case label does not reduce to an integer constant

一开始发现这个问题，是在switch中使用了const int类型的量作为case，举例如下：

int main(void)
{
    
    

	const int c[3] = {
    
    0, 1, 2};

	int a = 0;

	switch (a)
		{
    
    
		case c[0]:
			a = c[1];
			break;
		case c[1]:
			a = c[2];
			break;
		case c[2]:
			a = c[0];
			break;
		default:
			a = c[0];
		}

	return 0;
}

GCC编译出现了错误： case label does not reduce to an integer constant。

回到switch的用法看看，case后面能够使用什么量呢？
答案是：int型常量、char型常量、enum型常量、sizeof表达式&经过强制类型转换后的浮点型常量。

…it is worth looking briefly at what an integral constant expression is, since that is what must follow the case labels in a switch statement. Loosely speaking, it is any expression that does not involve any value-changing operation (like increment or assignment), function calls or comma operators. The operands in the expression must all be integer constants, character constants, enumeration constants, sizeof expressions and floating-point constants that are the immediate operands of casts. Any cast operators must result in integral types.
——https://publications.gbdirect.co.uk/c_book/chapter3/flow_control.html

也就是说这样写case是可以的：

typedef enum{
    
    
	ZERO,
	ONE
}numbers;

case 0: //integer constant
	break;
case 'A': //character constant
	break;
case ONE: //enumeration constant
	break;
case sizeof(int): //sizeof expressions
	break;
case (int)5.0:
	break;

那么const在C语言中的作用是？
——准确地说，const in C does not mean something is constant. It just means a variable is read-only，表示只读而非常量，因此是不能用在case之后的。

BTW，C语言中常量除了可以用enum还可以用#define定义。