std::string::append函數

string& append (const string& str);
string& append (const string& str, size_t subpos, size_t sublen);   
string& append (const char* s);
string& append (const char* s, size_t n);
string& append (size_t n, char c);

/*
std::string stra("helloworld");
std::string str;
str.append(stra);
str.append(stra,0,5);
*/

/*
char pch[15]="helloworld";
std::string str;
str.append(pch);
str.append(pch,5);
*/

/*
std::string str;
str.append(5,'c');
*/