C++ 中的 += 运算符

+= Operator in C++

提问by Itban Saeed

Someone please tell me the difference between the following codes which add two variables of datatype int. I want to know which one is better.

有人请告诉我添加两个数据类型变量的以下代码之间的区别int。我想知道哪个更好。

Code A:

代码 A：

sum = sum + value;

Code B:

代码 B：

sum += value;

We usually prefer ++operator over += 1. Is there any specific reason behind that as well ?

我们通常更喜欢++运算符而不是+= 1. 这背后有什么具体原因吗？

I want to know the difference between the above codes with respect to the conventions or efficiency level. Which one is recommended ?

我想知道上述代码在约定或效率级别方面的区别。推荐哪一款？

回答by Some programmer dude

While the end result of the e.g. someVar++operator is the same as someVar += 1there are other things playing in as well.

虽然 egsomeVar++运算符的最终结果与someVar += 1其他东西相同。

Lets take a simple statement like

让我们做一个简单的陈述，比如

foo = bar++;

It's actually equivalent (but not equal) to

它实际上相当于（但不等于）

temp = bar;
bar += 1;
foo = temp;

As for the prefix and suffix increment or decrement operators, they have different operator precedence, which will affect things like pointer arithmeticusing those operators.

至于前缀和后缀递增或递减运算符，它们具有不同的运算符优先级，这将影响使用这些运算符的指针算术等内容。

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As for the difference between

至于两者的区别

foo += 1;

and

和

foo = foo + 1;

there's no different for primitive types (like intor float) or pointer types, but there's a very big difference if foois an object with operator overloading. Then

原始类型（如intor float）或指针类型没有什么不同，但是如果foo是带有运算符重载的对象，则有很大的不同。然后

foo += 1;

is equal to

等于

foo.operator+=(1);

while

尽管

foo = foo + 1;

is equal to

等于

temp = foo.operator+(1);
foo.operator=(temp);

Semantically a very big difference. Practically too, especially if any of the operator overload functions have side-effects, or if the copy-constructor or destructor have some side-effects (or you forget the rules of three, five or zero).

语义上有很大的不同。实际上也是如此，特别是如果任何运算符重载函数有副作用，或者如果复制构造函数或析构函数有一些副作用（或者你忘记了三、五或零的规则）。

回答by Paul Evans

One calls operators =and +the later calls operator +=.

一个调用 operator =，+后者调用 operator +=。

operators ++and +=are preferred because of readability - most programmers know what they mean.

运算符++，+=因为可读性而受到青睐——大多数程序员都知道它们的意思。

On the other hand most modern compilers will generate the same code for += 1as ++and +/=as +=for builtin types;

另一方面，大多数现代编译器将为+= 1as++和+/ =as生成与+=内置类型相同的代码；

But for user defined classs, the actual operators will be called and it's up to the implementer of those classs to make sense of it all. In these cases ++and +=can be optimal.

但是对于用户定义的classs，将调用实际的运算符，由这些classs的实现者来理解这一切。在这些情况下++，+=可以是最佳的。

回答by Pétur Ingi Egilsson

cout << sum++;Would print out the value of sum before it was incremented. Also, depending on what you are doing, you can overwrite the operators +=and +.

cout << sum++;将在 sum 增加之前打印出 sum 的值。此外，根据您在做什么，您可以覆盖运算符+=和+.

回答by Grantly

When you minimize code, you reduce the chance of an error (a typographical error or a logical error).

当您最小化代码时，您会减少出现错误（印刷错误或逻辑错误）的机会。

By using

通过使用

sum += value;

you reduce the chance - ever so slightly - of an error while typing

你减少了打字时出错的机会 - 非常轻微 -

sum = sum + value;

The same with value++;

与 value++;

value += 1;

could be more easily confused with

可能更容易与

value += l;where l is a variable....

value += l;其中 l 是一个变量....

Its more about consistency that it is about right or wrong, but reducing code is a major bonus for maintainability.

它更多的是关于一致性，它关乎对或错，但减少代码是可维护性的主要奖励。

Care must be taken with precendence of operators however, in complex statements.

但是，在复杂语句中，必须注意运算符的优先级。

回答by Bob Jarvis - Reinstate Monica

In the case shown there's no particular reason to prefer one method of incrementing the value over another except perhaps for readability purposes. In this case I think I'd prefer sum += valueover sum = sum + valueas it's a bit briefer and (I think) clearer, but YMMV on that.

在所示的情况下，除了可能出于可读性目的之外，没有特别的理由更喜欢一种增加值的方法而不是另一种方法。在这种情况下，我认为我更喜欢它sum += value，sum = sum + value因为它更简洁并且（我认为）更清晰，但是 YMMV 对此。

As far as prefering ++over += 1, (IMO again) ++is preferable when incrementing a value as part of an expression, e.g. sum += array[index++]- but if the entire point of what's being done is adding one to a value I'd prefer index += 1. But let's face it, a great deal of this is personal preference and spur-of-the-moment choice. I always try to write what I think, at that moment, is the simplest and clearest code possible - but I must admit that when I go back and read some of my own code later I have more "What was I thinkin'?!?" moments than I'd care to admit to. :-)

至于prefering++过+= 1，（IMO再次）++增加值作为表达式，如部分时最好sum += array[index++]-但如果得到了些什么，整个点增加一个到一个值我喜欢index += 1。但让我们面对现实，这在很大程度上是个人喜好和一时冲动的选择。我总是试着写出我当时认为最简单、最清晰的代码——但我必须承认，当我回去阅读我自己的一些代码时，我有更多的“我在想什么？！？ ” 比我愿意承认的时刻。:-)

YMMV.

天啊。

Best of luck.

祝你好运。

回答by James Wu

Code A and B do the same thing. The advantage to using Code B is that it's quicker to type and easier to read.

代码 A 和 B 做同样的事情。使用代码 B 的优点是键入速度更快且更易于阅读。

As for using the ++operator over += 1, again it is for readability. Although there is a difference between foo++and ++foo. The former is read first and then incremented, while the latter is incremented first and then read from.

至于使用++运算符 over += 1，再次是为了可读性。虽然foo++和之间有区别++foo。前者先读后递增，后者先递增后读。

回答by Sazzad Hissain Khan

A compound assignment expression of the form E1 op= E2is equivalent to E1 = (T)((E1) op (E2)), where Tis the type of E1, except that E1is evaluated only once.

形式的复合赋值表达式E1 op= E2等价于E1 = (T)((E1) op (E2))，其中T是 的类型E1，只是E1只计算一次。

An example cited from Java's +=, -=, *=, /= compound assignment operators

引用自Java 的 +=、-=、*=、/= 复合赋值运算符的示例

[...] the following code is correct:

[...] 以下代码是正确的：

short x = 3;
x += 4.6;

and results in x having the value 7 because it is equivalent to:

并导致 x 的值为 7，因为它等价于：

short x = 3;
x = (short)(x + 4.6);

回答by Saleem Ullah

They both are same up to that, both can be used for incrementing the value of a variable (stored in it).

它们都是相同的，都可以用于增加变量的值（存储在其中）。

x++will increment the value of xby one (1) every run time.

x++将在x每次运行时将 的值增加一 (1)。

+=adds right operand to the left operand and stores the result in left operand. Something like following: C += Ais just same as C = C + A

+=将右操作数添加到左操作数并将结果存储在左操作数中。类似于以下内容： C += A与C = C + A

The difference between both ++and +=is that the first can increment by one (1) only, while +=can be used to increment more than one in just one line.

二者之间的差++和+=是通过一（1）在第一罐增量而已，而+=可以使用在仅一个线递增一个以上。

e.g:

例如：

x += 1; // will increment by 1 every run time
x += 4; // will increment by 4 every run time
x += 10; // will increment by 10 every run time

回答by washington

There is no difference between the two in terms of functionality. A += B actually means A = A + B. The first one is just a shorter way of writing the second.

两者在功能上没有区别。A += B 实际上意味着 A = A + B。第一个只是写第二个的更短的方式。

回答by washington

Its basically the same thing. Its both an operator.

它基本上是一样的。它既是运营商。

One of it calls =and +. And the other +=..

其中之一调用=和+。而另一个+=..

So if you did value +=5. Value goes up by 5. += is better and more organized. And shortens your code whitch is better and more professional.

所以如果你确实价值+=5。值增加 5。+= 更好，更有条理。并缩短您的代码，使其更好更专业。