J-字符串处理，这样写为什么错了呀？

#include <iostream>

using namespace std;
int cnt;
int main()
{
    string a,b;
    cin >> a;
   
    int l = a.size();
    for(int i = 0;i < l;i ++)
    {
        if(a[i] >= 'A' && a[i] <= 'Z') b[cnt ++] = a[i];
        else if(a[i] >= 'a' && a[i] <= 'z') b[cnt ++] = a[i] - 32;
            else continue;
    }
    for(int i = 0;i < cnt;i ++) cout << b[i];
    return 0;
}