. If the two-argument version is chosen, maybe a better name should be chosen, like: template< typename From, typename To > bool lexical_convert(From&& from, To& to);