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Creating loggers

There are two ways to make a logger: the factory helpers, which build a sink and register the logger in one call, and explicit construction, for when you want to own the sinks yourself or avoid the global registry entirely.

Factory helpers​

FactorySink createdRegistered
spdlog::stdout_color_mt(name)Colored console (stdout)Yes
spdlog::stderr_color_mt(name)Colored console (stderr)Yes
spdlog::basic_logger_mt(name, path)Plain file, no rotationYes
spdlog::rotating_logger_mt(name, path, max_size, max_files)Size-based rotating fileYes
spdlog::daily_logger_mt(name, path, hour, minute)Date-stamped daily fileYes

Every one of these both creates the logger and registers it, so spdlog::get(name) finds it afterward from anywhere in the program.

_mt vs _st​

Every factory has an _mt (mutex-protected sink) and _st (single-threaded, unguarded) variant — stdout_color_st, basic_logger_st, and so on.

Using an _st logger from two threads is a data race

_st sinks skip locking entirely for speed. If more than one thread can reach the same _st logger, you have a data race the sanitizers will eventually find — or worse, won't. Default to _mt; only drop to _st for a logger you've confirmed is genuinely single-threaded.

Explicit construction​

Build the sink and logger yourself when you want to configure the sink before it's attached, share a sink between loggers, or skip automatic registration:

auto sink = std::make_shared<spdlog::sinks::stdout_color_sink_mt>();
sink->set_level(spdlog::level::warn);

auto logger = std::make_shared<spdlog::logger>("app", sink);
logger->set_level(spdlog::level::debug);

spdlog::register_logger(logger);   // optional — makes spdlog::get("app") work

Duplicate names​

The factory helpers throw spdlog::spdlog_ex if a logger with that name is already registered. Either spdlog::drop(name) first, or use explicit construction and skip register_logger if you don't need the name to be globally reachable — see The registry for what registration actually buys you.

Per-logger configuration​

set_level, set_pattern, and flush_on called on a logger apply to all of its sinks unless an individual sink has its own override — a sink-level set_pattern/set_level wins for that sink. See Multi-sink loggers for the common case of different settings per destination.

See also​