Facet that ignores all alphabetical characters

alphafmt.cpp

#include <iostream>
#include <sstream>

class whitespace_fmt : public std::ctype<char>
{
    mask table[table_size];
public:
    whitespace_fmt(size_t refs = 0)
        : std::ctype<char>(table, false, refs)
    {
        for (auto c : "abcdefghijklmnopqrstuvwyzABCDEFGHIJKLMNOPQRSTUVWYZ")
        {
            table[static_cast<int>(c)] |= space;
        }
    }
};

int main()
{
    std::stringstream ss("abcdef12345ghij");
    ss.imbue(std::locale(ss.getloc(), new whitespace_fmt));
    
    std::string str;
    ss >> str;
    std::cout << str << std::endl;
}