Very fast - performance is the primary goal (see benchmarks below).
Headers only.
No dependencies - just copy and use.
Feature rich call style using the excellent fmt library.
ostream call style is supported too.
Extremely fast asynchronous mode (optional) - using lockfree queues and other tricks to reach millions of calls/sec.
Custom formatting.
Multi/Single threaded loggers.
Various log targets:
- Rotating log files.
- Daily log files.
- Console logging (colors supported).
- Linux syslog.
- Easily extendable with custom log targets (just implement a single function in the sink interface).
Severity based filtering - threshold levels can be modified in runtime as well as in compile time.

Benchmarks

Below are some benchmarks comparing popular log libraries under Ubuntu 64 bit, Intel i7-4770 CPU @ 3.40GHz

Synchronous mode

Time needed to log 1,000,000 lines in synchronous mode (in seconds, the best of 3 runs):

threads	boost log 1.54	glog	easylogging	spdlog
1	4.169s	1.066s	0.975s	0.302s
10	6.180s	3.032s	2.857s	0.968s
100	5.981s	1.139s	4.512s	0.497s

Asynchronous mode

Time needed to log 1,000,000 lines in asynchronous mode, i.e. the time it takes to put them in the async queue (in seconds, the best of 3 runs):

threads	g2log ^{async logger}	spdlog ^{async mode}
1	1.850s	0.216s
10	0.943s	0.173s
100	0.959s	0.202s

Usage Example

#include <iostream>
#include "spdlog/spdlog.h"

int main(int, char* [])
{
    namespace spd = spdlog;
    try
    {
        // console logger (multithreaded and with color)
        auto console = spd::stdout_logger_mt("console", true);
        console->info("Welcome to spdlog!") ;
        console->info("An info message example {}..", 1);
        console->info() << "Streams are supported too  " << 1;

        //Formatting examples
        console->info("Easy padding in numbers like {:08d}", 12);
        console->info("Support for int: {0:d};  hex: {0:x};  oct: {0:o}; bin: {0:b}", 42);
        console->info("Support for floats {:03.2f}", 1.23456);
        console->info("Positional args are {1} {0}..", "too", "supported");

        console->info("{:<30}", "left aligned");
        console->info("{:>30}", "right aligned");
        console->info("{:^30}", "centered");

        //
        // Runtime log levels
        //
        spd::set_level(spd::level::info); //Set global log level to info
        console->debug("This message shold not be displayed!");
        console->set_level(spd::level::debug); // Set specific logger's log level
        console->debug("Now it should..");

        //
        // Create a file rotating logger with 5mb size max and 3 rotated files
        //
        auto file_logger = spd::rotating_logger_mt("file_logger", "logs/mylogfile", 1048576 * 5, 3);
        for(int i = 0; i < 10; ++i)
		      file_logger->info("{} * {} equals {:>10}", i, i, i*i);

        //
        // Create a daily logger - a new file is created every day on 2:30am
        //
        auto daily_logger = spd::daily_logger_mt("daily_logger", "logs/daily", 2, 30);

        //
        // Customize msg format for all messages
        //
        spd::set_pattern("*** [%H:%M:%S %z] [thread %t] %v ***");
        file_logger->info("This is another message with custom format");

        spd::get("console")->info("loggers can be retrieved from a global registry using the spdlog::get(logger_name) function");

        //
        // Compile time debug or trace macros.
        // Enabled #ifdef SPDLOG_DEBUG_ON or #ifdef SPDLOG_TRACE_ON
        //
        SPDLOG_TRACE(console, "Enabled only #ifdef SPDLOG_TRACE_ON..{} ,{}", 1, 3.23);
        SPDLOG_DEBUG(console, "Enabled only #ifdef SPDLOG_DEBUG_ON.. {} ,{}", 1, 3.23);

        //
        // Asynchronous logging is very fast..
        // Just call spdlog::set_async_mode(q_size) and all created loggers from now on will be asynchronous..
        //
        size_t q_size = 8192; //queue size must be power of 2
        spdlog::set_async_mode(q_size);
        auto async_file= spd::daily_logger_st("async_file_logger", "logs/async_log.txt");
        async_file->info() << "This is async log.." << "Should be very fast!";

        //
        // syslog example. linux only..
        //
        #ifdef __linux__
        std::string ident = "spdlog-example";
        auto syslog_logger = spd::syslog_logger("syslog", ident, LOG_PID);
        syslog_logger->warn("This is warning that will end up in syslog. This is Linux only!");
        #endif
                
    }
    catch (const spd::spdlog_ex& ex)
    {
        std::cout << "Log failed: " << ex.what() << std::endl;
    }
}


// Example of user defined class with operator<<
class some_class {};
std::ostream& operator<<(std::ostream& os, const some_class& c) { return os << "some_class"; }

void custom_class_example()
{
    some_class c;
    spdlog::get("console")->info("custom class with operator<<: {}..", c);
    spdlog::get("console")->info() << "custom class with operator<<: " << c << "..";
}

Documentation

Documentation can be found in the wiki pages.