c++三元运算符

发布于 2024-09-10 05:25:49 字数 1264 浏览 7 评论 0原文

因此，我遇到了一些关于三元运算符的有趣的事情，但我没有意识到（至少在 Visual C++ 98-2010 中）。正如 http://msdn.microsoft.com/ 中指出的en-us/library/e4213hs1(VS.71).aspx 如果表达式和条件表达式均为左值，则结果为左值。

当然，通常在 c/c++ 中你会写这样的内容：

int value = (x == 1) ? 1 : 0;

甚至从不关心右值/左值的参与，在这种情况下，1 和 0 都不能转换为左值。

然而，采取类似：

int value = (x == 1) ? y : z;

y 和 z 都是左值，它们，或者更准确地说，其中之一是三元运算符的实际结果（不是它的存储值），这不一定是显而易见的（至少我从来没有认真思考过这个问题）。

但是，这会导致能够编写以下

代码 (x == 1 ? y : z) = 99;

如果 x == 1，则将 99 分配给 y；如果 x !，则将 99 分配给 z = 1

我从未在任何地方以及我读过的有关使用（或者通常是否使用）三元运算符的所有讨论中看到过这种描述。

当然，只有当表达式和条件表达式都是左值时，例如

(x == 1 ? 0 : z) = 99;

无法编译，因为 0 是右值，编译器高兴地指出。

只有包含括号

x == 1 才有效？ y : z = 99;

是完全不同的东西，它仅在 (x != 1) 时才将 99 分配给 z，而美妙的部分是两边仍然是左值，因此存在严重的鼠洞像 (x == 1 ? y : z = 99) = 100 这样的事情（它根据 x == 1 的真实情况将 100 分配给 y 或 z，如果满足 z = 99 的赋值，则踩踏 z = 99 的赋值） x==1 是 false）

所以，这引出了我的问题：

A）这是否是实际的 C++ 标准的一部分（看起来应该是）而不仅仅是 Microsoft 的东西 - 我已经看过但失败了，到目前为止，找到这个信息。

B) 如果这被广泛认识到，而我却一直生活在岩石下呢？我从未见过它在我能记得的任何代码中使用过，也从未在讨论三元运算符时见过它被提及。

C) 我需要更频繁地出去吗？

原文

So I ran into something interesting that I didn't realize about the ternary operator (at least in Visual C++ 98-2010). As pointed out in http://msdn.microsoft.com/en-us/library/e4213hs1(VS.71).aspx if both the expression and conditional-expression are l-values the result is an l-value.

Of course normally in c/c++ you'd write something like:

int value = (x == 1) ? 1 : 0;

and never even care about the r-value/l-value involvment, and in this case neither 1 nor 0 are convertible to l-values.

However, take something like:

int value = (x == 1) ? y : z;

both y and z are l-values and they, or more precisely, one of them is the actual result of the ternary operator (not its stored value) which isn't necessarily obvious (at least I had never thought about it at any length).

But, what that leads to is the ability to write the following

(x == 1 ? y : z) = 99;

Which assigns 99 to y if x == 1 or 99 to z if x != 1

I've never seen that described anywhere and in all the discussions I've read about the use (or, usually, whether to use) the ternary operator.

Of course it only works if both the expression and conditional-expression are l-values something like

(x == 1 ? 0 : z) = 99;

fails to compile because 0 is an r-value as happily pointed out by the compiler.

And this only works if you include the parenthesis

x == 1 ? y : z = 99;

is something entirely different which assigns 99 to z only if (x != 1) and the beautiful part is that both sides are still l-values so there is the serious rat-hole of what things like (x == 1 ? y : z = 99) = 100 do (it assigns 100 to y or z depending on the truth of x == 1, stomping on the z = 99 assignment if x==1 is false)

So, this leads me to my questions:

A) Is this part of the actual c++ standard (which seems like it would be) and not just a Microsoft thing -- I've looked but have failed, so far, to find this info.

B) If this is widely realized and I've been living under a rock? I've never seen it used in any code that I can recall, and never seen it mentioned when the ternary operator is discussed.

C) Do I need to get out more often?

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