函数 stoi 未声明

I'm trying to use stoi to convert a string to an integer, however it says it's not declared. I have the standard library and the <string> included, but it still says [Error] 'stoi' was not declared in this scope

代码如下:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string>

using namespace std;

int main()
{
string end, init;
cout << "Introduction" << endl;
cout << "Start time (xx:yy)" << endl;
cin >> init;
string hours0 = init.substr(0,2);
int hours = stoi(hours0);
cout << hours << endl;
system("pause");
return 0;

}

请告诉我为什么它不起作用，或者给我第二个选择.