class some_class {};
namespace shared {
    std::ostream& operator<<(std::ostream& os, const some_class& c) {
        return os << "some_class";
    }
}
using namespace shared; // not necessary

namespace fmt {
    using namespace shared;
}

void custom_class_example() {
    some_class c;
    auto console = spd::stdout_logger_mt("console");
    console->info("custom class with operator<<: {}..", c);
    console->info() << "custom class with operator<<: " << c << "..";
}