a =（a + b） - （b = a）; C ++ vs php [英] a = (a + b) - (b = a); C++ vs php

问题描述

我一直在寻找，发现公式： a =（a + b） - （b = a）它应该交换两个变量一些情况）。但是我用C ++和php测试它们，这些给我不同的结果。

php：

< $ a = 10;
$ b = 20;
$ a =（$ a + $ b） - （$ b = $ a）; $ b $ d echo $ a，，$ b;


打印 20 10

C ++

  int a = 10; 
 int b = 20; 
 a =（a + b） - （b = a）; 
 std :: cout<< a<< < b; 
  

打印 10 10

代码看起来一样，但输出不同，我一直在想两个原因：

$ b

有人可以解释为什么C ++和php输出在这种情况下不同？

解决方案

规则是在PHP中，但是在C ++中，单个子表达式的顺序没有严格定义，或者作为技术术语，它是未指定 - 换句话说，允许编译器计算 b = a 之前或之后 a + b 。只要在减法之前它 a + b 和 b = a 使用未指定行为允许编译器在某些情况下生成更高效的代码，或者只是为某些体系结构构建一个编译器。





这也意味着如果你有一个表达式重新计算表达式中的值，并且在表达式的其他地方使用它，你会得到未定义的行为简称）。 UB意味着，行为没有定义 - 几乎任何事情都可能发生，包括你看到的和许多其他选择（例如，编译器允许产生42作为结果，即使逻辑说，答案不会42在这种情况下[这是错误的问题！]）。





我也建议如果你想在PHP中交换两个值：

  $ t = $ a; 
 $ a = $ b; 
 $ b = $ t; 
  

和在C ++中：

  #include< algorithm> 
 
 std :: swap（a，b）; 
  

或如果您坚持写自己的：

  int t = a; 
 a = b; 
 b = t; 
  

想要聪明，执行它没有临时变量几乎肯定会使它慢使用临时（当然是在像C ++这样的编译语言中） - 在像PHP这样的解释性语言中，创建一个新的变量可能会增加一点额外的开销，但是它不可能是那么大，相比之下， 。




I've been looking around and found formula: a = (a + b) - (b = a) it is supposed to swap two variables (or objects in some cases). However I tested it with C++ and php, these gave me different result.

php:

$a = 10;
$b = 20;
$a = ($a + $b) - ($b = $a);
echo $a, " ", $b;

This prints 20 10

C++

int a = 10;
int b = 20;
a = (a + b) - (b = a);
std::cout << a << " " << b;

This prints 10 10

Code looks the same but outputs are different, I've been thinking about two reasons:

1. C++ code is compiling and php is interpreting.
2. This formula is useless because it leads to undefined behavior.

Can somebody explains, why C++ and php output differs in this situation?

解决方案

I'm not sure what the rules are in PHP, but in C++, the order of individual sub-expressions isn't strictly defined, or as the technical term is, it is "unspecified" - in other words, the compiler is allowed to calculate b = a before or after it does a + b. As long as it does a + b and b = a before the subtraction. The use of "unspecified" behaviour allows the compiler to produce more efficient code in some cases, or simply that it's possible to build a compiler for some architectures.

It also means that if you have an expression that "recalculates" a value within the expression itself, and also using it elsewhere in the expression, you get unedefined behaviour (UB for short). UB means just that, the behaviour is not defined - almost anything could happen, including what you are seeing and many other alternatives (e.g. the compiler is allowed to produce 42 as a result as well, even if logic says the answer wouldn't be 42 in this case [it's the wrong question for that!]).

I would also suggest that if you want to swap two values, in PHP:

 $t = $a;
 $a = $b;
 $b = $t;

and in C++:

 #include <algorithm>

 std::swap(a, b); 

or if you insist on writing your own:

 int t = a;
 a = b;
 b = t; 

Trying to be clever and perform it "without temporary variable" is almost certainly going to make it slower than the use of a temporary - certainly in a compile language like C++ - in a interpreted language like PHP, creating a new variable may add a bit of extra overhead, but it's unlikely to be that big, compared to the extra effort in the logic required.

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