+ =和= + C赋值运算符有什么区别 [英] What is the difference between += and =+ C assignment operators

问题描述

我想知道 = + 和 + = (以及其他赋值运算符)之间是否存在差异.我尝试过并且都做了同样的事情.那么有区别还是有约定?两者都能工作是因为我的编译器不检查标准字体吗?

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?

我犯了一个错误.在测试期间，我使用了错误的输入，导致我认为它们都在做相同的事情.原来他们是两回事.

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.

+ = 运算符将右值添加到左值

+= operator adds rvalue to lvalue

x += y;
x = x + y;

= + 只需将分配给左值

x =+ y;
x = +y;
x = y;

推荐答案

在现代C甚至是中古的C语言中， + = 是复合赋值运算符，而 = + 被解析为两个单独的标记. = 和 + .标点符号允许相邻.

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.

所以，如果您写:

x += y;

等效于

x = x + y;

除了 x 仅被评估一次(如果它是一个更复杂的表达式，这可能很重要).

except that x is only evaluated once (which can matter if it's a more complicated expression).

如果您写:

x =+ y;

然后将其解析为

x = + y;

和 + 是一元加号运算符.

非常的C早期版本(大约1970年代中期，1978年K& R1出版之前)使用了不同的符号进行化合物赋值.在现代C使用 + = 的地方，早期C使用 = + .早期的C没有一元 + 运算符，但确实有一元-运算符，并且使用 =-会引起问题.程序员会写 x = -y 的意思是 x = -y ，但是它被默默地解释为 x =-y .为了避免该问题，在1975年至1978年之间对语言进行了更改.直到1999年，我使用了一个编译器(VMS上的VAXC)，该编译器会警告 =-的使用模棱两可，但会使用较早的含义.除非您是一个业余爱好者，并且正在使用一些非常旧的软件和/或硬件，否则现在不必担心.

Very early 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=-y intending 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 very old software and/or hardware.

(A 1975 C参考手册显示了旧的 =-， = + 等复合赋值运算符的形式，由Kernighan和Ritchie出版的第一版 C编程语言1978年，显示了现代的-= ， + = 等，但在时代性"下提到了较旧的形式.)

(A 1975 C Reference Manual shows the old =-, =+, et al forms of the compound assignment operators. The first edition of The C Programming Language by Kernighan and Ritchie, published in 1978, shows the modern -=, +=, et al, but mentions the older forms under "Anachronisms".)

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