Conversion Constructor vs. Conversion Operator: Which Takes Precedence in C ?

Conversion Constructor vs. Conversion Operator: Precedence

In C , both conversion constructors and conversion operators can be used to convert objects between different classes, but their precedence in overload resolution can sometimes lead to unexpected results.

Question 1: Precedence Establishment

In the code snippet provided in the question, the compiler chooses to call the conversion operator operator B() despite the presence of a matching conversion constructor. This suggests that the conversion operator has precedence in this case.

According to the C standard (8.5/14), "user-defined conversion sequences that can convert from the source type to the destination type [...] are enumerated as described in 13.3.1.4, and the best one is chosen through overload resolution (13.3)."

Question 2: Object-Oriented Philosophy

From an object-oriented perspective, one might argue that the class that defines the conversion (in this case, A) should have more say in how the conversion is performed. However, in C , the compiler primarily considers type safety and efficiency.

The conversion operator is typically more efficient because it can be inlined, while the conversion constructor requires an explicit call. Additionally, the conversion operator can be made const, which allows for binding to rvalues, giving it an advantage in certain situations.

Choosing the Conversion Method

The precedence between conversion constructors and conversion operators is established by the overload resolution process described in 13.3.3.2/3 of the C standard. In the case of the provided code, the conversion operator is chosen because it has fewer const qualifiers and can bind to the rvalue A() more efficiently.

In general, choosing between a conversion constructor and a conversion operator depends on the specific requirements of the code. If efficiency and binding to rvalues are important, a conversion operator may be a better choice. However, if the conversion logic is complex or should be controlled by the destination class, a conversion constructor might be more appropriate.

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