报错 string subscript out of range，怎么办？

#include
#include
using namespace std;
int main()
{

string P = {'c','b'};
for (int i = 0;i<sizeof P;++i)
{
    cout << P[i];
}
cout << "string is over";

}

新学C++，求问老铁们为什么报错“ string subscript out of range”？
该怎么改？

// Q765981.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"

#include <iostream>
#include <string>

using namespace std;
int main()
{
string P = "cb";
for (int i = 0;i< P.length();++i)
{
    cout << P[i];
}
cout << "string is over";
}