Thorsten Ottosen wrote:
"Jonathan Turkanis" <technews@kangaroologic.com> wrote:
4. Thorsten asks why the widening and narrowing functions shouldn't be non-member functions. One answer is that code conversion can be (slightly) more efficient if a large buffer is used. Making the core conversion functions member functions allows buffers to be used for several string conversions.
I think the added flexibility of the overloads taking iterators is more significant than the ability to buffer.
I can't really figure out how this buffering should work. Buffering of what?
Look at the interface of std::codecvt, e.g. at the member function in. This function takes a Byte array as input and write wide characters to a second Byte array. Since in() is a virtual function, it's slightly faster to call it once or twice per string than to call it once for each character in a string. Similar remarks hold for out. To make this work, you need (i) the input to be presented as a character array (std::string or const char* is fine, but a pair of forward iterators isn't) (ii) a good sized buffer for output The example I presented doesn't satisfy (i). But in the absense of performance data I won't worry about it.
I agree that there should be iterator versions underneith the range interface (that's how it all works.)
Jonathan:
template<typename WideStr> basic_string<typename Codecvt::extern_type> narrow(const WideStr& str) { string_converter<> cvt; return cvt::narrow(str); }
The could be specified as
template< class NarrowString, class ReadableWideForwardRange, class Codecvt > NarrowString narrow( const ReadableWideForwardRange& r, const Codecvt& cc ); ...
First, as much as I love Boost.Range and would like to see it standardized, I don't think the code conversion proposal should use it. The second problem is that it's awkard to specify the codecvt instance when you just want a codecvt to be grabbed from the globale locae: std::locale loc = locale::global(); const std::codecvt<wchar_t, char, std::mbstate_t>& cvt = std::use_facet< std::codecvt<wchar_t, char, std::mbstate_t> >(loc); // etc. std::string s = narrow(ws, cvt); Most people will want to be able to write std::string s = narrow(ws); Perhaps a good solution would be to have overloads, in addition to the ones I showed, with a signature including a codecvt instance, as in your example. Jonathan