replace hand-rolled lexicalCast #4473
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Can you quantify the change in performance in any way? |
This change does not have a directly measurable impact in performance and synthetic benchmarks are unlikely to be helpful. The C++ std::from_chars is intended to be as efficient and fast as possible and it's unlikely that it will be any slower than the code that it replaces. And given that it also reduces the amount of hand-rolled code it's a net gain. |
@gregtatcam why? |
I agree that it's probably not used in practice. But are we 100% sure that it's never used? And it's easy to add support for |
@dangell7 thoughts? |
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I'm not sure of any instance where someone would use a "+" in an integer string. I had thought of If we need to parse "+" we can always revert it. No rush let me check these 2 tests to make sure theres not a nefarious way to use this. |
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I like that this replaces some of our code with std::code.
On the '+' issue, I recommend just checking in[0] == '+' && isdigit(in[1]) prior to calling from_chars, being careful about strings of length 0 and 1 of course. It likely won't be used, but it is cheap to support this existing outward facing API.
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@dangell7 can you address the '+' issue as discussed above? |
| @@ -84,6 +84,9 @@ struct LexicalCast<Out, std::string> | |||
| auto first = in.data(); | |||
| auto last = in.data() + in.size(); | |||
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| if (first != last && *first == '+') | |||
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Have to check the next one is digit. See @HowardHinnant comment.
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I don't think so - the semantics seem correct to me. If it's a plus and the next character is not a digit, then std::from_chars will return some kind of failure and the check, which ensures that (a) not failure is returned; and (b) that the whole string was parsed and nothing was left over will work correctly.
Check out this example. As you can see, the only cases where the semantics don't match is when you have a numeric value prefixed with a + sign. The "new" code does what we want, and the old one just fails.
Thank you for this. Building and testing now. Is there a program you're using to identify these or any recommendations you can make? |
intelliot
added
Passed
No program. |
Replace hand-rolled code with std::from_chars for better maintainability. The C++ std::from_chars function is intended to be as fast as possible, so it is unlikely to be slower than the code it replaces. This change is a net gain because it reduces the amount of hand-rolled code.
Replace hand-rolled code with std::from_chars for better maintainability. The C++ std::from_chars function is intended to be as fast as possible, so it is unlikely to be slower than the code it replaces. This change is a net gain because it reduces the amount of hand-rolled code.
Replace hand-rolled code with std::from_chars for better maintainability. The C++ std::from_chars function is intended to be as fast as possible, so it is unlikely to be slower than the code it replaces. This change is a net gain because it reduces the amount of hand-rolled code.
High Level Overview of Change
Replace hand-rolled code with std::from_chars for better performance and maintainability. The only minor change in semantics is the inability to parse the + sign, but that functionality is unused in practice.
Context of Change
Type of Change