结构化绑定和基于范围的gcc中未使用的警告 [英] structured bindings and range-based-for; supress unused warning in gcc

问题描述

我想使用结构绑定遍历地图，而忽略键：

I want to traverse a map using structure bindings, ignoring the key:

for (auto& [unused, val] : my_map)
     do_something(val);

我用gcc-7.2.0尝试了不同的选择：

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]].

似乎 [[maybe_unused]] 属性尚未为结构绑定实现。

It seems that the [[maybe_unused]] attribute is not implemented yet for structure bindings.

有没有简单的解决方案？任何宏，gcc / gnu扩展名或任何杂用语可以暂时禁止特定警告对我有用；例如，在我使用基于范围的for的整个函数体中将其禁用，因为我使用的此函数非常短（它基本上是两个具有精确行为的不同地图上的两个范围进行循环）。

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).

我用来编译项目的（相关）选项是：

The (relevant) options I'm using to compile the project are:

-std=c++17 -Wall -Wextra -Werror -pedantic -pedantic-errors

我要继续做的是，但这很丑陋：

What I have current to go on is, but that's ugly:

for (auto& [unused, val] : my_map)
   (void)unused, do_something(val);

推荐答案

相关的GCC编译指示已记录在在此页面上。

The relevant GCC pragmas are documented on this page.

#include <map>

std::map<int, int> my_map;

void do_something(int);

void loop()
{
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-variable"
    for (auto & [unused, val]: my_map)
#pragma GCC diagnostic pop
        do_something(val);

}

这是我可以禁止的警告的最小范围已经并且仍然通过 -Wall -Wextra -Werror 取消了警告。

This is the smallest scope of the disabled warning that I could have, and still have the warning suppressed with -Wall -Wextra -Werror.

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