【VS2022 编译UE5.1 错误 C4834 】

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错误

使用VS2022编译UE5.1源码，错误 C4834 放弃具有 [[nodiscard]] 属性的函数的返回值

F:\UE\Engine\Plugins\Runtime\Steam\SteamVR\Source\SteamVRInputDevice\Private\SteamVRInputDeviceFunctionLibrary.cpp(513): error C4834: discarding return value of function with 'nodiscard' attribute

原因

• 编译器警告（等级 1）C4834
• 详细解释

This is a bug in the UE code on line 513.
It should probably be:

GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Red, FString::Printf(TEXT("Unable to find Action [%s] for Action Set [%s]"), *ActionName.ToString(), *ActionSet.ToString()));

instead of:

GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Red, (TEXT("Unable to find Action [%s] for Action Set [%s]"), *ActionName.ToString(), *ActionSet.ToString()));

Actually the problem is in the substring:

(TEXT("Unable to find Action [%s] for Action Set [%s]"), *ActionName.ToString(), *ActionSet.ToString())

This line is like:

const TCHAR *value = (TEXT("some value"), name1.CalculateString(), name2.CalculateString());

or

auto a = (op1(), op2(), op3());

the results of calling op1 and op2 will be ignored, but only op3() will be assigned to a.
So, if op1 or op2 has a “nodiscard” attribute, Visual Studio will generate an error/warning.
The previous version of Visual Studio did not have this behavior. New Visual Studio update - has.

The actual nodiscard attribute that can cause this error in this situation is UnrealString.h, line 324:

/*** Get pointer to the string** @Return Pointer to Array of TCHAR if Num, otherwise the empty string*/
UE_NODISCARD FORCEINLINE const TCHAR* operator*() const UE_LIFETIMEBOUND
{return Data.Num() ? Data.GetData() : TEXT("");
}

To sum up, the solution would be to use this line instead of line 513:

GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Red, FString::Printf(TEXT("Unable to find Action [%s] for Action Set [%s]"), *ActionName.ToString(), *ActionSet.ToString()));

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