Fast Logging for C++ with spdlog¶

1. Installation¶

Install spdlog and the dependencies fmt and cmake. You can also install the header-only version or the compiled version separately instead of using the package manager.

brew install spdlog

brew install fmt

brew install cmake

2. Include Headers¶

spdlog/spdlog.h and spdlog/sinks/basic_file_sink.h are key header files of the spdlog library.

#include <spdlog/spdlog.h>
#include <spdlog/sinks/basic_file_sink.h>

spdlog.h¶

This header file provides the core feature of the library. You can:

Call basic logging functions like spdlog::info, spdlog::warn, spdlog::error, etc.
Set the level of the log message (trace, debug, info, etc)
Create an global logger.
Format the log message.

example:

#include <spdlog/spdlog.h>

int main() {
    spdlog::info("This is an info message.");
    spdlog::warn("This is a warning message.");
    spdlog::error("This is an error message.");
}

basic_file_sink.h¶

This header file provides the “sink” functionality needed to output logs to a file. In spdlog, the log output target is defined as a “sink,” allowing logs to be sent to different destinations like console, files, or networks.

Sink refers to the output target that determines where log messages should be sent. It acts as the “exit point” where log messages flow.

Sink decides whether log messages are recorded to the console, files, etc.

A single logger can have 1+ sinks → same log messages can be sent to multiple destinations simultaneously.

Each Sink operates independently → you can format the messages and filter them for each sink.

example:

#include <spdlog/spdlog.h>
#include <spdlog/sinks/basic_file_sink.h>

int main() {
    auto file_logger = spdlog::basic_logger_mt("file_logger", "logs/example.log");
    file_logger->info("This message will be written to the file.");
    file_logger->warn("This is a warning logged to the file.");
}

The type for file_logger is std::shared_ptr<spdlog::logger>
Since it’s a smart pointer, you can pass it as a parameter.

Example Code¶

#include <spdlog/spdlog.h>
#include <spdlog/sinks/basic_file_sink.h>

int main() {
    spdlog::info("Hello, spdlog!");

    spdlog::debug("This is a debug message"); 
    spdlog::warn("Warning: Something might be wrong");
    spdlog::error("Error: Something went wrong!");
    spdlog::critical("Critical issue encountered!");

    auto file_logger = spdlog::basic_logger_mt("file_logger", "logs/example.log");
    file_logger->info("This message is logged to a file!");

    spdlog::set_pattern("[%Y-%m-%d %H:%M:%S] [%^%l%$] %v");
    spdlog::info("This is a log message with a custom format!");

    spdlog::set_level(spdlog::level::debug);
    spdlog::debug("Debug messages are now visible!");

    return 0;
}

I created example.cpp with the code above, and used the Makefile below. Make sure to modify your spdlog, fmt paths. You can check using brew info <libName>.

CXX = g++

CXXFLAGS = -std=c++17 -I/opt/homebrew/Cellar/spdlog/1.15.0_1/include -I/opt/homebrew/Cellar/fmt/11.1.3/include
LDFLAGS = -L/opt/homebrew/Cellar/spdlog/1.15.0_1/lib -L/opt/homebrew/Cellar/fmt/11.1.3/lib -lspdlog -lfmt

TARGET = app

SRC = example.cpp

$(TARGET): $(SRC)
    $(CXX) $(CXXFLAGS) $(SRC) -o $(TARGET) $(LDFLAGS)

clean:
    rm -f $(TARGET)

Build and run the executable.

make
./app

You should see the following output on the terminal, and also see that example.log was created.

Logging¶

spdlog::info("Hello, spdlog!");
spdlog::debug("This is a debug message");
spdlog::warn("Warning: Something might be wrong");
spdlog::error("Error: Something went wrong!");
spdlog::critical("Critical issue encountered!");

Log messages in different levels are printed to the console.
The basic level is set to info → if you log debug, you won’t be able to see it.

File Logging¶

auto file_logger = spdlog::basic_logger_mt("file_logger", "logs/example.log");
file_logger->info("This message is logged to a file!");

spdlog::basic_logger_mt creates a file logger named “file_logger". It saves the logs in to logs/example.log (spdlog creates it if it doesn’t exist).
Since spdlog::basic_logger_mt is a shared pointer, you can call the info method and log the message to the file.

Formatting¶

spdlog::set_pattern("[%Y-%m-%d %H:%M:%S] [%^%l%$] %v");
spdlog::info("This is a log message with a custom format!");

spdlog::set_pattern sets the format for all the logs.

format desc

%Y-%m-%d %H:%M:%S date and time

%^%l%$ output log levels in colours.
(%I: log level, %^ / %$: start / end colour)

%v log message content

Set Log Level¶

spdlog::set_level(spdlog::level::debug);
spdlog::debug("Debug messages are now visible!");

spdlog::set_level sets the log level (global)

format	desc
`%Y-%m-%d %H:%M:%S`	date and time
`%^%l%$`	output log levels in colours. (`%I`: log level, `%^` / `%$`: start / end colour)
`%v`	log message content