Why does the conversion operator take precedence over the conversion constructor when converting an object from one type to another?

Conversion Constructor vs. Conversion Operator: Precedence and Philosophical Implications

In object-oriented programming, conversion constructors and operators facilitate the transformation of objects from one type to another. However, there may arise scenarios where multiple viable conversion paths exist, posing the question of which one takes precedence.

In the given code snippet, the conversion operator is invoked when assigning an A object to a B variable (B b = A();), despite the presence of both a conversion operator and a conversion constructor. The following sections delve into the rationale behind this behavior and its philosophical implications.

Precedence According to the C Standard

The C standard establishes precedence for resolving ambiguous conversion calls in Section 13.3.1.4, which states that overload resolution is employed to select the "best" user-defined conversion. In this case, candidate functions include both the conversion constructor of B and the conversion operator of A.

When the source and destination types are different classes (S and T), the compiler initializes an object of type T (via copy initialization) by applying a series of conversion steps. The standard considers both the conversion functions of S and its base classes. The conversion function that yields a type matching or derived from T without additional cv-qualifiers becomes a candidate. This step results in the selection of both operator B() and B(const A&) as candidates in our case.

Subsequently, the C standard applies Overload Resolution to determine the best match for the arguments. The argument list consists of the initializer expression in this instance. In our code, the argument is an A object, which is an lvalue.

The crucial factor in determining precedence is encapsulated in Section 13.3.3.2/3: "If two candidate functions are both reference bindings and refer to compatible types, the one with the least cv-qualification on the reference is preferred."

In our code, operator B() takes an lvalue of type A by virtue of being a member function, while B(const A&) takes a const reference. Since operator B() has fewer const qualifications, it is selected as the superior candidate by the compiler, resulting in the invocation of the conversion operator.

Object-Oriented Philosophical Implications

From a philosophical standpoint, the choice of whether A or B has more knowledge about how the conversion should occur is debatable.

On the one hand, one could argue that B (the target type) is the more knowledgeable party because it is responsible for defining how to construct itself from an A object. The conversion constructor in B can encapsulate specific conversion rules or validations.

On the other hand, one could advocate that A (the source type) possesses greater insight into its own representation and how it can be interpreted as a B object. The conversion operator in A allows the object to control how it is transformed into B.

Ultimately, the C standard has opted for the approach where the conversion operator has precedence, giving priority to the source object's representation. This choice aligns with the notion that the object itself has the best understanding of its own internal state and how it can be converted to other types. However, it is essential to note that this precedence rule is subject to the rules of Overload Resolution, whereby the final decision can be influenced by other factors such as const-qualification, as demonstrated in our example code.

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