使用枚举变量定义一个数组作为数组大小

Question

What does it mean to define an array with enum variable as array size?

For example I have the following code:

typedef enum
{
  D_ROM_RDE_GROUP_0 = 0x0,
  D_ROM_RDE_GROUP_1,   

  D_ROM_RDE_MAX_GROUPS

}E_ROM_RDE_GROUPS;

U_08 pPlaneCopy[D_ROM_RDE_MAX_GROUPS];

I don't understand it...

Thanks for the help.

Answer 1

The first thing to remember is that enumeration values are compile-time constant. The other thing is that enumeration values (unless initialized to a specific value) increases. So in your case D_ROM_RDE_GROUP_0 is equal to 0 , D_ROM_RDE_GROUP_1 is equal to 1 and D_ROM_RDE_MAX_GROUPS is equal to 2 .

This means that when you declare the array, it's basically the same thing as

U_08 pPlaneCopy[2];

Answer 2

The enum value D_ROM_RDE_MAX_GROUPS is actually an integer value, in your case: 2. So you are defining an array with two elements. Probably the two elements will be accessed by the indices : D_ROM_RDE_GROUP_0 and D_ROM_RDE_GROUP_1

Answer 3

This is a techique often used to define an array that is in some way related to the values in an enum. For a simple example:

enum  {
   ZERO, 
   ONE,
   TWO,
   THREE,
   MAX_VALUE //has value of 4
};

because the natural value of an enumerated element starts at zero, and increments by one for each member, the final value will always be useful to initialize an array, of any type ( strings , ints , floats , etc) for a number of elements equal to that final value. eg.

int numbers[MAX_VALUE];//an array with 4 elements
int i;

You can then use MAX_VALUE to guarantee treating the array in a loop without going beyond the bounds of the array, eg. something like:

for(i=0;i<MAX_VALUE;i++)//will loop 4 times
{
    numbers[i]=i;//assigned a value equal to its namesake, for i < MAX_VALUE
}  

Given this explanation, the enum in your original post is simply being initialized with a value of 2:

U_08 pPlaneCopy[D_ROM_RDE_MAX_GROUPS];  

is the same as

U_08 pPlaneCopy[2];

Answer 4

An unsigned integer is automatically assigned to each enum elements if not explicitly specified otherwise. Thus, with your enum declaration, D_ROM_RDE_GROUP_0 equals to 0 , D_ROM_RDE_GROUP_1 equals to 1 , and D_ROM_RDE_MAX_GROUPS equals to 2.

Thus, the declaration

U_08 pPlaneCopy[D_ROM_RDE_MAX_GROUPS];

is equivalent to

U_08 pPlaneCopy[2];

Which means, you declared an array with one U_08 element corresponding to each possible value in E_ROM_RDE_GROUPS assuming D_ROM_RDE_MAX_GROUPS is not a value used otherwise.