I want to traverse a map using structure bindings, ignoring the key:
for (auto& [unused, val] : my_map)
do_something(val);
I have tried different options with gcc-7.2.0:
// The warning is issued
for ([[maybe_unused]] auto& [unused, val] : my_map)
do_something(val);
// Syntax error
for (auto& [[[maybe_unused]] unused, val] : my_map)
do_something(val);
// The same two combinations above with [[gnu::unused]].
It seems that the [[maybe_unused]]
attribute is not implemented yet for structure bindings.
Is there any simple solution to this? Any macro, gcc/gnu extension, or any pragma to temporarily supress that specific warning is fine to me; for example, disabling it during the whole function body where I'm using the range-based-for, because the function I'm using this is pretty short (it is basically two range-for-loops over two different maps with exact behaviour).
The (relevant) options I'm using to compile the project are:
-std=c++17 -Wall -Wextra -Werror -pedantic -pedantic-errors
What I have current to go on is, but that's ugly:
for (auto& [unused, val] : my_map)
(void)unused, do_something(val);
It seems that you are right that the maybe_unused
attribute is not implemented yet for structured bindings in gcc 7.2.0, but it's worth noting that it seems to be implemented for gcc 8.0 trunk (g++ 8.0.0 20171026 experimental).
Compiling using gcc 8.0 trunk, the following will emit a -Wunused-variable
warning:
// warning: unused structured binding declaration [-Wunused-variable]
for (auto& [unused, val] : my_map) { }
Whereas this will not:
// no warning
for ([[maybe_unused]] auto& [unused, val] : my_map) { }
Peculiarly, removing [[maybe_unused]]
but making use of at least one of the bounded variables will also not yield a warning (intended?).
// no warning
for (auto& [unused, val] : my_map)
{
do_something(val);
}
// no warning
for (auto& [unused, val] : my_map)
{
(void)unused;
}
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