基础知识
位 & 字节
计算机存储的是二进制（比如101010）。每一个数字就是一位 (bit), 每 8 位是一个字节 (Byte), 位是计算机传输的最小单位，而字节是编码的最小单位。
字符
每个文字都是一个字符，不同的编码对应同样字符所需的字节数也是不同的。（英文占一个字节，中文占两个字节）
编码
编码就是字节到字符的规则。（比如 0001 代表 1，也可以用 01 代表 1 这个字符）
解码
有编码就有解码，数据在网络中是二进制形式传输，加密与解密配套时可以正确解码二进制流信息。
总结
位和字节都是单位，字符是看到的结果，解码编码则是固定的规则
NSString & NSData & char
Definition
NSData
- 遵循 NSCopying NSCoding 协议, 它提供面向对象的数组存储为字节，即二进制数据流类型。
- 读写文件需要一个缓冲区，而 NSData 就提供缓存区。
Coding
char <-> NSString
逐
char打印时，中文乱码是因为无法一次打印两个, 后面unicode部分有解决方法。const char *chars = "this is a string, and contain 中文"; // char to NSString NSString *string = [[NSString alloc] initWithCString:chars encoding:NSUTF8StringEncoding]; NSLog(@"%lu, %@", string.length, string); // 32, this is a string, and contain 中文 // NSString to char chars = [string cStringUsingEncoding:NSUTF8StringEncoding]; NSLog(@"%lu, %s", strlen(chars), chars); // 36, this is a string, and contain ‰∏≠Êñáchar -> NSData
NSData *dataWithBytes = [NSData dataWithBytes:chars length:strlen(chars)]; NSLog(@"%@", dataWithBytes); // <74686973 20697320 61204320 73747269 6e672c20 616e6420 636f6e74 61696e20 e4b8ade6 9687>NString <-> NSData
NSString *string = @"this is a string, and contain 中文"; // NSString to data NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding]; NSLog(@"%@", data); // <74686973 20697320 61207374 72696e67 2c20616e 6420636f 6e746169 6e20e4b8 ade69687> // NSData to NSString string = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]; NSLog(@"%@", string); // this is a string, and contain 中文NSString <-> ASCII
// NSString to int ASCII NSString *charString = @"A"; int asciiCode = [charString characterAtIndex:0]; NSLog(@"%i", asciiCode); // 65 // int to NSString asciiCode = 66; charString = [NSString stringWithFormat:@"%c", asciiCode]; NSLog(@"%@", charString); // Bchar <-> ASCII
// char to int ASCII char character = 'A'; int asciiCode = (int)character; NSLog(@"%i", asciiCode); // 65 // int to char asciiCode = 66; character = (char)asciiCode; NSLog(@"%c", character); // B
unicode
%c
8-bit unsigned character (unsigned char), printed by NSLog() as an ASCII character, or, if not an ASCII character, in the octal format \ddd or the Unicode hexadecimal format \udddd, where d is a digit.
%C
16-bit Unicode character (unichar), printed by NSLog() as an ASCII character, or, if not an ASCII character, in the octal format \ddd or the Unicode hexadecimal format \udddd, where d is a digit.
NSString *theString = @"g"; unichar theChar = [theString characterAtIndex:0]; NSString *theString1 = [NSString stringWithFormat:@"%c", theChar]; NSString *theString2 = [NSString stringWithFormat:@"%C", theChar]; NSLog(@"%@, %d, %@, %@",theString, theChar, theString1, theString2); // g, 103, g, g theString = @"家"; theChar = [theString characterAtIndex:0]; theString1 = [NSString stringWithFormat:@"%c", theChar]; theString2 = [NSString stringWithFormat:@"%C", theChar]; NSLog(@"%@, %d, %@, %@",theString, theChar, theString1, theString2); // 家, 23478, ¶, 家
