什么是 std::identity 以及它是如何使用的? [英] what is std::identity and how it is used?

问题描述

我只想知道 std::identity 的目的是什么?我在网上找不到任何有用的东西.我知道它是如何实现的:

I just want to know what is the purpose of std::identity? I could not find anything useful on web. I know how it is implemented :

template <typename T>
struct identity
{
    T operator()(T x) const { return x; }
};

为什么我们真的需要这个?

why do we actually need this?

推荐答案

Standard (up to C++20) 没有 std::identity，所有提到它的提案都被删除了.最初建议时，它应该用于与接受标准中的 std::forward 服务相同的目的，但它与非标准扩展相冲突，并且经过多次迭代最终被删除.

Standard (up to C++20) doesn't have std::identity, all proposals mentioning it have been removed. When initially suggested, it was supposed to serve the same purpose as std::forward serves in accepted standard, but it conflicted with non-standard extensions and after several iterations was finally removed.

C++20 支持 std::identity:https://en.cppreference.com/w/cpp/utility/functional/identity

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