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Clarify lifetimes of named_arg parameters (#1051)
* Clarify usage of fmt::arg Document that fmt::arg takes a non-owning reference, even if that reference is to a temporary. As such, users should make sure the lifetime of the reference lasts as long as the named argument. * Clean up language Remove mentions of `std::reference_wrapper` and rvalues in favor of more common terminology like dangling references.
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@ -1200,6 +1200,7 @@ const unsigned long long format_arg_store<Context, Args...>::TYPES =
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Constructs an `~fmt::format_arg_store` object that contains references to
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arguments and can be implicitly converted to `~fmt::format_args`. `Context`
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can be omitted in which case it defaults to `~fmt::context`.
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See `~fmt::arg` for lifetime considerations.
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\endrst
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*/
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template <typename Context = format_context, typename... Args>
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@ -1384,6 +1385,12 @@ typename buffer_context<Char>::type::iterator vformat_to(
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\rst
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Returns a named argument to be used in a formatting function.
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The named argument holds a reference and does not extend the lifetime
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of its arguments.
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Consequently, a dangling reference can accidentally be created.
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The user should take care to only pass this function temporaries when
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the named argument is itself a temporary, as per the following example.
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**Example**::
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fmt::print("Elapsed time: {s:.2f} seconds", fmt::arg("s", 1.23));
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