http://www.cplusplus.com/reference/sstream/ostringstream/
https://en.cppreference.com/w/cpp/io/basic_stringstream
https://www.cnblogs.com/hdk1993/p/5853233.html
https://blog.csdn.net/qq1987924/article/details/7671154
template<class T>
void to_string(string & result,const T& t)
{
ostringstream oss;//创建一个流
oss<<t;//把值传递如流中
result=oss.str();//获取转换后的字符转并将其写入result
}
std::ostringstream os;
BigInt c;
os << c;
std::string str = os.str();