On Sep 28, 2010, at 4:26 PM, David Levin wrote:

This came up before: https://lists.webkit.org/pipermail/webkit-dev/2010-May/012873.html but I'd like to understand it a bit better.

It feels there were two points of view:
Use explicit only when necessary to prevent an undesirable implicit conversion like int to vector.
Use explicit except when it is desirable to allow an implicit conversion that makes the code simpler. For example, the String <-> AtomicString makes the bindings generator code simpler since it doesn't need to know which the underlying method takes.

Are there any reasons beyond personal preference to select either of these?

Starting list:

Pro's for #1
It is a pain to remember to put explicit every time you have a constructor with one argument.

Pro's for #2
It would prevent accidental mistakes that happen with implicit constructors.

3. Do not explicit when the single-argument constructor can be thought of as a type conversion - the class will be in some sense an alternate form of its sole parameter. Do use explicit when the single-argument constructor is *not* reasonably thought of as a type conversion - the single argument just happens to be the sole initialization parameter. Or to put it another way - can you imagine having a type conversion operator overload that does the same thing as this constroctor?

Applying this rule to your two examples, Vector(int) should be explicit, because it doesn't convert the int to a vector, it uses the int as a size instead of the default one. But String(AtomicString) or AtomicString(String) need not be explicit, since they convert from one string type to another, carrying largely the same data.