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char,unsigned char,uint8_t,uint16_t
Summary
- Basics
In C/C + +, char is used to define character type variables, accounting for only one byte (8 bits).
In C language such as int, long, short and so on, if you do not specify signed or unsigned, the default is signed, but char is not specified as signed or unsigned in the standard. The compiler can be compiled as signed or unsigned.
But I feel as if most of them are signed without specifying signed or unsigned.
If it is signed, its value range is - 128 ~ + 127 (- 27 ~ 27-1), the symbol occupies 1 bit, others occupy 7 bits.
unsigned char means unsigned, with the range of 0 ~ 255 (0 ~ 2 ^ 8-1)
- Range of values
signed char: -2^7 ~ 2^7-1 unsigned char : 0 ~ 2^8-1
Pow (2,8), a function to calculate the power
- Writing method
char *a = "abcd";//No warning signed char *b = "abcd";//Compiler warning "initializing 'signed char *' with an expression of type 'char [5]' converts between points to integer types with different sign" unsigned char *c = "abcd"; //Compiler warning "initializing 'unsigned char *' with an expression of type 'char [5]' converts between points to integer types with different sign"
The compilation warning is because * b and * c use signed and unsigned respectively to modify the data of type char as integer, and then assign the string of char, so the warning is given
- Character array
char string[] = "abcd"; NSLog(@"%s",string);
- The method of judging the compiler's default char symbol
The method of judging the compiler's default char symbol( This method comes from Baidu Encyclopedia)
char c = -1;
if(c < 200){
printf("signed\n");
}
else{
printf("unsigned\n");
}
My compiler alarms. In line 2, "Comparison of constant 200 with expression of type 'char' is always true", Xcode's Clang compiler is signed by default, so always true.
- cont char
char string[] = "abc"; const char *run = string;
Printing uses% s.
- unsigned char
unsigned char uc = 100; NSLog(@"uc %c",uc); // -> d NSString *str = [NSString stringWithFormat:@"%c",uc]; NSLog(@"str %@",str); // -> d
The definition in the first line is equivalent to
unsigned char uc = 'd';
Printing uses% c.
Turn NSString
char -> NSString
//Define char type characters char *a = "abcd"; //Print character a NSLog(@"char %s",a); //Convert char type to NSString type NSString *str = [NSString stringWithFormat:@"%s",a]; NSLog(@"string : %@",str);
char[] -> NSString
char string[] = "abcd"; NSLog(@"%s",string); NSString *str = [NSString stringWithFormat:@"%s",string]; NSLog(@"string : %@",str);
- How to use NSString
[NSString stringWithUTF8String:(nonnull const char *)]
perhaps
[NSString stringWithCString:(nonnull const char *) encoding:(NSStringEncoding)]
The input parameters are of const char * type, and the return values are of NSString type.
char string[] = "abcd"; NSString *str = [NSString stringWithUTF8String:string]; NSLog(@"string : %@",str);
char *a = "abcd"; NSString *str = [NSString stringWithUTF8String:a]; NSLog(@"string : %@",str);
uint8_t,uint16_t
- uint8_t
The definition in iOS is as follows:
#ifndef _UINT8_T #define _UINT8_T typedef unsigned char uint8_t; #endif /* _UINT8_T */
typedef unsigned char, which defines uint8 Φ t as an unsigned int integer.
In essence, it is an integer with a value range of 0-225.
- uint16_t
Definition in iOS:
#ifndef _UINT16_T #define _UINT16_T typedef unsigned short uint16_t; #endif /* _UINT16_T */
Of type unsigned short.
On the range of value
1 byte = 8 bits (stored in binary), understanding the relationship between bytes and bits helps to understand the storage of data in the computer.
The 32-bit operating system corresponds to 4 bytes and the 64 bit corresponds to 8 bytes. That is to say, on the 32-bit operating system, the computer allocates 4 bytes of memory to store a data.
Because the number of bytes in memory is limited, the data stored is limited, so different types of data are stored in a range on the computer.
char
Unsigned value range: binary is 00000000 ~ 11111111, with 8-bit binary data storage
The value range of char is 0 ~ 255 (0 ~ 2 ^ 8-1)
The range of signed value: - 2 ^ 7 ~ (2 ^ 7) - 1 is - 128 ~ 127, and the sign occupies one place.
int
- Signed 32-bit int
Signed 32-bit int integers range from - 2147483648 (- 2 ^ 31) to 2147483647 (2 ^ 31-1)
int max = pow(2, 31)-1; // -> 2147483647 int min = pow(-2, 31); // -> -2147483648 NSLog(@"%d~%d",min,max);
My computer is 32-bit, and when the calculation is out of range, the value returned is the minimum.
int max = pow(2, 100); // -> -2147483648 int min = pow(-2, 100); // -> -2147483648 NSLog(@"%d~%d",min,max);
pow() must have been processed in this function, so the program does not flash back directly.
- Unsigned 32-bit int
0 ~ 2^32-1
Calculation method of value range
sizeof()
int result_int = sizeof(int); NSLog(@"%d",result_int);// -> 4 int result_f = sizeof(float); NSLog(@"%d",result_f);// -> 4 int result_char = sizeof(char); NSLog(@"%d",result_char);// -> 1 int result_uint8_t = sizeof(uint8_t); NSLog(@"%d",result_uint8_t);// -> 1 int result_uint16_t = sizeof(uint16_t); NSLog(@"%d",result_uint16_t);// -> 2 int result_uint32_t = sizeof(uint32_t); NSLog(@"%d",result_uint32_t);// -> 4 int result_uint64_t = sizeof(uint64_t); NSLog(@"%d",result_uint64_t);// -> 8
1 byte = 8 bits
Some system macros
iOS provides some macros that can directly get the value range of some data types
/* C90/99 5.2.4.2.1 */ #define SCHAR_MAX __SCHAR_MAX__ #define SHRT_MAX __SHRT_MAX__ #define INT_MAX __INT_MAX__ #define LONG_MAX __LONG_MAX__ #define SCHAR_MIN (-__SCHAR_MAX__-1) #define SHRT_MIN (-__SHRT_MAX__ -1) #define INT_MIN (-__INT_MAX__ -1) #define LONG_MIN (-__LONG_MAX__ -1L) #define UCHAR_MAX (__SCHAR_MAX__*2 +1) #define USHRT_MAX (__SHRT_MAX__ *2 +1) #define UINT_MAX (__INT_MAX__ *2U +1U) #define ULONG_MAX (__LONG_MAX__ *2UL+1UL) #ifndef MB_LEN_MAX #define MB_LEN_MAX 1 #endif #define CHAR_BIT __CHAR_BIT__ #ifdef __CHAR_UNSIGNED__ /* -funsigned-char */ #define CHAR_MIN 0 #define CHAR_MAX UCHAR_MAX #else #define CHAR_MIN SCHAR_MIN #define CHAR_MAX __SCHAR_MAX__ #endif /* C99 5.2.4.2.1: Added long long. C++11 18.3.3.2: same contents as the Standard C Library header <limits.h>. */ #if __STDC_VERSION__ >= 199901L || __cplusplus >= 201103L #undef LLONG_MIN #undef LLONG_MAX #undef ULLONG_MAX #define LLONG_MAX __LONG_LONG_MAX__ #define LLONG_MIN (-__LONG_LONG_MAX__-1LL) #define ULLONG_MAX (__LONG_LONG_MAX__*2ULL+1ULL) #endif
Some macros about int
/* 7.18.2.1 Limits of exact-width integer types */
#define INT8_MAX 127
#define INT16_MAX 32767
#Define int 32_max 2147483647 / / the maximum value of signed 32-bit int data
#define INT64_MAX 9223372036854775807LL
#define INT8_MIN -128
#define INT16_MIN -32768
/*
Note: the literal "most negative int" cannot be written in C --
the rules in the standard (section 6.4.4.1 in C99) will give it
an unsigned type, so INT32_MIN (and the most negative member of
any larger signed type) must be written via a constant expression.
*/
#Define int 32 min (- int32 MAX-1) / / the minimum value of signed 32-bit int data
#define INT64_MIN (-INT64_MAX-1)
#define UINT8_MAX 255
#define UINT16_MAX 65535
#Define uint32? Max 4294967295u / / the maximum value of unsigned 32-bit int data
#define UINT64_MAX 18446744073709551615ULL
/* 7.18.2.2 Limits of minimum-width integer types */
#define INT_LEAST8_MIN INT8_MIN
#define INT_LEAST16_MIN INT16_MIN
#define INT_LEAST32_MIN INT32_MIN
#define INT_LEAST64_MIN INT64_MIN
#define INT_LEAST8_MAX INT8_MAX
#define INT_LEAST16_MAX INT16_MAX
#define INT_LEAST32_MAX INT32_MAX
#define INT_LEAST64_MAX INT64_MAX
#define UINT_LEAST8_MAX UINT8_MAX
#define UINT_LEAST16_MAX UINT16_MAX
#define UINT_LEAST32_MAX UINT32_MAX
#define UINT_LEAST64_MAX UINT64_MAX
/* 7.18.2.3 Limits of fastest minimum-width integer types */
#define INT_FAST8_MIN INT8_MIN
#define INT_FAST16_MIN INT16_MIN
#define INT_FAST32_MIN INT32_MIN
#define INT_FAST64_MIN INT64_MIN
#define INT_FAST8_MAX INT8_MAX
#define INT_FAST16_MAX INT16_MAX
#define INT_FAST32_MAX INT32_MAX
#define INT_FAST64_MAX INT64_MAX
#define UINT_FAST8_MAX UINT8_MAX
#define UINT_FAST16_MAX UINT16_MAX
#define UINT_FAST32_MAX UINT32_MAX
#define UINT_FAST64_MAX UINT64_MAX
/* 7.18.2.4 Limits of integer types capable of holding object pointers */
#if __WORDSIZE == 64
#define INTPTR_MAX 9223372036854775807L
#else
#define INTPTR_MAX 2147483647L
#endif
#define INTPTR_MIN (-INTPTR_MAX-1)
#if __WORDSIZE == 64
#define UINTPTR_MAX 18446744073709551615UL
#else
#define UINTPTR_MAX 4294967295UL
#endif
/* 7.18.2.5 Limits of greatest-width integer types */
#define INTMAX_MIN INT64_MIN
#define INTMAX_MAX INT64_MAX
#define UINTMAX_MAX UINT64_MAX
/* 7.18.3 "Other" */
#if __WORDSIZE == 64
#define PTRDIFF_MIN INT64_MIN
#define PTRDIFF_MAX INT64_MAX
#else
#define PTRDIFF_MIN INT32_MIN
#define PTRDIFF_MAX INT32_MAX
#endif
//Signed int data value range (32-bit operating system) NSLog(@"%d~%d",INT_MIN,INT_MAX); // -> -2147483648~2147483647 NSLog(@"%d~%d",INT_LEAST32_MIN,INT_LEAST32_MAX); // -> -2147483648~2147483647 //Unsigned 32-bit int maximum, i.e. 2 ^ 32-1 NSLog(@"%u",UINT32_MAX); // -> 4294967295 NSLog(@"%f",pow(2, 32)-1); // -> 4294967295.000000
