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1.   ʲôÊÇ¿ÕÖ¸Õë³£Á¿£¨null pointer constant£©?
[ 6.3.2.3-3] An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.
ÕâÀï¸æËßÎÒÃÇ£º0¡¢0L¡¢'\0'¡¢3 - 3¡¢0 * 17 £¨ËüÃǶ¼ÊÇ“integer constant expression”£©ÒÔ¼° (void*)0 £¨tyc£ºÎÒ¾õµÃ(void*)0Ó¦¸ÃËãÊÇÒ»¸ö¿ÕÖ¸Õë°É£¬¸üÇ¡µ±Ò»µã£©µÈ¶¼ÊÇ¿ÕÖ¸Õë³£Á¿£¨×¢Òâ (char*) 0 ²»½Ð¿ÕÖ¸Õë³£Á¿£¬Ö»ÊÇÒ»¸ö¿ÕÖ¸ÕëÖµ£©¡£ÖÁÓÚϵͳѡȡÄÄÖÖÐÎʽ×÷Ϊ¿ÕÖ¸Õë³£Á¿Ê¹Óã¬ÔòÊÇʵÏÖÏà¹ØµÄ¡£Ò»°ãµÄ C ϵͳѡÔñ (void*)0 »òÕß 0 µÄ¾Ó¶à£¨Ò²Óиö±ðµÄÑ¡Ôñ 0L£©£»ÖÁÓÚ C++ ϵͳ£¬ÓÉÓÚ´æÔÚÑϸñµÄÀàÐÍת»¯µÄÒªÇó£¬void* ²»ÄÜÏó C ÖÐÄÇÑù×ÔÓÉת»»ÎªÆäËüÖ¸ÕëÀàÐÍ£¬ËùÒÔͨ³£Ñ¡ 0 ×÷Ϊ¿ÕÖ¸Õë³£Á¿£¨tyc: C++±ê×¼ÍƼö£©£¬¶ø²»Ñ¡Ôñ (void*)0¡£
2.   ʲôÊÇ¿ÕÖ¸Õ루null pointer£©?
[6.3.2.3-3] If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.
Òò´Ë£¬Èç¹û p ÊÇÒ»¸öÖ¸Õë±äÁ¿£¬Ôò p = 0;¡¢p = 0L;¡¢p = '\0';¡¢p = 3 - 3;¡¢p = 0 * 17; ÖеÄÈκÎÒ»ÖÖ¸³Öµ²Ù×÷Ö®ºó£¨¶ÔÓÚ C À´Ëµ»¹¿ÉÒÔÊÇ p = (void*)0;£©£¬ p ¶¼³ÉΪһ¸ö¿ÕÖ¸Õ룬ÓÉϵͳ±£Ö¤¿ÕÖ¸Õë²»Ö¸ÏòÈκÎʵ¼ÊµÄ¶ÔÏó»òÕߺ¯Êý¡£·´¹ýÀ´Ëµ£¬ÈκζÔÏó»òÕߺ¯ÊýµÄµØÖ·¶¼²»¿ÉÄÜÊÇ¿ÕÖ¸Õë¡££¨tyc: ±ÈÈçÕâÀïµÄ(void*)0¾ÍÊÇÒ»¸ö¿ÕÖ¸Õë¡£°ÑËüÀí½âΪnull pointer»¹ÊÇnull pointer constant»áÓÐ΢ÃëµÄ²»Í¬£¬µ±È»Ò²²»ÊǽôÒªÁË£©
3.   ʲôÊÇ NULL£¿
[6.3.2.3-Footnote] The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant
¼´ NULL ÊÇÒ»¸ö±ê×¼¹æ¶¨µÄºê¶¨Ò壬ÓÃÀ´±íʾ¿ÕÖ¸Õë³£Á¿¡£Òò´Ë£¬³ýÁËÉÏÃæµÄ¸÷ÖÖ¸³Öµ·½Ê½Ö®Í⣬»¹¿ÉÒÔÓà p = NULL; À´Ê¹ p ³ÉΪһ¸ö¿ÕÖ¸Õë¡££¨tyc£ººÜ¶àϵͳÖеÄʵÏÖ£º#define NULL (void*)0£¬ÓëÕâÀïµÄ“a null pointer constant”²¢²»ÊÇÍêÈ«Ò»Öµģ©
4.   ¿ÕÖ¸Õ루null pointer£©Ö¸ÏòÁËÄÚ´æµÄʲôµØ·½£¨¿ÕÖ¸ÕëµÄÄÚ²¿ÊµÏÖ£©£¿
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