对身边的大神写的快速读入int和string方法的解读

===========骨灰级菜鸟的第一篇技术博客，勿喷=========
同学写了一个快速读入int和string的程序，用较底层的fread读入数据，我理解了一下，现贴上源码以及自己添加的注释，表示对大神的崇敬

===========骨灰级菜鸟的第一篇技术博客，勿喷=========

/* * $File: fast-read.cc * $Date: Fri Dec 21 14:44:05 2012 +0800 * $Author: Xinyu Zhou <zxytim[at]gmail[dot]com> * * A template for fast read of integer and string. * * This implementation is in MACRO format because it * yields a slightly faster code that that of using * function calls. * * This version of code uses a fixed size buffer to store * inputs by *fread*, and analyze the string manually to convert it * to int or string. Also, update the buffer when it is exhausted. * * sample usage: *    READ_INT(num); *    READ_STR(str); // str is a char*, which no boundary checking *//* {{{ fast read*/#include <cstdio>#include <cctype>FILE *fast_read_fin = stdin;const int N_BUF_MAX = 10000;char fast_read_buffer[N_BUF_MAX];char *fast_read_ptr = fast_read_buffer + N_BUF_MAX,     *fast_read_buf_end = fast_read_buffer + N_BUF_MAX;/*都是宏定义，包含了很多宏定义的技巧*/#define PTR_NEXT() \    do { \        if (fast_read_ptr != fast_read_buf_end) \            break; \        fast_read_ptr = fast_read_buffer; /*效果：当指针指到末尾时重新指到流的开头*/\        fread(fast_read_buffer, N_BUF_MAX, 1, fast_read_fin); /*读入*/\    } while (0) /*只读入一次，使用这个循环体，是为了可以中途break；*//*关于宏定义的技巧，最后没有打分号，为了和调用时的语法 PTR_NEXT(); 后面的分号兼容，*/#define READ_INT(__num__)  \    do { \        int ret = 0, sign = 1; \        for (; ;) /*将fast_read_ptr指向第一个数字或负号*/\        { \            PTR_NEXT(); \            if (isdigit(*fast_read_ptr) || (*fast_read_ptr) == '-') \                break; \            fast_read_ptr ++; \        } \        if ((*fast_read_ptr) == '-') /*若fast_read_ptr指向负号，标记并指向下一个数字*/\        { \            sign = -1; \            fast_read_ptr ++; \        } \        for (; ;) /*计算输入的数字*/\        { \            PTR_NEXT(); \            if (!isdigit(*fast_read_ptr)) \                break; \            ret = ret * 10 + (*(fast_read_ptr ++)) - '0'; \        } \        (__num__) = ret * sign; \    } while (0)#define READ_STR(__str__) \    ({ \        char *start = (__str__), \             *ret = start; \        do { \            for (; ;) /*将fast_read_ptr指向第一个非空白字符*/\            { \                PTR_NEXT(); \                if (!isspace(*fast_read_ptr)) \                    break; \                fast_read_ptr ++; \            } \            for (; ;) /*将输入的非空白字符存起来*/\            { \                PTR_NEXT(); \                if (isspace(*fast_read_ptr)) \                    break; \                *(ret ++) = *(fast_read_ptr ++); /*将输入的非空白字符存起来*/\            } \            (*ret) = 0; \        } while (0); \        ret - start;  /*这是逗号表达式的最后一个表达式，其值就是ret - start*/\    }) /*逗号表达式  (a1, a2, a3);  a1,a2,a3均为表达式（函数调用或者语句）*/        /*逗号表达式的值就是最后一个表达式的值*//* }}}*/