Conversion Sequence Ambiguity: When Does a Conversion Operator Trump a Constructor?

Conversion Sequence Ambiguity: Precedence of Conversion Constructors vs. Operators

When initializing an object of class B from an object of class A, ambiguity can arise if both a conversion constructor and a conversion operator are defined. The interaction between these functions determines which one will be invoked.

Precedence Establishment

According to the C standard (8.5/14), the user-defined conversion sequences that can convert from A to B are enumerated, and the best one is chosen through overload resolution. Conversion operators are considered only if the conversion constructor is not applicable.

Parameter Transformation

Overload resolution transforms the conversion functions and constructors into lists of parameters that match the call arguments. For a conversion operator, the implicit object parameter is generated, creating a binding between the operator's function parameter and the source object.

Winning Candidate

The conversion operator wins due to the "fewest const qualification" rule (13.3.3.2/3). In this case, the constructor B(const A&) has a const reference as its parameter, while the conversion operator operator B() has a non-const reference. Since the source object is an rvalue (temporary), the non-const reference can bind to it.

Object-Oriented Perspective

From an object-oriented standpoint, the default choice of A's conversion operator over B's constructor suggests that the onus is on the source class (A) to define the conversion procedure. However, it remains debatable whether B, as the destination class, should have the primary responsibility for this conversion.

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