C++入出力１

mondai1.cpp

#include <iostream>
#include <sstream>
#include <string>
#include <climits>

using namespace std;

template <typename T>
string numToString(const T &num)
{
    ostringstream oss;
    oss << num;
    return oss.str();
}

int main()
{
    unsigned long ul;
    cout << "正の整数の上限を設定してください: ";
    cin >> ul;
    if (ul < 0 || ul > ULONG_MAX)
    {
        cout << "範囲外の数値です" << endl;
        return 1;
    }
    int idx = 0;
    size_t npos = 0;
    int cnt = 0;
    for (unsigned long i = 1; i <= ul; i++)
    {
        string s = numToString(i);
        while (idx < s.length())
        {
            if ((npos = s.find("7", idx)) != string::npos)
            {
                idx += (npos+1);
                cnt++;
            }
            else
            {
                idx++;
            }
        }
        npos = 0;
        idx = 0;
    }
    cout << "７の個数: " << cnt << endl;
    return 0;
}