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目標：為string對象提供一個printf形式的格式化函數(shù) 方法：vsprintf先輸出到一個char[]，再拷貝到string變量中 問題：vsprintf輸出的char[]緩沖區(qū)該開多大？ 開小了可能不夠，開大了浪費空間，多大是夠大？ 有沒有函數(shù)能預先計算出目標串的長度？ 有沒有其他思路解決這個問題？ char fmt_buf[4096]; int stringprintf( string& str, char* fmt, ... ) { va_list args; va_start(args, fmt); if ( vsnprintf ( fmt_buf, sizeof(fmt_buf)-1, fmt, args ) < 0 ) return -1; str = fmt_buf; va_end(args); return ( str.size() ); } 奉上完美解決方案（例子見main函數(shù)）： #include <stdio.h>#include <stdlib.h>#include <stdarg.h>#include <string.h>#define ZEROPAD 1#define SIGN 2#define PLUS 4#define SPACE 8#define LEFT 16#define SPECIAL 32#define LARGE 64#define PAGE_SIZE (0x1 << 12)#define is_digit(c) ((c) >= '0' && (c) <= '9')static size_t strnlen(const char * s, size_t count){ const char *sc; for (sc = s; count-- && *sc != '\0'; ++sc) /* nothing */; return sc - s;}static int skip_atoi(const char **s){ int i=0; while (is_digit(**s)) i = i*10 + *((*s)++) - '0'; return i;}static int do_div(long &n, int &base){ int y = n%base; n = n/base; return y;}static char * number(char * buf, char * end, long num, int base, int size, int precision, int type){ char c,sign,tmp[66]; const char *digits; static const char small_digits[] = "0123456789abcdefghijklmnopqrstuvwxyz"; static const char large_digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; int i; digits = (type & LARGE) ? large_digits : small_digits; if (type & LEFT) type &= ~ZEROPAD; if (base < 2 || base > 36) return 0; c = (type & ZEROPAD) ? '0' : ' '; sign = 0; if (type & SIGN) { if (num < 0) { sign = '-'; num = -num; size--; } else if (type & PLUS) { sign = '+'; size--; } else if (type & SPACE) { sign = ' '; size--; } } if (type & SPECIAL) { if (base == 16) size -= 2; else if (base == 8) size--; } i = 0; if (num == 0) tmp[i++]='0'; else while (num != 0) tmp[i++] = digits[do_div(num,base)]; if (i > precision) precision = i; size -= precision; if (!(type&(ZEROPAD+LEFT))) { while(size-->0) { if (buf <= end) *buf = ' '; ++buf; } } if (sign) { if (buf <= end) *buf = sign; ++buf; } if (type & SPECIAL) { if (base==8) { if (buf <= end) *buf = '0'; ++buf; } else if (base==16) { if (buf <= end) *buf = '0'; ++buf; if (buf <= end) *buf = digits[33]; ++buf; } } if (!(type & LEFT)) { while (size-- > 0) { if (buf <= end) *buf = c; ++buf; } } while (i < precision--) { if (buf <= end) *buf = '0'; ++buf; } while (i-- > 0) { if (buf <= end) *buf = tmp[i]; ++buf; } while (size-- > 0) { if (buf <= end) *buf = ' '; ++buf; } return buf;}int my_vsnprintf(char *buf, size_t size, const char *fmt, va_list args){ int len; unsigned long num; int i, base; char *str, *end, c; const char *s; int flags; int field_width; int precision; int qualifier; if((int) size < 0) { printf("improper call of my_vsnprintf!\n"); return 0; } str = buf; end = buf + size - 1; if (end < buf - 1) { end = ((char *) -1); size = end - buf + 1; } for (; *fmt ; ++fmt) { if (*fmt != '%') { if (str <= end) *str = *fmt; ++str; continue; } /* process flags */ flags = 0; repeat: ++fmt; /* this also skips first '%' */ switch (*fmt) { case '-': flags |= LEFT; goto repeat; case '+': flags |= PLUS; goto repeat; case ' ': flags |= SPACE; goto repeat; case '#': flags |= SPECIAL; goto repeat; case '0': flags |= ZEROPAD; goto repeat; } /* get field width */ field_width = -1; if (is_digit(*fmt)) field_width = skip_atoi(&fmt); else if (*fmt == '*') { ++fmt; /* it's the next argument */ field_width = va_arg(args, int); if (field_width < 0) { field_width = -field_width; flags |= LEFT; } } /* get the precision */ precision = -1; if (*fmt == '.') { ++fmt; if (is_digit(*fmt)) precision = skip_atoi(&fmt); else if (*fmt == '*') { ++fmt; /* it's the next argument */ precision = va_arg(args, int); } if (precision < 0) precision = 0; } /* get the conversion qualifier */ qualifier = -1; if (*fmt == 'h' || *fmt == 'l' || *fmt == 'L' || *fmt =='Z' || *fmt == 'z') { qualifier = *fmt; ++fmt; if (qualifier == 'l' && *fmt == 'l') { qualifier = 'L'; ++fmt; } } /* default base */ base = 10; switch (*fmt) { case 'c': if (!(flags & LEFT)) { while (--field_width > 0) { if (str <= end) *str = ' '; ++str; } } c = (unsigned char) va_arg(args, int); if (str <= end) *str = c; ++str; while (--field_width > 0) { if (str <= end) *str = ' '; ++str; } continue; case 's': s = va_arg(args, char *); if ((unsigned long)s < PAGE_SIZE) s = "<NULL>"; len = strnlen(s, precision); if (!(flags & LEFT)) { while (len < field_width--) { if (str <= end) *str = ' '; ++str; } } for (i = 0; i < len; ++i) { if (str <= end) *str = *s; ++str; ++s; } while (len < field_width--) { if (str <= end) *str = ' '; ++str; } continue; case 'p': if (field_width == -1) { field_width = 2*sizeof(void *); flags |= ZEROPAD; } str = number(str, end, (unsigned long) va_arg(args, void *), 16, field_width, precision, flags); continue; case 'n': /* FIXME: * What does C99 say about the overflow case here? */ if (qualifier == 'l') { long * ip = va_arg(args, long *); *ip = (str - buf); } else if (qualifier == 'Z' || qualifier == 'z') { size_t * ip = va_arg(args, size_t *); *ip = (str - buf); } else { int * ip = va_arg(args, int *); *ip = (str - buf); } continue; case '%': if (str <= end) *str = '%'; ++str; continue; /* integer number formats - set up the flags and "break" */ case 'o': base = 8; break; case 'X': flags |= LARGE; case 'x': base = 16; break; case 'd': case 'i': flags |= SIGN; case 'u': break; default: if (str <= end) *str = '%'; ++str; if (*fmt) { if (str <= end) *str = *fmt; ++str; } else { --fmt; } continue; } if (qualifier == 'L') num = va_arg(args, long); else if (qualifier == 'l') { num = va_arg(args, unsigned long); if (flags & SIGN) num = (signed long) num; } else if (qualifier == 'Z' || qualifier == 'z') { num = va_arg(args, size_t); } else if (qualifier == 'h') { num = (unsigned short) va_arg(args, int); if (flags & SIGN) num = (signed short) num; } else { num = va_arg(args, unsigned int); if (flags & SIGN) num = (signed int) num; } str = number(str, end, num, base, field_width, precision, flags); } if (str <= end) *str = '\0'; else if (size > 0) /* don't write out a null byte if the buf size is zero */ *end = '\0'; /* the trailing null byte doesn't count towards the total * ++str; */ return str-buf;}int my_vsprintf(char *buf, const char *fmt, va_list args){ return my_vsnprintf(buf, (~0U)>>1, fmt, args);}/* haha your function here! */int stringprintf(char **buf, char* fmt, ... ){ va_list args; va_start(args, fmt); /* get need buf size */ char testbuf[1]; int needsize = my_vsnprintf(testbuf,0,fmt,args); /* alloc buf */ *buf = (char*)malloc(needsize+1); /* do it */ my_vsprintf(*buf, fmt,args); va_end(args); return needsize;}/* sample */void main(){ char *buf = NULL; int i = stringprintf(&buf, "%s%d", "it's so nice!", 55); printf("%s\n", buf); free(buf);} |
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