c++ 字符串和数字相互转化

       在C++11中string 类中有stoi、stod、stof、stol和stoll等函数分别是把字符串转化为int、double、float、long和long、long型。

       而数字转字符串则用to_string

       用法举例如下：

#include <iostream>   // std::cout
#include <string>     // std::string, std::stod

int main ()
{
std::string str_bin = "2456"
int i_bin = std::stoi (str_bin);
std::string s=std::to_string(i_bin);

}

另外，stoi还可以指定向2、8、10、16进制转化：

// stoi example
#include <iostream>   // std::cout
#include <string>     // std::string, std::stoi

int main ()
{
std::string str_dec = "2001, A Space Odyssey";
std::string str_hex = "40c3";
std::string str_bin = "-10010110001";
std::string str_auto = "0x7f";

std::string::size_type sz;   // alias of size_t

int i_dec = std::stoi (str_dec,&sz);
int i_hex = std::stoi (str_hex,nullptr,16);
int i_bin = std::stoi (str_bin,nullptr,2);
int i_auto = std::stoi (str_auto,nullptr,0);

std::cout << str_dec << ": " << i_dec << " and [" << str_dec.substr(sz) << "]\n";
std::cout << str_hex << ": " << i_hex << '\n';
std::cout << str_bin << ": " << i_bin << '\n';
std::cout << str_auto << ": " << i_auto << '\n';

return 0;
}