c++ - 为什么 `std::invoke` 不是 constexpr？

Question

不应该std::invoke成为 constexpr 尤其是在 constexpr lambdas in C++17 之后？

是否有任何障碍可以阻止这种情况发生？

Answer 1

更新: P1065将使它成为 constexpr。

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由于历史原因保留原帖:

来自 the proposal :

Although there is possibility to implement standard conforming invoke function template as a constexpr function, the proposed wording does not require such implementation. The main reason is to left it consistent with existing standard function objects, that could have such definition, like std::mem_fn, std::reference_wrapper and operator wrappers. Furthermore imposing such requirement will block the implementation of invoke that refers to std::mem_fn.

This proposal assumes that constexpr addition to the header would be applied consistently by a separate proposal.

Both constexpr and standard library based implementation are presented in Implementability section of the proposal.

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相关 CWG 问题 #1581:When are constexpr member functions defined? .