Re: Converting double to widestring
* Ed:
We have an application that converts a double to widestring
(std::wstring). This happens thousands of time. We also need to provide
how many decimal places to be used. The decimal point needs to be a
period regardless of the locale of the machine. The function we call has
signature like ConverDouble(std::wstring & str, double d, long digits)
What is the fastest way to do it?
First, be aware that you're likely heading for a case of evil premature
optimization. Thousands of times is nothing, absolutely nothing, not even an
eyeblink for the user. But given that you have measured and found that this is
indeed a bottleneck that really really needs some optimization:
be aware that dynamic allocation for the std::wstring is likely to use orders of
magnitude more time than the double->string conversion. So I'd suggest using a
static buffer. For that, be aware of thread safety issues. An alternative, if
this function is called in a tight loop in one place (or two, or three), is to
only let the function resize the string and optimize at the caller site, where
the same string variable should then be re-used repeatedly.
And given that, I'd prefer to suggest using the internal function that swprintf
uses, namely _cfltcvt, which is just a macro that in turn references the first
function pointer in array _cfltcvt_tab. However, when checking that out (that's
how I found the names) the linker protested that it couldn't find this symbol.
Not sure what needs to be linked, or how the heck it's found for printf itself.
If you find out how to use the _cfltcvt function, please report the details (of
linking) here! :-)
Given that the _cfltcvt function is practically unavailable, I suggest plain
wsprintf, which doesn't really add that much extra processing, only a little. A
suitable format might be "%.*g", with precision as int argument. Note that your
function signature has long precision (assuming "digits" means precision), and
that that is most probably extreme overkill and Bad Design, but that to get rid
of the dynamic allocation for wstring the signature may need to be adjusted
anyway -- and I'd also suggest fixing the name, which says nothing whatsoever.
Then, again, measure.
Cheers, & hth.,
- Alf
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