How Do Deleted Functions in C 11 Influence Overload Resolution?

Overload Resolution in C 11 with Deleted Functions

C 11 allows for the use of the = delete syntax to mark functions as "deleted." This syntax prevents the compiler from generating the function body and prohibits its direct invocation. However, these deleted functions still participate in overload resolution, a process that determines the most appropriate function to call given a set of input parameters.

Why is this feature useful? Why are deleted functions hidden instead of being completely removed from the code?

The primary reason is to disable specific overloads based on parameter types. By participating in overload resolution, deleted functions prevent implicit conversions that could lead to unintended behavior. Consider the following example:

struct onlydouble {
  onlydouble(std::intmax_t) = delete;
  onlydouble(double);
};

If delete entirely removed the function, it would effectively make the = delete syntax equivalent to:

struct onlydouble2 {
  onlydouble2(double);
};

With this change, the following code would be legal:

onlydouble2 val(20);

The compiler would attempt to implicitly convert the integer literal 20 to a double before invoking the constructor that takes a double.

By contrast, in the original example with the deleted constructor, the compiler would detect an exact match between the function parameters (std::intmax_t) and the argument (20). It would then raise an error because it has selected a deleted function for invocation.

The = delete syntax, therefore, allows for stronger control over which functions are called based on parameter types. It prevents implicit conversions that could result in unintended consequences.

Additionally, the concept of "this does not exist" is not necessary in the C specification because:

• Special grammar exists for deleting functions ("I forbid this")
• Implicitly defined functions (e.g., copy constructor) exist by default
• Overload resolution handles function "existence" by identifying mismatches or ambiguities
• The primary purpose of deleted functions is to control overload resolution, rather than indicating non-existence

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