• std::string length() and size() member functions.
  As per the documentation, these are just synonyms.

• Error: invalid operands of types ‘const char [35]’ and ‘const char [2]’ to binary ‘operator+’

std::string str = "Hello " + "world"; // bad!
std::string str = std::string("Hello ") + "there " + "world"; // ok

• char* 与 string https://www.cnblogs.com/devilmaycry812839668/p/6353807.html

const char *st = "hello";
// 赋值转换 
string st1 = st;
// 构造转换 
string s1(st, st + strlen(st));

// string转char *
string st = "My test";
//char *st1 = st;           // 错误类型不同 
//char *st1 = st.c_str();   // 错误类型不同, const char* c_str() const; st对象被析构时，其内容将丢失！
char *st1 = const_cast<char *>(st.c_str()) ;