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Fix UB in format_arg_store implementation. #3833

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Jan 31, 2024
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Fix UB in format_arg_store implementation. #3833

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15 changes: 14 additions & 1 deletion include/fmt/base.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1636,7 +1636,9 @@ template <typename Context, size_t NUM_ARGS, size_t NUM_NAMED_ARGS,
struct format_arg_store {
// args_[0].named_args points to named_args to avoid bloating format_args.
// +1 to workaround a bug in gcc 7.5 that causes duplicated-branches warning.
arg_t<Context, NUM_ARGS> args[1 + (NUM_ARGS != 0 ? NUM_ARGS : +1)];
static constexpr size_t ARGS_ARR_SIZE = 1 + (NUM_ARGS != 0 ? NUM_ARGS : +1);

arg_t<Context, NUM_ARGS> args[ARGS_ARR_SIZE];
named_arg_info<typename Context::char_type> named_args[NUM_NAMED_ARGS];

template <typename... T>
Expand All @@ -1649,6 +1651,17 @@ struct format_arg_store {
0,
(init_named_arg(named_args, arg_index, named_arg_index, values), 0)...};
}

format_arg_store(format_arg_store&& rhs) {
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This move constructor should be noexcept

args[0] = {named_args, NUM_NAMED_ARGS};
for (size_t i = 1; i < ARGS_ARR_SIZE; ++i) args[i] = rhs.args[i];
for (size_t i = 0; i < NUM_NAMED_ARGS; ++i)
named_args[i] = rhs.named_args[i];
}

format_arg_store(const format_arg_store& rhs) = delete;
format_arg_store& operator=(const format_arg_store& rhs) = delete;
format_arg_store& operator=(format_arg_store&& rhs) = delete;
};

// A specialization of format_arg_store without named arguments.
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