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Custom formatters

When no built-in pattern flag carries the field you need — a request id, a thread name, a tenant — the supported move is adding your own pattern flag, not stuffing that field into every individual log message by hand.

custom_flag_formatter​

thread_name_flag.hpp
class thread_name_flag : public spdlog::custom_flag_formatter {
public:
    void format(const spdlog::details::log_msg&, const std::tm&,
                spdlog::memory_buf_t& dest) override {
        std::string name = current_thread_name();   // your own lookup
        dest.append(name.data(), name.data() + name.size());
    }

    std::unique_ptr<custom_flag_formatter> clone() const override {
        return spdlog::details::make_unique<thread_name_flag>(*this);
    }
};

Wiring it up​

auto f = std::make_unique<spdlog::pattern_formatter>();
f->add_flag<thread_name_flag>('*').set_pattern("[%*] %v");
spdlog::set_formatter(std::move(f));

'*' is the flag character you're choosing to bind — pick one that isn't already a built-in flag (see Pattern flags for the reserved set).

clone() is not optional​

Each sink clones the formatter; a clone() that drops state produces silently wrong output

spdlog clones the formatter once per sink so each sink can format independently. If clone() doesn't copy every piece of state your flag depends on, sinks beyond the first silently format with default-initialized (often empty or wrong) state — no crash, no warning, just quietly incorrect output in the second and later sinks.

Replacing the whole formatter​

For a wire format that isn't line-oriented at all — say, a fixed binary layout for a metrics collector — implement spdlog::formatter directly instead of composing pattern flags. That's more work than custom_flag_formatter, but it's the right tool when the pattern-flag model (a sequence of fields in a text line) doesn't match what you're producing.

Where formatters live​

A formatter can be set per logger or per sink, following the same override rules as patterns — see Multi-sink loggers. Whichever level you set it at, remember it gets cloned once per sink; see Sink overview for how a sink actually consumes its formatter.

See also​