Field-level input validation error collector

#include <exception>
#include <map>
#include <string>
#include <vector>

using Errors = std::map<std::string, std::vector<std::string>>;

Errors validate(const std::map<std::string, std::string>& input) {
    Errors errs;
    auto get = [&](const char* k) { auto it = input.find(k); return it == input.end() ? std::string{} : it->second; };
    if (get("name").empty()) errs["name"].push_back("required");
    const std::string email = get("email");
    if (email.find('@') == std::string::npos) errs["email"].push_back("must contain @");
    if (!get("age").empty()) {
        try { if (std::stoi(get("age")) < 18) errs["age"].push_back("must be 18 or older"); }
        catch (const std::exception&) { errs["age"].push_back("must be a number"); }
    }
    return errs;
}