問題描述

一個主要是為了好玩/好奇的問題:如何在 C++ 中編寫一個 for 循環來迭代 bool 的兩個值(即 true 和 false)，僅使用 bool 的操作(即不轉換為其他類型)?

A question mostly for fun/curiosity: how to write a for loop in C++ that would iterate over two values of a bool (i.e. true and false), using only operations with bool (i.e. without conversions to other types)?

背景是我想檢查像 (A && B) || 這樣的方程有多少解(!B && !C && !D) == true，然后開始寫類似 for (bool A=false; ??? ; ++A) for (bool B=false; ...) 等，但立即被 ??? 卡住 - 即繼續循環的條件是什么?當然，我將它改寫為使用 int，而且我也知道 do ... while 循環會起作用，但是我很好奇是否有可能編寫這樣的 for 循環?由于 SO 似乎沒有答案，我決定問:)

The background is that I wanted to check how many solutions exists for an equation like (A && B) || (!B && !C && !D) == true, and started to write something like for (bool A=false; ??? ; ++A) for (bool B=false; ...) etc but immediately got stuck by ??? - i.e. what would be the condition to continue the loop? Of course I rewrote it to use int, and I also know that a do ... while loop will work, but I got curious if it's ever possible to write such a for loop? And since SO does not seem to have an answer, I decided to ask :)

更新:請注意，在至少兩個現已刪除的答案中建議的明顯"變體 for(bool A=false; !A; A=true) 只會運行一次迭代，因為對于第二個條件 !A 變為 false 并且循環結束.

Update: note that an "obvious" variant for(bool A=false; !A; A=true) suggested in at least two now-removed answers will only run one iteration, because for the second one the condition !A becomes false and the loop ends.

經過深思熟慮后，我相信在沒有第二個變量或像 Dietmar Kühl 建議的基于指針的構造的情況下，在 C++03 中是不可能做到的.條件應在所需的執行中測試 3 次，因此布爾值的兩個值根本不夠.do-while 循環之所以有效，是因為第一次迭代是無條件執行的，條件只檢查兩次，因此可以使用 bool 值在繼續和退出之間進行選擇.

After some pondering, I believe it's impossible to do it in C++03 without a second variable or a pointer based construct like suggested by Dietmar Kühl. The condition should be tested three times in a desired execution, so two values of a bool are simply not enough. And the do-while loop works because the first iteration is executed unconditionally, the condition is only checked twice and so a bool value can be used to select between continuing and exiting.

推薦答案

C++11: for (bool b : { false, true }) {/* ... *