3.3 自增/自减操作符，和Side effects

心已赠人 2021-12-14 03:04 175阅读 0赞

from [http://www.learncpp.com/cpp-tutorial/33-incrementdecrement-operators-and-side-effects/][http_www.learncpp.com_cpp-tutorial_33-incrementdecrement-operators-and-side-effects]

一个变量自增1与自减1是如此的平常，以至于在C中它们有了自己的操作符。每一个操作符都有两个版本——前缀与后缀。

<table> 
 <tbody> 
  <tr> 
   <th>Operator</th> 
   <th>Symbol</th> 
   <th>Form</th> 
   <th>Operation</th> 
  </tr> 
  <tr> 
   <td>Prefix increment</td> 
   <td>++</td> 
   <td>++x</td> 
   <td>Increment x, then evaluate x</td> 
  </tr> 
  <tr> 
   <td>Prefix decrement</td> 
   <td>––</td> 
   <td>––x</td> 
   <td>Decrement x, then evaluate x</td> 
  </tr> 
  <tr> 
   <td>Postfix increment</td> 
   <td>++</td> 
   <td>x++</td> 
   <td>Evaluate x, then increment x</td> 
  </tr> 
  <tr> 
   <td>Postfix decrement</td> 
   <td>––</td> 
   <td>x––</td> 
   <td>Evaluate x, then decrement x</td> 
  </tr> 
 </tbody> 
</table>

如：

1: int x = 5;

2: int y = ++x; // x is now equal to 6, and 6 is assigned to y

1: int x = 5;

2: int y = x++; // x is now equal to 6, and 5 is assigned to y

再举一例来显示前置与后置之间的差别：

1: int x = 5, y = 5;

2: cout << x << " " << y << endl;

3: cout << ++x << " " << --y << endl; // prefix

4: cout << x << " " << y << endl;

5: cout << x++ << " " << y-- << endl; // postfix

6: cout << x << " " << y << endl;

结果为：

5 5
    6 4
    6 4
    6 4
    7 3

**Side effects**

**A side effect is a result of an operator, exp****ression, statement, or function that presists even after the operator, expression, statement, or function has finished being evaluated. **

**(能不能解释为，表达式，函数，或语句等求值以后的结果)**

如：

1: x = 5;

是有用的。

而：

1: int x = 5;

2: int nValue = Add(x, ++x);

C++并没有定义函数中哪个参数先被求值。可能产生两种结果：1、Add(5,6)   2、Add(6,6).

一个通用的规则是避免使用会引起类似结果的操作符。所有的赋值操作符、自增自减。会引起side effect的操作符因该放在独立的语句中。

**有谁能解释一下side effects不？？**

转载于:https://www.cnblogs.com/grass-and-moon/archive/2012/05/12/2496984.html

[http_www.learncpp.com_cpp-tutorial_33-incrementdecrement-operators-and-side-effects]: http://www.learncpp.com/cpp-tutorial/33-incrementdecrement-operators-and-side-effects/