问题描述
说我有
[[nodiscard]] int foo ()
{
return 0;
}
int main ()
{
foo ();
}
然后
error: ignoring return value of ‘int foo()’, declared with attribute nodiscard [-Werror=unused-result]
，但如果
int x = foo ();
然后
error: unused variable ‘x’ [-Werror=unused-variable]
是否存在告诉编译器我要舍弃此 [[nodiscard]] 值的一种干净方法？
Is there a clean way of telling the compiler "I want to discard this [[nodiscard]] value"?
推荐答案
讨论了通过强制转换为无效来使诊断静音的理由。它说强制转换为空是鼓励（如果不是非规范性的）使它静音的方法，该方法遵循现有实现对 __ attribute __（（warn_unused_result））的作用：
The WG14 nodiscard proposal discusses the rationale for allowing the diagnostic to be silenced by casting to void. It says casting to void is the encouraged (if non-normative) way to silence it which follows what the existing implementation do with __attribute__((warn_unused_result)):
此属性的语义很大程度上取决于a的概念。用途，
的定义由实施人员自行决定。但是，WG21指定的非规范性指南中的
鼓励
实现在可能评估的舍弃值表达式中使用nodiscard函数
调用时发出警告诊断。
，除非它是明确转换为空的。这意味着不鼓励
实现执行数据流分析（就像
初始化但未使用的局部变量诊断一样）。
...
The semantics of this attribute rely heavily on the notion of a use, the definition of which is left to implementation discretion. However, the non-normative guidance specified by WG21 is to encourage implementations to emit a warning diagnostic when a nodiscard function call is used in a potentially-evalulated discarded-value expression unless it is an explicit cast to void. This means that an implementation is not encouraged to perform dataflow analysis (like an initialized-but- unused local variable diagnostic would require). ...
C ++方式为 static_cast< void> 。
请参阅C ++标准草案[[：
See the draft C++ standard [[dcl.attr.nodiscard]p2:
这是一条注释，因此不是规范性的，但基本上这是现有实现对 __ attribute __（（warn_unused_result））。另外，请注意，对 nodiscard 的诊断也是非规范的，因此，对违反 nodiscard 的诊断不是错误的形式，而是实现质量，就像通过强制转换来抑制一样
This is a note, so non-normative but basically this is what existing implementations do with __attribute__((warn_unused_result)). Also, note a diagnostic for nodiscard is also also non-normative, so a diagnostic for violating nodiscard is not ill-formed but a quality of implementation just like suppressing via a cast to void is.
参见：
这篇关于如何有意丢弃[[nodiscard]]返回值？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！