如何有意丢弃[[nodiscard]]返回值？ [英] How can I intentionally discard a [[nodiscard]] return value?

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

WG14 nodiscard提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)):

[[nodiscard]]属性在现实世界中具有广泛的用途，由Clang和GCC实施为__attribute __（（warn_unused_result））
，但由WG21以[[nodiscard]]名称进行了标准化。这项建议选择标识符nodiscard
，因为与此名称的偏离会导致与C ++不必要的不​​兼容。

The [[nodiscard]] attribute has extensive real-world use, being implemented by Clang and GCC as __attribute__((warn_unused_result)) , but was standardized under the name [[nodiscard]] by WG21. This proposal chose the identifier nodiscard because deviation from this name would create a needless incompatibility with C++.

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[[ dcl.attr.nodiscard] p2 ：

See the draft C++ standard [[dcl.attr.nodiscard]p2:

[注意：无丢弃调用是一个函数调用表达式，该函数调用先前声明为nodiscard的函数，或者其返回类型是可能的cv限定类或标记为nodiscard的枚举类型。
不建议将nodiscard调用显示为可能评估的舍弃值表达式，除非明确将其强制转换为void。
在这种情况下，实现应发出警告。
这通常是因为丢弃nodiscard调用的返回值会产生令人惊讶的后果。
-注释[end note]

[ Note: A nodiscard call is a function call expression that calls a function previously declared nodiscard, or whose return type is a possibly cv-qualified class or enumeration type marked nodiscard. Appearance of a nodiscard call as a potentially-evaluated discarded-value expression is discouraged unless explicitly cast to void. Implementations should issue a warning in such cases. This is typically because discarding the return value of a nodiscard call has surprising consequences. — end note]

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.

参见在nodiscard上提交的clang文档，warn_unused_result ：

Clang支持诊断a的结果何时的能力。在可疑的情况下，将丢弃函数调用表达式。当函数或其返回类型标记为[[nodiscard]]（或__attribute __（（warn_unused_result）））并且函数调用显示为可能评估的舍弃表达式（未明确转换）时，将生成诊断。无效。

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