[面试] 写出 string 类的构造，析构，复制构造函数，复制操作符！

#include <iostream> // 。。。
#define BUG puts("here!!!");

using namespace std;
class String {
private :
char *m_data;
public :
String(const char *str = NULL); // 构造函数
String(const String &other);    // 拷贝构造函数
~String();
String& operator= (const String &other); // 赋值函数
};
String::String(const char* str) {
if(str == NULL) {
m_data = new char[1];
m_data[0] = '\0';
}
else {
int len = static_cast<int> (strlen(str));
m_data = new char[len + 1];
strcpy(m_data, str);
}
}
String::String(const String &other) {
if(other.m_data == NULL) {
m_data = new char[1];
*m_data = '\0';
}
else {
int len = strlen(m_data);
m_data = new char[len+1];
strcpy(m_data, other.m_data);
}
}
String& String::operator= (const String& other) {
if(this == &other) return *this;
delete []m_data;
if(other.m_data == NULL) {
m_data = new char[1];
m_data[0] = '\0';
}
else {
int len = strlen(other.m_data);
m_data = new char[len+1];
strcpy(m_data, other.m_data);
}
return *this;
}
String::~String() {
delete []m_data;
}
int main() {
String sa, sb;
sa = "abcde";
String sc(sa);
return 0;
}