Why Can\'t You Compare `std::function` Objects for Equality?

Why std::function is Not Equality Comparable

In C , std::function is a generic callable wrapper that allows you to work with arbitrary function objects. Despite its utility, std::function does not support equality comparison (==). Boost::function and std::tr1::function exhibit the same behavior.

Historical Context

In earlier drafts of the C 11 standard, std::function did have overloads for equality operators. However, these overloads were later declared as deleted, with the rationale of closing a "possible hole in the type system."

The "Loophole"

The "loophole" referred to in the C 11 draft concerns the potential for implicit bool conversions to lead to unexpected equality comparisons. In C 03, the safe-bool idiom was introduced to avoid this issue, and in C 11, an explicit bool conversion function was introduced. However, these measures do not fully eliminate the possibility of unexpected equality comparisons.

Specific Considerations for std::function

Unlike std::shared_ptr, which has well-defined equality semantics (equality implies pointing to the same object), std::function does not have a clear concept of equality. Consider the following:

• Equivalent functions constructed with different argument bindings would compare as unequal.
• Functions with different underlying types (e.g., lambdas vs. function pointers) would compare as unequal.

Consequences of Incomparability

The inability to compare std::function objects as equal has certain consequences:

• Difficulties in writing generic equality-checking code.
• Potential ambiguity when using std::function in set-like containers.
• Increased potential for logical errors when chaining functions.

Conclusion

While the lack of equality comparison for std::function may be inconvenient in certain scenarios, it ultimately prevents potential type system vulnerabilities and ensures the consistency of function behavior across different implementations. For applications requiring equality comparison, alternative solutions can be employed, such as using a wrapper class with explicit equality overrides.

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