C语言 += 和 =+ C 赋值运算符有什么区别
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What is the difference between += and =+ C assignment operators
提问by Michael
I was wondering if there was a difference between =+and +=(and other assignment operators too). I tried and both did the same thing. So is there a difference or is there a convention? Do both work because my compilers dont check for standarts?
我想知道=+和+=（以及其他赋值运算符）之间是否有区别。我试过了，两者都做了同样的事情。那么有区别还是有约定？两者是否都有效，因为我的编译器不检查标准？
Edit: I made a mistake. I used bad inputs during my testing which led me to thinking they are both doing the same thing. Turns out they are two different things.
编辑：我犯了一个错误。我在测试期间使用了错误的输入，这让我认为他们都在做同样的事情。原来它们是两种不同的东西。
+=operator addsrvalue to lvalue
+=运算符将右值添加到左值
x += y;
x = x + y;
=+simply assignsrvalue to lvalue
=+简单地将右值赋给左值
x =+ y;
x = +y;
x = y;
回答by Keith Thompson
In modern C, or even moderately ancient C, +=is a compound assignment operator, and =+is parsed as two separate tokens. =and +. Punctuation tokens are allowed to be adjacent.
在现代 C 语言中，甚至+=是较古老的 C 语言中，是复合赋值运算符，并被=+解析为两个单独的标记。=和+。标点符号可以相邻。
So if you write:
所以如果你写：
x += y;
it's equivalent to
它相当于
x = x + y;
except that xis only evaluated once (which can matter if it's a more complicated expression).
除了x它只计算一次（如果它是一个更复杂的表达式，这可能很重要）。
If you write:
如果你写：
x =+ y;
then it's parsed as
然后它被解析为
x = + y;
and the +is a unary plus operator.
并且+是一元加运算符。
Veryearly versions of C (around the mid 1970s, before the publication of K&R1 in 1978) used different symbols for compound assignments. Where modern C uses +=, early C used =+. Early C had no unary +operator, but it did have a unary -operator, and the use of =-caused problems; programmers would write x=-yintending it to mean x = -y, but it was silently interpreted as x =- y. The language was changed some time between 1975 and 1978 to avoid that problem. As late as 1999, I worked with a compiler (VAXC on VMS) that would warn about an ambiguous use of =-, but would use the older meaning. That shouldn't be a concern now unless you're a hobbyist playing with some veryold software and/or hardware.
非常的C早期版本（绕70年代中期，K＆R1的在1978年出版之前）用于化合物分配不同的符号。现代 C 使用的地方+=，早期的 C 使用=+。早期的 C 没有一元运算+符，但它确实有一个一元运算-符，并且使用=-引起了问题；程序员会写x=-y它的意思x = -y，但它被默默地解释为x =- y. 为了避免这个问题，语言在 1975 年和 1978 年之间的某个时间发生了变化。直到 1999 年，我还在使用一个编译器（VMS 上的 VAXC），它会警告 的歧义使用=-，但会使用旧的含义。除非您是玩一些非常旧的软件和/或硬件的业余爱好者，否则现在不必担心。
(A 1975 C Reference Manualshows the old =-, =+, et al forms of the compound assignment operators. The first edition of The C Programming Languageby Kernighan and Ritchie, published in 1978, shows the modern -=, +=, et al, but mentions the older forms under "Anachronisms".)
（1975年C参考手册显示旧=-，=+等形式的复合赋值运算符。第一版C语言编程由Kernighan和Ritchie，1978年出版的，显示了现代-=，+=等，但提到了旧的形式在“时代错误”下。）
