这段代码是否滥用了STL的find_if？

发布于 2024-07-04 13:19:17 字数 343 浏览 6 评论 0原文

假设我有一个存储在向量中的服务器名称列表，我想一次与他们联系，直到他们成功响应为止。我正在考虑按以下方式使用STL的find_if算法：

find_if(serverNames.begin(), serverNames.end(), ContactServer());

其中ContactServer是谓词函数对象。
一方面，存在一个问题，因为谓词并不总是为相同的服务器名称返回相同的结果（由于服务器停机、网络问题等......）。然而，无论使用谓词的哪个副本（即谓词没有真实状态），都将返回相同的结果，因此状态保持谓词的原始问题与这种情况无关。

你怎么说？

原文

Let's say I have a list of server names stored in a vector, and I would like to contact them one at a time until one has successfully responded. I was thinking about using STL's find_if algorithm in the following way:

find_if(serverNames.begin(), serverNames.end(), ContactServer());

Where ContactServer is a predicate function object.
On one hand, there's a problem since the predicate will not always return the same result for the same server name (because of server downtime, network problems, etc...). However, the same result will be returned regardless of which copy of the predicate is used (i.e. the predicate has no real state), so the original problem with state-keeping predicates is not relevant in this case.

What do you say?

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假装爱人 2024-07-11 13:19:17

std::for_each 可能是更好的选择。

1）在每个元素上使用相同的函数对象进行复制之后，并且在处理所有元素之后，将可能更新的函数对象的副本返回给用户。

2）我认为这也会提高调用的可读性。

函数对象和 for_each 调用如下所示：


struct AttemptServerContact {
  bool        server_contacted;
  std::string active_server;    // name of server contacted

  AttemptServerContact() : server_contacted(false) {}

  void operator()(Server& s) {
    if (!server_contacted) {
      //attempt to contact s
      //if successful, set server_contacted and active_server
    }
  }
};

AttemptServerContact func;
func = std::for_each(serverNames.begin(), serverNames.end(), func);
//func.server_contacted and func.active_server contain server information.

std::for_each might be a better candidate for this.

1) After being copied in the same function object is used on each element and after all elements are processed a copy of the potentially updated function object is returned to the user.

2) It would improve the readability of the call in my opinion as well.

The function object and the for_each call would look something like:


struct AttemptServerContact {
  bool        server_contacted;
  std::string active_server;    // name of server contacted

  AttemptServerContact() : server_contacted(false) {}

  void operator()(Server& s) {
    if (!server_contacted) {
      //attempt to contact s
      //if successful, set server_contacted and active_server
    }
  }
};

AttemptServerContact func;
func = std::for_each(serverNames.begin(), serverNames.end(), func);
//func.server_contacted and func.active_server contain server information.

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说好的呢 2024-07-11 13:19:17

find_if 在这里似乎是正确的选择。在这种情况下谓词不是有状态的。

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画▽骨i 2024-07-11 13:19:17

在我看来，std::find_if 的这种用法有点误导。当我阅读这段代码时，我不希望发生任何副作用，我只是希望找到一个服务器名称。 find_if 的结果被丢弃这一事实也让我怀疑代码是否真的正确。也许谓词的另一个名称会让意图更清晰，但我认为问题更根本。

对于大多数人来说，find_if 是一种查询算法，而不是修改算法。即使您实际上没有修改迭代的值，您也正在修改应用程序的全局状态（在这种情况下，您甚至可能修改远程服务器的状态）。

在这种情况下，我可能会坚持使用手动循环，特别是现在 C++11 引入了基于范围的 for 循环：

for (std::string const & name : serverNames)
{
    if (ContactServer(name)) break;
}

另一个解决方案是将其封装在一个函数中，其名称更能清楚地传达意图，例如 < code>apply_until 或类似的东西：

template <typename InputIterator, typename Function>
void apply_until(InputIterator first, InputIterator last, Function f)
{
    std::find_if(first, last, f);
    // or
    // while (first != last)
    // {
    //     if (f(*first)) break;
    //
    //     ++first;
    // }
}
}

但也许我过于纯粹了:)！

In my opinion, this use of std::find_if is a bit misleading. When I read this piece of code, I don't expect any side effects to occur, I just expect a server name to be found. The fact that the result of find_if is discarded would also make me wonder if the code is really correct. Perhaps another name for the predicate would make the intent clearer, but I think the problem is more fundamental.

For most people, find_if is a querying algorithm, not a modifying algorithm. Even though you are not actually modifying the values iterated upon, you are modifying the global state of your application (in this case, you are even possibly modifying the state of distant servers).

In such a case, I would probably stick with a manual loop, especially now that C++11 introduced range-based for loops:

for (std::string const & name : serverNames)
{
    if (ContactServer(name)) break;
}

Another solution would be to encapsulate this in a function with a name conveying more clearly the intent, such as apply_until or something like that:

template <typename InputIterator, typename Function>
void apply_until(InputIterator first, InputIterator last, Function f)
{
    std::find_if(first, last, f);
    // or
    // while (first != last)
    // {
    //     if (f(*first)) break;
    //
    //     ++first;
    // }
}
}

But perhaps I'm being over-puristic :)!

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