当不抛出零成本时，noexcept没用吗？ [英] Is noexcept useless when not throwing is zero-cost?

问题描述

如果您的实现具有零成本（如果未抛出任何异常）异常模型，则 noexcept 说明符是否无用？缺少 noexcept 会导致后果的例子是什么？

Is the noexcept specifier useless if your implementation has a zero-cost (if nothing is thrown) exception model? What is an example where lacking noexcept has a consequence?

推荐答案

如果您的实现具有零成本（如果未抛出任何异常）异常模型，则 noexcept 指定符是否无用？

否，即使异常对性能完全没有影响， noexcept 还是有用的。

No, noexcept would be useful even if exceptions had no performance impact at all.

一个例子是 noexcept 可以表示可以使用哪种算法。例如，使用 std :: move_if_noexcept 或许多标准 type_traits 的行为可能会有所不同，具体取决于 noexcept 的存在。

One example is that noexcept can signal which algorithm can be used. For example algorithms that use std::move_if_noexcept or many of the standard type_traits could behave differently depending on the presence of noexcept.

这可能对 std :: vector之类的常用功能产生重大影响code>如果您元素的move构造函数不是 noexcept ，则速度会慢得多。如果移动构造函数不是 noexcept ， std :: vector 将在重新分配时复制所有元素，而不是移动它们。强有力的例外保证。

This can have a significant impact for common features like std::vector which will be much slower if you elements' move constructor isn't noexcept. std::vector will copy all elements when reallocating instead of moving them if the move constructor is not noexcept to preserve the strong exception guarantee.

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