Each logger's sink has a formatter which formats the messages to its destination.

spdlog's default logging format is in the form of:

[2014-10-31 23:46:59.678] [my_loggername] [info] Some message

There are 2 ways to customize a logger's format:

• Set the pattern string (recommended):

set_pattern(pattern_string);

• Or implement custom formatter that implements the formatter interface and call

set_formatter(std::make_unique<my_custom_formatter>());

Customizing format using set_pattern(..)

Format can be applied globally to all registered loggers:

spdlog::set_pattern("*** [%H:%M:%S %z] [thread %t] %v ***");

or to a specific logger object:

some_logger->set_pattern(">>>>>>>>> %H:%M:%S %z %v <<<<<<<<<");

or to a specific sink object:

some_logger->sinks()[0]->set_pattern(">>>>>>>>> %H:%M:%S %z %v <<<<<<<<<");
some_logger->sinks()[1]->set_pattern("..");

Efficiency

Whenever the user calls set_pattern("..") the library "compiles" the new pattern to an internal efficient representation - This way the performance stays excellent even with complex patterns (no re-parsing of the pattern on each log call).

Pattern flags

Pattern flags are in the form of %flag and resembles the strftime function:

Aligning

Each pattern flag can be aligned by prepending a width number(up to 64).

Use-(left align) or = (center align) to control the align side:

Optionally add ! to truncate the result if its size exceeds the specified width:

Note: To truncate function names use '!!'. For example %10!! will limit function names to 10 chars.

Extending spdlog with your own flags

You can define your own flags by inheriting the custom_flag_formatter class and implementing the clone() and format(...) abstract methods.

The following example adds a new flag %* - which will be bound to a <my_formatter_flag> instance.

#include "spdlog/pattern_formatter.h"
class my_formatter_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 some_txt = "custom-flag";
        dest.append(some_txt.data(), some_txt.data() + some_txt.size());
    }

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

void custom_flags_example()
{    
    auto formatter = std::make_unique<spdlog::pattern_formatter>();
    formatter->add_flag<my_formatter_flag>('*').set_pattern("[%n] [%*] [%^%l%$] %v");
    spdlog::set_formatter(std::move(formatter));
}

Note: The clone() method should return a deep copy of the object and is required because spdlog passes a new copy of the object to each sink in use (for performance reasons as this enables to have a state in this object without worrying about race conditions or thread safety across sinks).

Note: You can also override spdlog's built in flags in this way.

Source location flags

If you need to use source location flags like %s, %g, %#, %!, is necessary to define this compiler flag:

#define SPDLOG_ACTIVE_LEVEL SPDLOG_LEVEL_TRACE

(change the log level according to your needs) and use these macros:

SPDLOG_LOGGER_TRACE(some_logger, "trace message");
SPDLOG_LOGGER_DEBUG(some_logger, "debug message");
SPDLOG_LOGGER_INFO(some_logger, "info message");
SPDLOG_LOGGER_WARN(some_logger, "warn message");
SPDLOG_LOGGER_ERROR(some_logger, "error message");
SPDLOG_LOGGER_CRITICAL(some_logger, "critical message");

NOTE: if you've set the flag to SPDLOG_LEVEL_TRACE but you don't see any trace and/or debug messages, it's most likely something like this https://github.com/gabime/spdlog/issues/2764. In that case define it directly as a compiler flag. For example, using cmake:

add_compile_definitions(SPDLOG_ACTIVE_LEVEL=SPDLOG_LEVEL_TRACE)