UE4 – logging with UE_LOG

Logging is extremely important for outputting internal game data. Using log tools lets you print information into a handy little Output Log window in the UE4 editor.

Getting ready

When coding, we may sometimes want to send some debug information out to the UE log window. This is possible using the UE_LOG macro. Log messages are an extremely important and convenient way to keep track of information in your program as you are developing it.

How to do it...

1. In your code, enter a line of code using the form:

   UE_LOG(LogTemp, Warning, TEXT("Some warning message") );
   

2. Turn on the Output Log inside the UE4 editor to see your log messages printed in that window as your program is running.
   

How it works...

The UE_LOG macro accepts a minimum of three parameters:

• The Log category (we used LogTemp here to denote a log message in a temporary log)
• The Log level (we used a warning here to denote a log message printed in yellow warning text)
• A string for the actual text of the log message itself

Do not forget the TEXT() macro around your log message text! It promotes the enclosed text to Unicode (it prepends an L) when the compiler is set to run with Unicode on.

UE_LOG also accepts a variable number of arguments, just like printf() from the C programming language.

int intVar = 5;
float floatVar = 3.7f;
FString fstringVar = "an fstring variable";
UE_LOG(LogTemp, Warning, TEXT("Text, %d %f %s"), intVar, floatVar, *fstringVar );

There will be an asterisk * just before FString variables when using UE_LOG to dereference the FString to a regular C-style TCHAR pointer.

Tip

TCHAR is usually defined as a variable type where, if Unicode is being used in the compile, the TCHAR resolves to wchar_t. If Unicode is off (compiler switch _UNICODE not defined), then TCHAR resolves to simply char.

Don't forget to clear your log messages after you no longer need them from the source!