字符串

size_t strlen( const char *str ){size_t length = 0;char *p = str; //保持str指向串首不变while( *p++ )++length;return length;}

char* strcpy( char *dest, const char *source ){assert( NULL != dest && NULL != source ); //对源地址和目的地址加非NULL断言char *pd = dest;char *ps = source;while( *pd++ = *ps++ ); //先执行*,再执行赋值,再判断*pd，最后++. 所以退出循环前'\0'已被赋值return dest;}

void* memcpy( void *dest, const void *source, size_t n ){assert( NULL != dest && NULL != source ); //首先判断指针是否有效char *pd = dest;char *ps = source;while( n-- > 0 )*pd++ = *ps++;return dest; //注意:返回的是原来的void* 类型的dest指针}

void* memmove( void *dest, const void *source, size_t n ){assert( NULL != dest && NULL != source );char *pd = dest;char *ps = source;if( pd < ps || ps + n -1 < pd ) //如果两块内存不重叠则正常操作{while( n-- ){*pd++ = *ps++;}}else{pd += n - 1; //否则找到最后一个元素,反向操作ps += n - 1;while( n-- ){*pd-- = *ps--;}}return dest;}

char* strcat( char *str1, char *str2 ){assert( NULL != str1 && NULL != str2 );char *p1 = str1;char *p2 = str2;while( *p1 ) p1++; //pd指向str1的'\n'while( *p1++ = *p2++ ); //类似strcpy的操作return str1;}

int strcmp( const *str1, const *str2 ){assert( NULL != str1 && NULL != str2 );char *p1 = str1;char *p2 = str2;while( *p1++ == *p2++ )//先执行*,然后==,然后运行循环体,最后++{if( *p1 == '\0' )  //其实此时:*p1 == *p2 == '\0',且之前的字符全相等return 0;}return ( (*p1 < *p2) ? -1 : 1 ); //比较不相同字符间大小}

char* strchr( const char *str, char ch ){char *p = str;while( *p++ != '\0' ){if( *p == ch )return p;}return ( *p == ch ? p : NULL ); //ch也有可能是'\0'}

char* strstr( const char *str1, const char *str2 ){char *p = str1;size_t len = strlen(str2);while( *p++ != '\0' ) //p=strchr(p,*s2){if( strncmp( p,str2,len ) == 0 )return p;}return NULL;}

void* memset( void *ptr, int ch, size_t n ){unsigned char c = ch;unsigned char *p = ptr;while( n-- ){*p++ = c;}return ptr;}

************************* class string *************************#include <iostream>#include <"string.h"> //使用标准库函数using namespace std;class String{friend ostream& operator << ( ostream &os, String &str ); //友元函数friend istream& operator >> ( istream &is, String &str );public:String();String( char *s );                         //使用字符串变量/常量初始化String( const String &other );             //由于数据成员为指针，需自定义拷贝构造函数~String();                                 //动态分配的内存空间，需手动释放String& operator = ( const String &other );//this=other,返回this的引用String operator + ( const String &other ); //new=this+other,返回函数中局部变量newbool operator == ( const String &other );  //判断是否相等char& operator [] ( unsigned int index );  //下标取值,返回字符的引用size_t size();                             //返回string的长度(不含'\0')private:char * ptr; //指针变量,用于指向动态存储空间中的字符数组};String::String(){ptr = NULL;}String::String( char *str ){size_t len = strlen(str);ptr = new char[len+1]; //动态分配内存strcpy( ptr, str );}String::String( const String &other ){size_t len = strlen(other.ptr);ptr = new char[len+1]; //动态分配内存,深拷贝strcpy( ptr, other.ptr );}String::~String(){if( ptr != NULL )delete [] ptr;     //释放堆空间(注意：采用new[]分配的空间必须采用delete[]释放)ptr = NULL;}String& String::operator = ( const String &other ){if( this != &other ){if( ptr != NULL )delete [] ptr; //释放原空间,再重新开辟ptr = new char[strlen(other.ptr)+1];stpcpy( ptr, other.ptr );}return *this;          //返回String类型引用}String String::operator + ( const String &other ){String tmp;            //局部变量,返回时复制给临时变量tmp.ptr = new char[strlen(this.ptr)+strlen(other.ptr)+1];strcpy( tmp.ptr, this.ptr );strcat( tmp.ptr, other.ptr );return tmp;            //由于采用深拷贝,返回值传递过程都会重新开辟空间并复制,不会发生重复释放内存}bool String::operator == ( const String &other ){return (strcmp( this.ptr, other.ptr ) == 0); //采用string.h中的标准库函数实现}char& String::operator [] ( unsigned int index ){if( index >= 0 && index < strlen(this.ptr) )return this.ptr[index];                  //将String[]取值转换成了数组下标取值}size_t String::size(){return strlen(this.ptr);}ostream& operator << ( ostream &os, String &str ){os << str.ptr;   //使用iostream类的<<函数return os;}istream& operator >> ( istream &is, String &str ){char tmp[100];   //临时字符串数组if( is >> tmp )  //如果有输入{delete [] str.ptr;str.ptr = new char[strlen(tmp)+1];strcpy( str.ptr, tmp );}return is;}