Why double parameter constructor begins with an explicit keyword?

Question

My buddy and I have been recently reading leveldb source code. And we encounter this problem. In leveldb db/skiplist.h file, there is a constructor declaration:

explicit SkipList(Comparator cmp, Arena* arena);

I know explicit constructor with single parameter means no implicit type conversion for constructor parameter. But what does double parameters constructor with explicit keyword mean? Is it new rule of C++11?

Thanks.

Is this the exact declaration? Has arena no default value? — Gorpik
– Gorpik, Commented Jul 15, 2013 at 6:55
@Gorpik I dont know if the OP has exactly the same interface but look here. — TobiMcNamobi
– TobiMcNamobi, Commented Jul 15, 2013 at 6:59
@alexbuisson yes, as I mentions in my question. But what about mutiple parameter constructor? — lulyon
– lulyon, Commented Jul 15, 2013 at 7:06

Qaz · Accepted Answer · 2016-04-11 19:30:03Z

16

With C++11, you can use braced-init-lists in place of some other expressions, and that makes a difference. For instance, you can use them in return statements:

SkipList foo() {
    return {{}, nullptr}; //does not compile with explicit constructor
    return SkipList{{}, nullptr}; //compiles with or without explicit constructor
}

edited Apr 11, 2016 at 19:30

answered Jul 15, 2013 at 7:01

Qaz

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6 Comments

lulyon Over a year ago

By uniform initialization You mean initializing multiple parameter like initializing an array?

Qaz Over a year ago

@lulyon, Uniform initialization generally results from using braces. You can use them to call constructors, as shorthand for TypeName(), and to eliminate the most vexing parse. It's actually quite a significant feature of C++11. You should look up some information on it.

M.M Over a year ago

This is called list initialization

Qaz Over a year ago

@M.M, Thanks, I changed it to use braced-init-list instead to match the "top level" of wording, even though it is list-initialized when all is said and done.

T.C. Over a year ago

I wouldn't be surprised if this constructor originally had a default argument (or was adapted from a single-argument constructor). There's not that many cases where you'd actually want to disable copy-list-initialization.

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