函數原型：
string to_string (int val);
string to_string (long val);
string to_string (long long val);
string to_string (unsigned val);
string to_string (unsigned long val);
string to_string (unsigned long long val);
string to_string (float val);
string to_string (double val);
string to_string (long double val);

功能：

將數值轉化為字符串。返回對應的字符串。

例子：

#include <iostream>   
#include <string>     
using namespace std;

int main()
{
    string pi = "pi is " + std::to_string(3.1415926);
    string perfect = to_string(1 + 2 + 4 + 7 + 14) + " is a perfect number";
    cout << pi << '\n';
    cout << perfect << '\n';
    system("pause");
    return 0;
}

參考：https://blog.csdn.net/u010455041/article/details/50097251