logger.hpp

Header-only, thread-safe, cross-platform logging utility (C++17).

Two independent outputs:

• output.log: general-purpose logging with levels
• diagnostics.log: crash-point detection based on function/block scopes

Usage

#include "logger.hpp"

void foo() {
  LOG_START;                          // scope logged to diagnostics.log
  LOG_MSG << "Doing stuff" << 123 << true;
  if (g()) {
    LOG_START1("if block");           // named scope
  }
  LOG_HERE("Halfway");                // checkpoint inside the current scope
}

int main() {
  SET_LOG_OUTPUT_FILE_PATH("app.log");
  SET_LOG_MIN_LEVEL(utils_log::Level::debug);

  foo();
  LOG_MSG_WARN << "Low disk space:" << 42 << "MB";
  LOG_MSG_ERROR << "Failed to open" << "config.json";
  LOG_MSGNF << "This only logs to console";

  utils_log::Log::terminate();        // optional: close the file explicitly
}

Arguments are space-separated by default. Use LOG_NOSPACE / LOG_SPACE to toggle:

LOG_MSG << "a" << LOG_NOSPACE << "b" << "c" << LOG_SPACE << "d";   // a bc d

Levels

Macro	Level	Console stream
LOG_MSG_DEBUG	debug	stdout
LOG_MSG_INFO	info	stdout
LOG_MSG_WARN	warn	stderr
LOG_MSG_ERROR	error	stderr
LOG_MSG	info (alias of LOG_MSG_INFO)	stdout
LOG_MSGNF	info, no file (console only)	stdout

• Default minimum level is info. Change at runtime with SET_LOG_MIN_LEVEL(level).
• Messages below the minimum level are skipped without evaluating their operands, so debug logging in hot paths is cheap.
• The LOG_MSG_* names are chosen to avoid collisions with <syslog.h> (LOG_INFO, ...) and windows.h (ERROR).
• Under Qt (QT_CORE_LIB), console output goes through qDebug() / qWarning().
• On Windows, console output is also sent to OutputDebugStringA.

Configuration

Macro	Effect
SET_LOG_OUTPUT_FILE_PATH(path)	Output log file (default output.log). Safe at runtime.
SET_LOG_DIAGNOSTICS_FILE_PATH(path)	Diagnostics file (default diagnostics.log).
SET_LOG_TO_FILE(bool)	Default file logging for new Log instances.
SET_LOG_TO_CONSOLE(bool)	Default console logging for new Log instances.
SET_LOG_MIN_LEVEL(level)	Minimum level to emit.

Set paths before the first log call when possible. Changing them later closes the current file and reopens at the new path on the next write.

Output format

output.log:

[2026-10-20 14:03:07.512] [WARN ] tid=1234567890 "Low disk space: 42 MB"

tid is a stable hash of the thread ID, consistent within a process.

diagnostics.log:

[2026-10-20 14:03:07] foo:start... C:\src\main.cpp |1
[2026-10-20 14:03:07] foo:end! C:\src\main.cpp |0

The number after | is the count of currently open scopes.

Rotation

• output.log rotates at runtime when it exceeds 5 MB. The current file becomes output.log.old (one backup, overwritten on each rotation).
• diagnostics.log rotates at startup only, when it exceeds 2 MB.
• Rotation failures (e.g. file locked by another instance) are ignored and never throw.

Crash detection

Every scope start/end is flushed to diagnostics.log immediately. On the next startup, if the last line shows open scopes (|N with N > 0), the process is assumed to have crashed and ## CRASH POINT ## is written. The lines above it show the scopes that were active.

Notes:

• Detection happens before rotation, so a crash is not lost to a rotated file.
• The depth counter is process-wide, not per-thread. With several threads, read the log as "scopes open in the process" rather than a per-thread call stack.
• LOG_START / LOG_START1 / LOG_HERE flush on every call. They are meant for diagnosing crashes, not for hot paths.

Notes

• Logging never throws. Failures inside ~Log() are swallowed.
• Console writes and file writes are serialized by a global mutex.
• Don't log from static destructors of other translation units, since the file stream may already be destroyed.
• Requires C++17 (<filesystem>, inline variables).