A deep dive into Unreal Engine log categories

1. How logging looks like

To start things off let's take a look at a simple example message: UE_LOG(LogTemp, Warning, TEXT("Hello world!")):

The first parameter of the UE_LOG macro is the category. LogTemp is a built-in category which you can use at any time to print messages quickly.

2. Creating a log category - standard version

If you want to keep your logs organised, you need to create a custom one. To achieve this you need to create a new .h and a .cpp file and use the DECLARE_LOG_CATEGORY_EXTERN & DEFINE_LOG_CATEGORY macros like this:

To stay consistent with Unreal's conventions, it's highly recommended to prefix your category's name with Log but you are free to do whatever you feel like.

Now you can use this log category anywhere you want by including the header file where needed.

3. Creating a log category - expedite version

If you need a custom log category fast, you can achieve this by using DEFINE_LOG_CATEGORY_STATIC. The downside is this category is only available in the .cpp file where it is defined and it looks like this:

This version is also create if you want a different log category in each class.

4. What is log verbosity

The second parameter of the UE_LOG macro is the verbosity. Most of the time, people go for Log but there are a few other options to consider as well:

• Fatal - crashes the application (please don't do it)
• Error - highlighted in red in the console output
• Warning - highlighted in yellow in the console output
• Display - shows up in the program's output
• Log - a simple message which shows up in the .log file and console output window
• Verbose - usually disabled, can be manually enabled if needed
• VeryVerbose - usually disabled, can be manually enabled if needed

Each category has verbosity level, if the verbosity of a UE_LOG is higher or equal to a category's level, the message will be printed, otherwise it will be ignored.

These can be controlled via the [Core.Log] properties inside the Engine's config - as well as many other.

5. Controlling log verbosity via command line parameters

Sometimes, we don't have the luxury of changing an .ini file and rebuilding.

For this cases, we have other commands to adjust the verbosity of an already packaged build, such as:

• Launch parameters: -LogCmds="LogTemp Error"
• Console commands: "log LogTemp Error"

(Credits to Ari Arnbjörnsson on Twitter for pointing these out)