How to create a string key at runtime?

[Randalphwa]

Migrated from the original repository
Original issue: serge-sans-paille/frozen#143
Originally reported by: @jgm-ktg

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Here is non-frozen code. I need the key being looked up to come from data outside the program... i.e. not a literal. Does frozen do this or does it not solve that gperf-like problem?

std::unordered_map<std::string_view, int> m = {
    {{"even_length",11},0},
    {{"length",6},1},
    {{"maybe",5},2},
    {{"symbol",6},3},
    {{"space_to_nonspace",17},4}
};
std::string line;
std::getline(std::cin, line);
fprintf(stderr,"%s\n", line.c_str());
fprintf(stderr,"%.*s index: %d\n", (int)line.length(), line.c_str(), m.at({line.c_str(),line.length()}));