String构造函数，析构函数和赋值函数

#include <assert.h>class String{public:String(const char*) ;String(const String&);String& operator=(const String&);~String();void Clone(const String&);private:char* buffer_;};String::String(const char* s){if (!s){buffer_ = new char[1];assert(buffer_ != NULL);buffer_[0] = '\0';}else{Clone(s);}}String::String(const String& rhs){Clone(s);}String& String::operator=(const String& rhs){if (this == &rhs ){return *this;}delete [] buffer_;Clone(s);return *this;}String::~String(){if (buffer_){delete [] buffer_;buffer_ = NULL;}}void String::Clone(const String& rhs){buffer_ = new char[strlen(rhs.buffer_)+1];assert(buffer_ != NULL);;strcpy(buffer_, rhs.buffer_);}