21. Filter Strings C++

#include <iostream>
#include <string>

std::string FilterString(const std::string&);

int main()
{

    std::string s;
    std::cin >> s;

    std::cout << FilterString(s) << std::endl;

    return 0;
}

std::string FilterString(const std::string& s)
{
    std::string out;
    for (char ch : s)
        if ('0' <= ch && ch <= '9')
            out += ch;

    return out;
}