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78 lines
2.3 KiB
Markdown
78 lines
2.3 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/modern-cpp/cpp17/cpp-nodiscard/) 的文章，由站点同步脚本导出。
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# C++17 [[nodiscard]] 属性
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自C++17引入[[nodiscard]]属性以来，我们在编写API时有了一种更强大的工具，用于标记那些在调用时不应该被忽略的函数返回值。在本文中，我们将深入探讨[[nodiscard]]的用法，并注意一些潜在的陷阱。
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## 什么是[[nodiscard]]？
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[[nodiscard]]是一种函数属性，它告诉编译器函数的返回值不应该被忽略。这在那些依赖于返回值的函数中尤为有用，例如谓词函数或工厂函数。
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## 如何使用[[nodiscard]]？
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在类、构造函数以及枚举上使用[[nodiscard]]都是可能的。下面是一些示例：
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```cpp
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class [[nodiscard]] A {};
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class B {
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public:
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[[nodiscard]] B() = default;
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};
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enum class [[nodiscard]] State {
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Default,
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Foo,
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Bar
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};
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```
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在上述例子中，编译器将在返回值被忽略时发出警告，有助于我们及早发现潜在的bug。
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## 警惕：[[nodiscard]]的陷阱
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尽管[[nodiscard]]可以有效地防止返回值被无意中忽略，但我们需要注意一些潜在的陷阱。一个常见的陷阱是使用[[nodiscard]]的函数返回一个类似于作用域守卫的对象，以在作用域结束时执行清理操作（比如提交事务）。
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```cpp
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[[nodiscard]] ScopeGuard createScopeGuard() {
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// Implementation of a scope guard
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}
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// 在这里，用户可以选择忽略返回值，导致作用域立即销毁，从而失去清理效果
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std::ignore = createScopeGuard();
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```
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尽管[[nodiscard]]表明函数返回值不应该被忽略，但用户仍然可以通过`std::ignore`等手段明确地忽略返回值，从而绕过清理操作。
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## 完整示例
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```cpp
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#include <tuple> // std::ignore
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class [[nodiscard]] A {};
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class B {
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public:
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[[nodiscard]] bool isOk() const { return true; }
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};
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// no need for nodiscard, A is nodiscard
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A foo() {
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return A();
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}
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// B is not nodiscard, mark it explicitly
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[[nodiscard]] B bar() {
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return B();
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}
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int main()
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{
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foo(); // warning: ignoring temporary created by a constructor declared with 'nodiscard' attribute
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bar(); // warning
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std::ignore = foo(); // ok
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auto b = bar(); // ok
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b.isOk(); // warning
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if (b.isOk()) {} // ok
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}
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``` |