[advanced C language] special user-defined type

1、 Bit segment

1.1、 What is a bit segment

1、 The member of the segment must be int、unsigned int 、 signed int or char It belongs to the integer family .
2、 There is a colon and a number after the member name of the bit segment
3、 The function of bit segment is to save memory space

1.2、 Bit segment memory allocation

1、 The space of the bit segment is as needed in 4 Bytes （int）, perhaps 1 Bytes （char） The way to open up .

2、 There are many uncertainties in the bit segment , Therefore, bit segments are not cross platform （ Focus on portable programs Bit segments should be avoided ）

1.3、 The cross platform problem of bit segment

1、int It's uncertain whether a bit segment is treated as a signed number or an unsigned number

2、 The number of the largest bits in the bit segment cannot be determined （32 The machine is the largest 32,64 The machine is the largest 64）

3、 The members in the bit segment are allocated from left to right in memory , Or right to left allocation criteria have not yet been defined

4、 When a structure contains two bit segments , The second segment is relatively large , Cannot hold the remaining bits of the first bit segment , Whether to discard the remaining bits or to use , This is uncertain

2、 enumeration

1、 Enumeration is a List , List all the possibilities

such as ：
One 、 There are seven days in a week , Monday to Sunday can be enumerated
Two 、 Month has 12 Months , January to December can be enumerated
wait

2.1、 Definition of enumeration type

1、 Enumeration is a special custom type
2、 The enumerated quantities are Constant

enum Day// Enumerate the days of the week 
{
    
	// Possible values of enumeration 
	Mon = 1,// Give Way Mon be equal to 1 The operation of , Is to assign an initial value to an enumeration constant , Does not mean that the enumeration is a variable 
	Tues,
	Wed,
	Thur,
	Fri,
	Sta,
	Sun
};

enum Sex// Enumerate gender 
{
    
	MALE,
	FEMALE,
	SECRET
};
int main()
{
    
	enum Day d = Sun;
	enum Sex s = SECRET;
	return 0;
}

2.2、 Advantages of enumeration

1、 Increase the readability and maintainability of the code

2、 and #define Defined identifier comparison , Enumeration has type checking , More rigorous

3、 Prevent naming pollution

4、#define Unable to debug , Enumeration can be debugged

3、 union （ Shared body ）

1、 Federation is also a special custom type

2、 Variables defined by this type also contain a series of members , The feature is that these members share a space

3、 The members of the consortium share a memory space , The size of such a joint variable , At least the size of the largest member .（ The consortium needs to be able to keep the largest members ）

4、 Use cases

5、 Use the union to judge the byte order of large and small ends

cheak_sys()
{
    
	union Un
	{
    
		char c;
		int i;
	}u;
	u.i = 1;
	return u.c;
}

int main()
{
    
	int ret = cheak_sys();
	if (1 == ret)
		printf(" The small end \n");
	else
		printf(" Big end \n");
	// If you return 1, Indicates the small endian byte order 
	// If you return 0, Indicates the big endian byte order 
	return 0;
}

3.1、 Calculation of the size of the consortium

1、 The size of the consortium is at least the largest member .
2、 When the maximum member size is not an integral multiple of the maximum number of alignments , It's about aligning to an integer multiple of the maximum number of alignments