Print Debug in UE4

// https://wiki.unrealengine.com/Logs,_Printing_Messages_To_Yourself_During_Runtime#Related_Tutorial
GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::White, TEXT("This message will appear on the screen!"))

Writing Logs in Unreal Engine

Logging using UE_LOG

UE_LOG(LogTemp, Warning, TEXT("Warning Text"));

Formatting Examples Quick Reference

Logging an FString

UE_LOG(LogTemp, Warning, TEXT("The Actor's name is %s"), *YourActor->GetName());

Logging a Bool

UE_LOG(LogTemp, Warning, TEXT("The boolean value is %s"), ( bYourBool ? TEXT("true") : TEXT("false") ));

Logging an Integer

UE_LOG(LogTemp, Warning, TEXT("The integer value is: %d"), YourInteger);

Logging a Float

UE_LOG(LogTemp, Warning, TEXT("The float value is: %f"), YourFloat);

Logging an FVector

UE_LOG(LogTemp, Warning, TEXT("The vector value is: %s"), *YourVector.ToString());

Logging with Multiple Specifiers

UE_LOG(LogTemp, Warning, TEXT("Current values are: vector %s, float %f, and integer %d"

Logging using UE_LOGFMT

WARNING

This macro is introduced with Unreal Engine 5.2 and may not be available in earlier revisions of the engine. In order to utilize UE_LOGFMT you will need to #include "Logging/StructuredLog.h" either in your current file or in a linked file.

Experienced Unreal Engine developers will be familiar with the UE_LOG macro, and in particular some of its quirks.

UE_LOG is extremely verbose, requiring the developer to constantly wrap log text in the TEXT macro. UE_LOG is also incapable of printing basic types, such as Booleans, or FStrings, Unreal's standard String type. UE_LOG requires awareness of types when printing different variables such as float, integer, booleans, strings.

The new UE_LOGFMT macro helps to alleviate many of these issues. At its most simple, UE_LOGFMT can be utilized in the following way:

UE_LOGFMT(LogTemp, Log, "This message will print to my log");

When printing variables to log, UE_LOGFMT provides a cleaner and more consistent syntax. Like so:

FString Name("SomeName");
int32 Value = 999;
UE_LOGFMT(LogTemp, Log, "Printing my Name {0} with Value {1}", Name, Value);

UE_LOGFMT accepts parameters in two ways:

Positional Parameters

The field values must exactly match the fields referenced by the log statement.

UE_LOGFMT(LogCore, Warning, "Loading '{0}' failed with error {1}", Package->GetName(), ErrorCode);

In this example, the first variable encountered in the log statement ({0}) is replaced by the value of Package->GetName(). The second variable ({1}) is replaced by ErrorCode.

The ordering of Package->GetName and ErrorCode in this example is implicitly important to deducing the values replaced in the log statement.

Named Parameters

The fields must contain every field referenced by Format. Order is irrelevant and extra fields are permitted.

UE_LOGFMT(LogCore, Warning, "Loading '{Name}' failed with error {Error}", ("Error", ErrorCode), ("Name", Package->GetName()), ("Flags", LoadFlags));

In this example, the first variable encountered in the log statement ({Name}) is replaced by the value bundled with the ("Name", ...) identifier. The second variable ({Error}) is replaced by the value bundled with the ("Error", ...) identifier.

The following table lists all available verbosity levels, from lowest verbosity to highest:

Verbosity Level	Printed in Console?	Printed in Editor's Log?	Notes
Fatal	Yes	N/A	Crashes the session, even if logging is disabled
Error	Yes	Yes	Log text is coloured red
Warning	Yes	Yes	Log text is coloured yellow
Display	Yes	Yes	Log text is coloured grey
Log	No	Yes	Log text is coloured grey
Verbose	No	No
VeryVerbose	No	No