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Clarify usage of fmt::arg

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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.
This commit is contained in:
Mike Lui 2019-02-21 10:22:38 -05:00
parent 82c24edcf6
commit 71e97b4819
1 changed files with 9 additions and 0 deletions
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9
include/fmt/core.h
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@ -1200,6 +1200,7 @@ const unsigned long long format_arg_store<Context, Args...>::TYPES =
Constructs an `~fmt::format_arg_store` object that contains references to Constructs an `~fmt::format_arg_store` object that contains references to
arguments and can be implicitly converted to `~fmt::format_args`. `Context` arguments and can be implicitly converted to `~fmt::format_args`. `Context`
can be omitted in which case it defaults to `~fmt::context`. can be omitted in which case it defaults to `~fmt::context`.
See `~fmt::arg` for lifetime considerations.
\endrst \endrst
*/ */
template <typename Context = format_context, typename... Args> template <typename Context = format_context, typename... Args>
@ -1384,6 +1385,12 @@ typename buffer_context<Char>::type::iterator vformat_to(
\rst \rst
Returns a named argument to be used in a formatting function. Returns a named argument to be used in a formatting function.
Usage is analogous to `std::reference_wrapper
<https://en.cppreference.com/w/cpp/utility/functional/reference_wrapper>`_,
except that rvalues references are not disabled.
The user should take care to only pass in temporaries when
the named argument is itself a temporary, as per the following example.
**Example**:: **Example**::
fmt::print("Elapsed time: {s:.2f} seconds", fmt::arg("s", 1.23)); fmt::print("Elapsed time: {s:.2f} seconds", fmt::arg("s", 1.23));
@ -1394,9 +1401,11 @@ inline internal::named_arg<T, FMT_CHAR(S)> arg(const S& name, const T& arg) {
return {name, arg}; return {name, arg};
} }
/// \cond
// Disable nested named arguments, e.g. ``arg("a", arg("b", 42))``. // Disable nested named arguments, e.g. ``arg("a", arg("b", 42))``.
template <typename S, typename T, typename Char> template <typename S, typename T, typename Char>
void arg(S, internal::named_arg<T, Char>) = delete; void arg(S, internal::named_arg<T, Char>) = delete;
/// \endcond
template <typename Container> struct is_contiguous : std::false_type {}; template <typename Container> struct is_contiguous : std::false_type {};