Why Should You Avoid `std::enable_if` in Function Signatures and When is it Applicable to Return Types?

Why Should You Avoid std::enable_if in Function Signatures and When is it Applicable to Return Types?

In his upcoming book, Scott Meyers advises against using std::enable_if in function signatures. This article delves into the reasons behind this recommendation and explores alternative approaches.

std::enable_if in Function Signatures

std::enable_if can be utilized within function signatures to selectively enable functions based on template arguments. However, this usage should be avoided because:

• Readability: Cluttered function signatures, where return types and template disambiguators are intertwined, can compromise comprehension.
• Limited Applicability: Some contexts, such as constructors and certain operators, do not allow for this placement of std::enable_if.

std::enable_if as Template Parameter

Meyers suggests placing std::enable_if as a template parameter instead. This technique offers several advantages:

• Enhanced Readability: The enable_if condition is closely associated with the template parameters, promoting clarity.
• Universal Applicability: SFINAE can be applied to any template, making this approach applicable in all situations.

std::enable_if as Return Type

While std::enable_if as a return type is not technically part of the function signature, it should also be avoided for consistency reasons. The use of std::enable_if for conditional return types can lead to confusing function declarations and inconsistent behavior across template specializations.

Member vs. Non-Member Function Templates

The recommendation to avoid std::enable_if in function signatures applies to both member and non-member function templates. However, member templates offer an additional advantage: they can be specialized on the class template arguments. This allows for more flexible and efficient template specialization without the need for enable_if.

Conclusion

For improved readability and universal applicability, programmers should prioritize using std::enable_if as a template parameter instead of in function signatures or return types. This approach simplifies the code structure and makes SFINAE applicability more consistent across different contexts.

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