C风格字符串等价于string 类型字符串字面值
string str1("Hello!"); //ok.
char *str2 = str1; //error.
char *str3 = str2.c_str(); //ok. but not quite.
//注意c_str()返回的数据类型是const char
const char *str4 = str2.c_str(); //ok.
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