我看了看，没错误，而且运行也没有报错啊

# include<iostream># include<stdlib.h># include<string.h>
using namespace std; class String{public:&nbsp&nbsp&nbsp&nbsp;String(const char *str = NULL);&nbsp&nbsp&nbsp&nbsp;String(const String &other);&nbsp&nbsp&nbsp&nbsp;~String();&nbsp&nbsp&nbsp&nbsp;String & operator =(const String &other);&nbsp&nbsp&nbsp&nbsp;String & operator +(const String &other);&nbsp&nbsp&nbsp&nbsp;void print(){cout<<m_data<<endl;}private:&nbsp&nbsp&nbsp&nbsp;char *m_data; // 用于保存字符串};
String::String(const char *str){&nbsp&nbsp&nbsp&nbsp;&nbsp&nbsp&nbsp&nbsp;if (NULL == str)&nbsp&nbsp&nbsp&nbsp;{&nbsp&nbsp&nbsp&nbsp;&nbsp&nbsp&nbsp&nbsp;m_data = new char[1];&nbsp&nbsp&nbsp&nbsp;&nbsp&nbsp&nbsp&nbsp;*m_data = '\0';&nbsp&nbsp&nbsp&nbsp;}&nbsp&nbsp&nbsp&nbsp;else&nbsp&nbsp&nbsp&nbsp;{&nbsp&nbsp&nbsp&nbsp;&nbsp&nbsp&nbsp&nbsp;int length = strlen(str);&nbsp&nbsp&nbsp&nbsp;&nbsp&nbsp&nbsp&nbsp;m_data = new char[length + 1];&nbsp&nbsp&nbsp&nbsp;&nbsp&nbsp&nbsp&nbsp;strcpy(m_data, str);&nbsp&nbsp&nbsp&nbsp;}}
String::String(const String &other){&nbsp&nbsp&nbsp&nbsp;int length = strlen(other.m_data);&nbsp&nbsp&nbsp&nbsp;m_data = new char[length +1];&nbsp&nbsp&nbsp&nbsp;strcpy(m_data, other.m_data);}
String::~String(){&nbsp&nbsp&nbsp&nbsp;delete []m_data;}
String & String::operator =(const String &other){&nbsp&nbsp&nbsp&nbsp;if (this == &other)&nbsp&nbsp&nbsp&nbsp;return *this; &nbsp&nbsp&nbsp&nbsp;delete []m_data;&nbsp&nbsp&nbsp&nbsp;&nbsp&nbsp&nbsp&nbsp;int length = strlen(other.m_data);&nbsp&nbsp&nbsp&nbsp;m_data = new char[length+1];&nbsp&nbsp&nbsp&nbsp;strcpy(m_data, other.m_data);&nbsp&nbsp&nbsp&nbsp; return *this; } String & String::operator +(const String &other){&nbsp&nbsp&nbsp&nbsp;String temp;&nbsp&nbsp&nbsp&nbsp;strcpy(temp.m_data, this->m_data);&nbsp&nbsp&nbsp&nbsp;delete []m_data;&nbsp&nbsp&nbsp&nbsp;this->m_data = new char[strlen(this->m_data)+strlen(other.m_data)+1];&nbsp&nbsp&nbsp&nbsp;strcpy(this->m_data, temp.m_data);&nbsp&nbsp&nbsp&nbsp;strcat(this->m_data, other.m_data);&nbsp&nbsp&nbsp&nbsp;return *this; //这里报错 求解释}
int main(){&nbsp&nbsp&nbsp&nbsp;String a = "123456";&nbsp&nbsp&nbsp&nbsp;a.print();&nbsp&nbsp&nbsp&nbsp;String b = a;&nbsp&nbsp&nbsp&nbsp;b.print();&nbsp&nbsp&nbsp&nbsp;String c= a+b;&nbsp&nbsp&nbsp&nbsp; c.print();&nbsp&nbsp&nbsp&nbsp;return 0;}