Fix format_as for non-const begin/end views #3955
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vitaut
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May 12, 2024
include/fmt/format.h
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| using base = formatter<detail::format_as_t<T>, Char>; | ||
| return base::format(format_as(value), ctx); | ||
| // format functions expect lvalue refs, not rvalues | ||
| auto val = format_as(value); |
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Can it introduce an extra copy? Can val be a reference?
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If format_as returns a local variable of type T by value then no extra copy will be made. This was my thought of the predominant use case.
However for a format_as function signature like: const std::string& format_as(T t). Let's say you have a lookup table of std::strings for T, then using auto val always copies, while auto&& val deduces the type of val as a const & so no copy is made.
https://godbolt.org/z/fqMrcY3c3
Thus I think your suggestion of using auto&& is a good one here! Will make the change
base::format does not accept rvalues, using the result of format_as directly means it is passed one. Doesn't matter for ranges with a const begin and end, but filter_view caches, so it only has mutable begin/end. Use auto&& to avoid copy if format_as returns const ref
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Merged, thanks! |
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Fixes issue #3752
base::formatdoes not accept rvalues, using the result offormat_asdirectly means it is passed one. Doesn't matter for ranges with a const begin and end, but sincestd::ranges::filter_viewcaches the next value to return, it only has a mutable begin/end iterator.If we just store the result of
format_asin a temporary variable, we can pass it as an lvalue.Since we use it in
fmt/include/fmt/ranges.h
Lines 485 to 490 in 1dc71f2
And we don't call
std::forwardanyway, I don't think we're performing any pessimisation by making this changeAnother way to fix this is by providing a
&&overload offmt/include/fmt/ranges.h
Lines 541 to 545 in 1dc71f2
However other control paths (that pass lvalues) then flow through this, which doesn't seem right, so didn't opt for that method