关于开关{}情况下用C？ [英] About switch{} case in C?

问题描述

我读 C语言一些文字。文说，开关{}情况下只能接受整型。

I am reading some text in C language. The text says that switch{} case can only accept integer type.

我只是好奇，为什么开关{}情况下不接受其他类型的如float或字符串。在这背后有什么原因呢？

I am just curious about why switch{} case does not accept other types such as float or string. Are there any reasons behind this?

非常感谢。

推荐答案

经典的原因可能是对整数值的决定前pressions，这是可以做到非常好的优化。

The classical reason is probably that for integer-valued "decision expressions", it's possible to do very nice optimizations.

基本上，你可以区分语句列表映射到包含地址的表，然后直接跳到基础上的价值。显然，对于花车和字符串不起作用。

Basically, you can map the list of case-statements to a table containing addresses, and then directly jump based on the value. Obviously, for floats and strings that doesn't work.

在GCC，你可以手工使用一些扩展像这样：

In GCC, you can do this by hand using some extensions like so:

const char * digit_name(int d)
{
  const void * handlers[] = { &&zero, &&one, &&two, &&three, &&four,
                              &&five, &&six, &&seven, &&eight, &&nine };
  goto *handlers[d]; /* Assumes d is in range 0..9. */

zero:  return "zero";
one:   return "one";
two:   return "two";
three: return "three";
four:  return "four";
five:  return "five";
six:   return "six";
seven: return "seven";
eight: return "eight";
nine:  return "nine";
 return NULL;
}

这是一般称为计算GOTO，它应该清楚如何开关基本上可以编译成非常类似的东西。在接通前pression的紧密定义有所帮助，如使用枚举。

This is in general called a "computed goto", and it should be clear how a switch can basically be compiled down to something very similar. Tight definition of the switched-on expression helps, such as using an enum.

此外，C并没有真正有多少串的概念在语言级别。

Also, C doesn't really have much of a concept of strings at the language level.

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