Unreal Engine provides an extensive set of debugging tools, including assertions that help developers catch issues early during development. While many are familiar with check() and ensure(), there are several lesser-known assertions that serve unique purposes. This article will dive into four such asserts: checkNoEntry(), checkNoReentry(), checkNoRecursion(), and unimplemented(). Understanding their use cases can improve code clarity, stability, and maintainability.

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1. checkNoEntry()

Purpose: Halts execution if the line is ever hit.
Use Case: This is a specialized version of check(false) intended for marking code paths that should never be executed.

void ProcessInput(int Input)
{
    switch (Input)
    {
    case 1:
        DoSomething();
        break;
    case 2:
        DoSomethingElse();
        break;
    default:
        checkNoEntry(); // Ensure no unexpected inputs
    }
}

Why Use It?
checkNoEntry() signals explicitly that the code path is unreachable, making it easier to understand intent and catch logic errors early.

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2. checkNoReentry()

Purpose: Halts execution if the line is hit more than once.
Use Case: Detects unintended re-entry into a code block, which can happen with multithreading or improper flow control.

void PerformCriticalOperation()
{
    checkNoReentry(); // Ensure this function is not re-entered
    // Perform operation
}

Why Use It?
This assert is invaluable for debugging multithreaded code or ensuring that functions critical to state integrity are not called recursively.

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3. checkNoRecursion()

Purpose: Halts execution if the line is hit more than once without leaving the current scope.
Use Case: Prevents unintended recursion, especially in complex algorithms or event-driven code.

void UpdateGraph(GraphNode* Node)
{
    checkNoRecursion(); // Ensure no recursive calls
    for (auto* Neighbor : Node->Neighbors)
    {
        UpdateGraph(Neighbor); // Recursive operation
    }
}

Why Use It?
Recursion can lead to stack overflows if not carefully managed. This assert helps developers catch such issues during testing.

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4. unimplemented()

Purpose: Halts execution if the line is hit.
Use Case: Marks virtual functions that must be overridden by derived classes.

class BaseCharacter
{
public:
    virtual void PerformAction()
    {
        unimplemented(); // This function must be overridden
    }
};

Why Use It?
Using unimplemented() explicitly communicates that the base implementation should never be called, reducing ambiguity and enforcing proper usage of virtual functions.