From 187bd1b8b2392d565d573c0984d0edd57e20ac8d Mon Sep 17 00:00:00 2001 From: Michael Lui Date: Fri, 22 Feb 2019 11:37:19 -0500 Subject: [PATCH] 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. --- include/fmt/core.h | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/include/fmt/core.h b/include/fmt/core.h index 3c101bb3480c..00f9ffb13438 100644 --- a/include/fmt/core.h +++ b/include/fmt/core.h @@ -1200,6 +1200,7 @@ const unsigned long long format_arg_store::TYPES = Constructs an `~fmt::format_arg_store` object that contains references to arguments and can be implicitly converted to `~fmt::format_args`. `Context` can be omitted in which case it defaults to `~fmt::context`. + See `~fmt::arg` for lifetime considerations. \endrst */ template @@ -1384,6 +1385,12 @@ typename buffer_context::type::iterator vformat_to( \rst Returns a named argument to be used in a formatting function. + The named argument holds a reference and does not extend the lifetime + of its arguments. + Consequently, a dangling reference can accidentally be created. + The user should take care to only pass this function temporaries when + the named argument is itself a temporary, as per the following example. + **Example**:: fmt::print("Elapsed time: {s:.2f} seconds", fmt::arg("s", 1.23));