switch case VS map

[xiaolomg]

今天想到这个问题，在网上搜了，真是出乎意料：
STL Map that comes with visual studio 2008 will give you O(log(n)) for each function call since it hides a tree structure beneath. With modern compiler (depending on implementation) , A switch statement will give you O(1) , the compiler translates it to some kind of lookup table. So in general , switch is faster.

However , consider the following facts:

The difference between map and switch is that : Map can be built dynamically while switch can't. Map can contain any arbitrary type as a key while switch is very limited to c++ Primitive types (char , int , enum , etc...).

By the way , you can use a hash map to achieve nearly O(1) dispatching (though , depending on the hash table implementation , it can sometimes be O(n) at worst case). Even though , switch will still be faster.

顿时打破我之前的观点，不知道Vs2008是否对switch进行如此优化，各位大神来讨论下啊。

[taodm]

鬼扯的英文。 你在1~100000的值域内随机取10个值switch自己试，看哪个编译器可以给你O（1）

[珍惜生命远离CPP]

switch在特定的情况下优化的很厉害。

[赵4老师]

不要低估编译器release版输出时对switch语句优化的能力。 VS IDE中想看release版输出时switch语句对应的汇编指令，可以在switch前临时加一句__asm int 3,然后先重建所有、再按F5、再按Alt+8

[帅得不敢出门]

在各case值相近的情况下，switch可能会比if else 快。

[jackzhhuang]

switch是比较快的，但文章也说了，map是动态的。

[swordtan]

这个还真不知道，估计switch优化的时候，采用类似hash的方式达到0（1）的速度，现在的编译器越做越好了