How to compare strings with conditional breakpoint in Unreal Engine

Problem

While debugging in Unreal Engine, you might want to break execution only when a specific string condition is met. For example, you have 100 NPCs in your game, and you want to break execution only when the name of the NPC is BP_NPC_Test.

You might think that the natural way to do this is to use a conditional breakpoint, so you create condition like:

GetString() == "String"

but this won’t work as expected. Depends on your debugger, it will freeze, throw an error, or behave like a regular breakpoint and trigger every time.

The reason is because of debugger evaluator can’t parse the string comparison correctly. Unreal Engine uses FString for strings, which is a custom type that the debugger doesn’t handle.

Couldn't parse conditional expression:
warning: result of comparison against a string literal is unspecified
error: invalid operands to binary expression ('FString' and 'const char [7]')
candidate function not viable: no known conversion from 'FString' to 'UE::CoreUObject::Private::FObjectHandlePrivate' for 1st argument
candidate function not viable: no known conversion from 'FString' to 'const FIoHash' for 1st argument
candidate function not viable: no known conversion from 'FString' to 'const wchar_t *' for 1st argument
candidate function not viable: no known conversion from 'FString' to 'const FLazyName' for 1st argument
...

Solution

The best way to compare strings here is to use wcscmp method:

wcscmp(str1, str2) == 0

This is comparing two wide-character strings (wchar_t*). It’s like strcmp, but for wide strings.

wcscmp(str1, str2) returns:

• 0 if the strings are identical
• < 0 if str1 is lexicographically less than str2
• > 0 if str1 is greater

Technically, you can use strcmp, but wchar_t* format is used for Unicode (wide) strings. Mixing them would result in a type mismatch or incorrect behavior. Unreal Engine uses wide strings everywhere with TEXT macro, so we should too.

The TEXT macro expands at compile time and isn’t available in runtime debugger expressions, so you can use L prefix for wide strings in C++:

L"Test"    // wide string (wchar_t*)
"Test"     // regular narrow string (char*)

We can’t use FString directly because it is a custom type in Unreal Engine, and the debugger doesn’t know how to handle it in a conditional breakpoint expression. So we will get raw char data from FString via:

StringVariable.Data.AllocatorInstance.Data

So the final conditional breakpoint expression will look like this:

wcscmp((wchar_t*)StringVariable.Data.AllocatorInstance.Data, L"StringToCompare") == 0