plog

ObservabilityLoggingC++

What it is

plog is a portable, simple, and extensible C++ logging library. It supports multiple logging destinations, severity levels, thread safety, and works across platforms with minimal dependencies.

plog uses macros for logging at different severity levels and supports multiple appenders (console, file, etc.).

Getting started

The smallest useful thing you can do with it, and what each part means.

Initialize and log

cpp

123456789101112#include <plog/Log.h>
#include <plog/Initializers/RollingFileInitializer.h>

int main() {
    plog::init(plog::debug, "log.txt");
    PLOGD << "This is a debug message";
    PLOGI << "This is an info message";
    PLOGW << "This is a warning";
    PLOGE << "This is an error";
    PLOGF << "This is fatal";
    return 0;
}

Initializes plog with rolling file logs and writes messages at different severity levels.

Advanced usage

Where the library earns its place over a simpler alternative.

Multiple appenders

cpp

plog::init(plog::debug, "log.txt").addAppender(new plog::ConsoleAppender<plog::TxtFormatter>());
PLOGI << "Log to both console and file";

Logs messages to both file and console simultaneously.

Custom formatter

cpp

123456789struct MyFormatter {
    static std::ostream& header(std::ostream& stream) { return stream; }
    static std::ostream& format(std::ostream& stream, const plog::Record& record) {
        return stream << record.getMessage() << " (custom)\n";
    }
};

plog::init(plog::debug, "log.txt", true, 1000, 3).addAppender(new plog::ConsoleAppender<MyFormatter>());
PLOGI << "Custom formatted log";

Demonstrates creating a custom log formatter.

Rolling file logging

cpp

plog::init(plog::debug, "log.txt", 1000, 5);
PLOGI << "Logs will roll over every 1000 lines, keeping 5 backups.";

Configures rolling logs with file size and count limits.

Errors and fixes

The failures you are most likely to hit, and what actually resolves them.

Logs not written to file: Check file path permissions and ensure `plog::init` is called before logging.
Too much log output: Adjust the severity threshold passed to `plog::init` (e.g., `plog::info`).
Thread-safety issues: plog is thread-safe, but custom appenders must ensure safe writes.

Best practices

Initialize logging early in the main function before other components run.
Use different severity levels consistently to filter logs easily.
Combine file and console appenders to keep persistent and real-time logs.
Use rolling file logging to avoid uncontrolled log growth.
Implement custom formatters if you need structured logs (e.g., JSON).

Background

Why it exists, and what it was reacting to.

plog was created to provide a lightweight and portable alternative to heavier C++ logging libraries like log4cxx and glog. It is header-only, easy to integrate, and flexible enough for both small applications and large systems. Its modular design allows developers to plug in custom appenders and formatters.

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plog

glog (Google Logging Library)