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

Why C 11-Deleted Functions Participate in Overload Resolution

C 11 introduced the = delete syntax to prevent the invocation of certain function overloads with specific parameter types. This behavior may seem unusual, as one might expect deleted functions to be entirely removed from consideration during overload resolution. However, there are specific reasons for this design decision.

Preventing Implicit Conversions

One primary purpose of = delete is to prohibit the use of implicit conversions in certain scenarios. When a function with deleted overloads participates in overload resolution, it ensures that the compiler will select an exact match or report an error. This prevents the use of implicit conversions to inadvertently call a deleted overload.

Consider the example below:

struct onlydouble {
  onlydouble(std::intmax_t) = delete; // Prevent implicit conversion from integer
  onlydouble(double);
};

If the deleted overload were entirely removed from consideration, the following code would be allowed:

onlydouble val(20);

The compiler would implicitly convert the integer literal to double and select the non-deleted overload. However, with deleted overloads participating in overload resolution, the compiler rejects the code because it identifies the deleted overload as an exact match for an integer literal argument.

Special Syntax vs. "This Does Not Exist"

One may question why C 11 defines = delete as "I forbid this" rather than "this does not exist." The answer lies in the lack of a need for explicit grammar to represent non-existence. Undefined identifiers and missing special member functions implicitly convey the concept of "this does not exist."

Conclusion

C 11-deleted functions participate in overload resolution to prevent implicit conversions and enforce the use of certain parameter types. This design feature allows for precise control over the invocation of function overloads, offering advantages in scenarios where implicit conversions are not desirable or lead to ambiguities.

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