如何在一个循环内只运行一次没有外部标志的代码？ [英] How to run code inside a loop only once without external flag?

问题描述

我想检查循环中的条件，并在第一次遇到时执行一段代码。之后，循环可能重复，但块应该被忽略。有这样的模式吗？当然，很容易在循环外声明一个标志。但我对一个完全生活在循环中的方法感兴趣。

I want to check a condition inside a loop and execute a block of code when it's first met. After that, the loop might repeat but the block should be ignored. Is there a pattern for that? Of course it's easy to declare a flag outside of the loop. But I I'm interested in an approach that completely lives inside the loop.

这个例子不是我想要的。有没有办法摆脱循环外的定义？

This example is not what I want. Is there a way to get rid of the definition outside of the loop?

bool flag = true;
for (;;) {
    if (someCondition() && flag) {
        // code that runs only once
        flag = false;
    }        
    // code that runs every time
}

推荐答案

这是相当hacky，但正如你所说的应用程序主循环，我认为它是一个一次调用函数，所以以下应该工作：

It's fairly hacky, but as you said it's the application main loop, I assume it's in a called-once function, so the following should work:

struct RunOnce {
  template <typename T>
  RunOnce(T &&f) { f(); }
};

:::

while(true)
{
  :::

  static RunOnce a([]() { your_code });

  :::

  static RunOnce b([]() { more_once_only_code });

  :::
}

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