blocks|key|5479439|text|标准(直到C%2B%2B20)没有std::identity，所有提到它的提案都被删除了。最初建议时，它应该与公认标准中的std::forward服务具有相同的目的，但它与非标准扩展相冲突，经过几次迭代后最终被删除。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5479440|C%2B%2B20让std::identity回来了：https://en.cppreference.com/w/cpp/utility/functional/identity|5479441|entityMap|0|LINK|mutability|MUTABLE|url|https://en.cppreference.com/w/cpp/utility/functional/identity^0|D|D|1L|C|0|6|D|N|1P|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|H|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

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

C++20 has <code>std::identity</code> back: <a href="https://en.cppreference.com/w/cpp/utility/functional/identity" rel="nofollow noreferrer">https://en.cppreference.com/w/cpp/utility/functional/identity</a>

blocks|key|2299060|text|代码中的结构是标识函数T+->+T，其中T是模板参数。该函数本身并不有用，但在需要传递函数参数的其他上下文中可能很有用，并且identify函数就是您希望在其中插入的函数。它通常是一种“什么都不做”的函数。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2299061|至于std::identity，我在C%2B%2B标准库中找不到这个结构存在的证据。|2299062|entityMap^0|B|6|0|2|D|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

The struct you have in your code is the identity function <code>T -&gt; T</code> where T is a template parameter. This function isn't useful on its own, but may be useful in other contexts where you need to pass a function parameter and the identify function is the one you want insert there. It's generally useful as a sort-of "do nothing" function.

As to <code>std::identity</code>, I can find no evidence that this struct exists in the C++ standard library.

blocks|key|2303875|text|虽然在提出问题时并不相关，但C%2B%2B20添加了似乎来自Ranges提案的std::identity。Here是来自Ranges+TS的前一个定义，其主要用法解释如下：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2303876|2303877|它被用作所有范围TS算法的默认投影。|blockquote|2303878|2303879|entityMap|0|LINK|mutability|MUTABLE|url|https://en.cppreference.com/w/cpp/utility/functional/identity|1|https://en.cppreference.com/w/cpp/experimental/ranges/functional/identity^0|Q|6|Z|D|1K|9|Z|D|0|1D|4|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]|$9|14|A|15|1|16]]|E|$]]|$1|F|3|-4|5|6|7|17|8|@]|D|@]|E|$]]|$1|G|3|H|5|I|7|18|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|19|8|@]|D|@]|E|$]]|$1|K|3|-4|5|6|7|1A|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]|S|$5|N|O|P|E|$Q|T]]]]

While not relevant at the time of the question being asked, C++20 adds <a href="https://en.cppreference.com/w/cpp/utility/functional/identity" rel="nofollow noreferrer"><code>std::identity</code></a> which seem to come from <code>Ranges</code> proposal. <a href="https://en.cppreference.com/w/cpp/experimental/ranges/functional/identity" rel="nofollow noreferrer">Here</a> is its previous definition from <code>Ranges TS</code> where the main usage of it is explained as:

<blockquote>
 It is used as the default projection for all Ranges TS algorithms.
</blockquote>

blocks|key|2303891|text|其他人已经回答了这个问题-它对于函数类型模板参数的默认值和haskell风格的函数式编程都很有用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2303892|但是您的示例实现不正确。您的代码将执行值复制，这是std::identity做不到的-它完美地转发。它也是constexpr的，并且是透明的。|offset|length|style|CODE|2303893|所以这是一个如何实现它的例子，我相信：|2303894|++++struct+identity
++++{
++++++++using+is_transparent+=+void;

++++++++template+<typename+T>
++++++++constexpr+T&&+operator()(T&&+t)+const+noexcept
++++++++{
++++++++++++return+std::forward<T>(t);
++++++++}
++++};|code-block|syntax|javascript|2303895|entityMap^0|0|P|D|1H|9|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@$D|S|E|T|F|G]|$D|U|E|V|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|X|8|@]|9|@]|A|$M|N]]|$1|O|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|P|$]]

Other people already answered the question - it's useful for things like a default for a function-type template parameter, and for haskell-style functional programming.
But your example implementation is not correct. Your code would perform a value copy, which <code>std::identity</code> does not do - it perfectly forwards. It's also <code>constexpr</code> and is transparent.
So this is an example of how it would be implemented, I believe:
<pre class="lang-cpp prettyprint-override"><code> struct identity
 {
 using is_transparent = void;

 template &lt;typename T&gt;
 constexpr T&amp;&amp; operator()(T&amp;&amp; t) const noexcept
 {
 return std::forward&lt;T&gt;(t);
 }
 };
</code></pre>

blocks|key|3368033|text|我的库中没有std::identity，但它应该是一个有用的工具，可以通过std::copy_if将一个向量复制到另一个没有nullptr对象的向量中。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3368034|entityMap^0|6|D|11|C|1Q|7|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

I don't have <code>std::identity</code> in my libraries, but it should be a useful tool to copy a vector into another one without the <code>nullptr</code> objects in it via <code>std::copy_if</code>.

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 :

<pre><code>template &lt;typename T&gt;
struct identity
{
 T operator()(T x) const { return x; }
};
</code></pre>

why do we actually need this?

what is std::identity and how it is used?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

文章

问答

视频

学习中心

腾讯云实验室

直播

竞赛

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋 

腾讯云代码助手

CODING DevOps

Cloud Studio

SDK中心

API中心

命令行工具

我只想知道std::identity的用途是什么？我在网上找不到任何有用的东西。我知道它是如何实现的：template <typename T>struct identity{ T operator()(T x) const { return x; }};为什么我们真的需要这个？

问什么是std::identity以及如何使用它？
EN

回答 5

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问什么是std::identity以及如何使用它？EN

回答 5

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问什么是std::identity以及如何使用它？
EN