Why are Undefined Behaviors Excluded from C Constant Expressions?

Understanding the Exclusion of Undefined Behavior in Constant Expressions

In the C draft standard, constant expressions hold a pivotal role in ensuring program correctness. However, within this realm, a particular exemption exists for undefined behavior. This raises questions: Why is this exclusion necessary, and what unique capabilities does it provide?

Essence of the Exclusion

Defining a constant expression, Section 5.19.2 of the draft standard stipulates that certain operations with potentially undefined behavior are prohibited as subexpression components. This includes signed integer overflow, certain pointer arithmetic, division by zero, and specific shift operations.

This exclusion stems from the requirement that constant expressions yield mathematically well-defined results within the representable value range for their data types. If an operation carries an inherent risk of undefined behavior, it cannot meet this requirement, necessitating its exclusion to maintain the integrity of constant expressions.

Benefits of the Exclusion

The exclusion of undefined behavior in constant expressions offers several advantages:

• Compile-Time Detection: Constant expressions are evaluated at compile-time. This exclusion ensures that any undefined behavior associated with a constant expression is detected and reported during compilation.
• Correctness Verification: By disallowing undefined behavior in constant expressions, the compiler can guarantee the mathematical correctness of these expressions, enhancing overall program reliability and reducing the potential for runtime errors.

Practical Applications

Leveraging the exclusion, developers can harness SFINAE to discern whether an addition expression could induce overflow. For instance, the following code, inspired by a solution proposed by dyp, demonstrates this technique:

template <typename T1, typename T2>
struct addIsDefined
{
    template <T1 t1, T2 t2>
    static constexpr bool isDefined()
    {
        return isDefinedHelper<t1, t2>(0);
    }

    template <T1 t1, T2 t2, decltype(t1 + t2) result = t1 + t2>
    static constexpr bool isDefinedHelper(int)
    {
        return true;
    }

    template <T1 t1, T2 t2>
    static constexpr bool isDefinedHelper(...)
    {
        return false;
    }
};

This construct effectively allows developers to detect potential overflow at compile-time, enhancing code safety.

Indication of Wider Intention

While the wording of Section 5.19.2 does not explicitly mandate the detection of undefined behavior in constant expressions, Issue 695 (Compile-time calculation errors in constexpr functions) provides insight into the committee's intent. This issue suggests that undefined behavior in constant expressions should result in a non-constant expression, with any subsequent diagnostic arising from its use in contexts requiring a constant expression.

Conclusion

The exclusion of undefined behavior from constant expressions serves a critical role in maintaining program correctness and facilitating reliable code. This exclusion allows compilers to detect and rectify undefined behavior during compilation and enables developers to utilize SFINAE for overflow detection. Understanding this exclusion and its implications in constant expressions is essential for enhancing code safety and ensuring robust software development.

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