Adding custom compile flag to build project plugin with installed version

[Thoran]

As a precondition to let you know, I am currently working on a installed version of the Unreal 5.2.1 engine.

To add a Blueprint Function Library with my own functions implemented in C++ I have created a small plugin in my project. The code I implemented however requires me to enable exception handling on Windows using the /EHsc flag for the compiler.

To be precise I only have this issue of compiling when I run the “Package project” for Windows in the Unreal 5 editor. The compilation of the C++ class in MSVC opened from the Editor is compiling correctly.

I get the error message:

“error C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc”
“Error executing C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.34.31933\bin\Hostx64\x64\cl.exe (tool returned code: 2)”

To achieve that I investigated other questions and came along with the solution to add this flag in the VCToolChain.cs file of my UnrealBuilderTool in the installed Unreal 5 engine. However it seems it is not picked up during packaging in the UnrealBuildTool compile step. My change in the VCToolChain.cs is done in the method

“protected virtual void AppendCLArguments_Global(CppCompileEnvironment CompileEnvironment, List Arguments)”

I added there a check if I am in MSVC and then add the flag

if (Target.WindowsPlatform.Compiler.IsMSVC())
{
    // Enable exception handling
    Arguments.Add("/EHsc");
    Arguments.Add("/DPLATFORM_EXCEPTIONS_DISABLED=0");
}

I also found this code part which seems to be also related to exception handling enabling, however I don’t know where I would need to define the condition for the branch to setup /EHsc

// Enable C++ exceptions when building with the editor or when building UHT.
if (CompileEnvironment.bEnableExceptions)
{
    // Enable C++ exception handling, but not C exceptions.
    Arguments.Add("/EHsc");
    Arguments.Add("/DPLATFORM_EXCEPTIONS_DISABLED=0");
}
else
{
    // This is required to disable exception handling in VC platform headers.
    AddDefinition(Arguments, "_HAS_EXCEPTIONS=0");
    Arguments.Add("/DPLATFORM_EXCEPTIONS_DISABLED=1");
}

Does anyone have some hints or ideas on how to resolve this?
Thanks
Thoran

[michael_chen2010]

Just modify cs file is not enough, you need ue5 source code to compile into UnrealBuildTool.exe, and copy the file over the same file on ue5 build.

[Thoran]

Thanks, for the information. I managed to get it working in the meantime exactly doing what you also proposed, which is effectively rebuilding the tool with the changed *.cs file.
I was initially, wrongly assuming the *.cs file is like a config file that is parsed during runtime, which is not the case.
Thanks again for answering.