C++ switch case vs if else
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switch case vs if else
提问by Jose Vega
I was wondering if there was any difference in the way the following code was compiled into assembly. I've heard that switch-case is more efficient than if else, but in this example I am not quite sure if that would be the case.
我想知道以下代码编译成程序集的方式是否有任何不同。我听说 switch-case 比 if else 更有效率，但在这个例子中，我不太确定是否会这样。
if(x==1){
...
}else if(x==2){
...
}else{
...
}
and
和
switch(x){
case 1:
...
break;
case 2:
...
break;
default:
...
}
回答by Jose Vega
A compiler will sometimes turn a switch into a jump-table, if the entries are contiguous (or nearly so). Or it could theoretically use a binary search to find the case instead of a linear series of tests, which would be faster if you had a large number of cases.
如果条目是连续的（或几乎是连续的），编译器有时会将开关转换为跳转表。或者理论上它可以使用二分搜索来查找案例而不是线性系列测试，如果您有大量案例，这会更快。
On the other hand, there's nothing stopping the compiler from doing the same optimisations on the same code converted into if/else.
另一方面，没有什么可以阻止编译器对转换为 if/else 的相同代码进行相同的优化。
So on a good compiler, switch can be faster in some cases. On a very good compiler, they'd be the same.
所以在一个好的编译器上，在某些情况下 switch 会更快。在一个非常好的编译器上，它们是一样的。
回答by Moishe Lettvin
Note too that the if/else construct can be more efficient if you know certain cases are more likely than others.
另请注意，如果您知道某些情况比其他情况更有可能，则 if/else 构造可能会更有效。
回答by Chris Jester-Young
In thisspecific case, the switchcan be turned into a jump table. The ifstatement (if you write your =as ==:-P) could still do the same thing if the compiler could tell that xisn't changing between the ifclauses (which is usually the case, unless xis volatileor something).
在这种特定情况下，switch可以变成一个跳转表。如果编译器可以判断在子句之间没有变化（通常是这种情况，除非是或某事），则该if语句（如果您将其编写=为==:-P）仍然可以做同样的事情。xifxvolatile
