为什么我不能使用“中断” C ++中的三元条件语句中的语句？ [英] Why can't I use a "break" statement inside a ternary conditional statement in C++?

问题描述

Node是一个非常简单的类，只有一个构造函数和一些变量：一个名称（实际上只是一个char）和两个子节点指针，称为左和右。

我刚刚开始写一些需要删除最左边节点的代码，当我想到这个时，我很高兴：

  Node * current = this-> root; 
 while（true）（current-> left！= nullptr）？ current = current-> left：break; 
  

看起来很简单：在无限循环中，检查当前是否有一个左边的孩子，如果是，将当前设置为该小孩，如果没有，则突破循环。这是一个很酷的小单线，不太可读。 （我已经评论了！）

嗯，我的编译器不喜欢它：

  iterator.cpp：20：70：error：expected expression 
 while（true）（current-> left！= nullptr）？ current = current-> left：break; 
 ^ 
 1生成错误。 
  

此外，只要在while循环中抛出一些大括号，并将三元运算符移动到自己的行，帮助（不出意料）。我不得不把它变成一个if / else来编译器来接受它。

有人可以解释一下它是如何解释单行的，为什么它是对象？ >

解决方案

三元条件运算符是将多个表达式组合成较大表达式的运算符。 break 是一个语句，而不是一个表达式，因此它不能在三元条件表达式中使用。 / p>

尽管如此，您可以重写代码：

  while （current-> left！= nullptr）current = current-> left; 
  

希望这有帮助！

Node is a very simple class with a just a constructor and a few variables: a "name" (actually just a char) and two child Node pointers called "left" and "right".

I was just starting to write some code that needs to drop to the left-most node, and I was quite pleased when I came up with this:

Node *current = this->root;
while (true) (current->left != nullptr) ? current = current->left : break;

Seems simple enough: in an infinite loop, check to see if current has a left child, if so, set current to that left child, if not, break out of the loop. It's a cool little one-liner, not too unreadable. (And I commented it!)

Well, my compiler doesn't like it:

iterator.cpp:20:70: error: expected expression
    while (true) (current->left != nullptr) ? current = current->left : break;
                                                                        ^
1 error generated.

Also, just throwing some braces into the while loop and moving the ternary operator to it's own line didn't help (unsurprisingly). I had to turn it into an if/else for the compiler to accept it.

Can someone explain how it's interpreting the one-liner and why it objects?

解决方案

The ternary conditional operator is an operator that combines multiple expressions into a larger expression. break is a statement and not an expression, so it can't be used inside a ternary conditional expression.

You could, though, rewrite your code like this:

while (current->left != nullptr) current = current->left;

Hope this helps!

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