Why Do Deleted Functions in C 11 Participate in Overload Resolution?

Why Deleted Functions Participate in Overload Resolution in C 11

In C 11, deleted functions are not erased entirely but instead participate in overload resolution. This feature serves two primary purposes.

Preventing Implicit Conversions

Half of the utility of the "= delete" syntax lies in prohibiting callers from invoking specific functions with certain parameter types. Its goal is to hinder implicit conversions under specific scenarios. For a particular overload to be disallowed, it must participate in overload resolution.

Ensuring Explicit Invocation

If the "= delete" syntax were to completely erase the function, it would essentially make it equivalent to omitting the declaration altogether. This would permit scenarios like:

onlydouble val(20); // Legal in C++

The compiler would consider all constructors and identify an implicit conversion that allows it to call the constructor intended for double parameters.

In contrast, with "= delete" participating in overload resolution:

onlydouble val(20); // Error in C++

The compiler evaluates all constructors, including the deleted one. It discovers an exact match for the integer literal and throws an error because it attempted to invoke a deleted function.

Clarifying the Role of "= delete"

The "= delete" syntax signifies "I forbid this," not merely "this does not exist." It is a stronger statement with specific implications. We do not require distinct syntax for "this does not exist" because it is already implied by omitting the declaration.

In rare cases, certain rule ambiguities can arise due to the absence of a clear way to indicate that the copy constructor does not exist. However, special constructors like the copy constructor are inherent to classes. Rather than indicating their non-existence, the concern lies in whether it is legal to invoke them.

Conclusion

Deleted functions in C 11 participate in overload resolution to facilitate clear error reporting and enforce explicit function invocation. This enables developers to prevent ambiguous and unintended conversions, ensuring program correctness and clarity.

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