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Friday, October 26, 2012

The Standard works in Circular Ways

Rummaging around in the C++11 standard isn't all fun and games, but from time to time you come across true gems.  For example, this is from 2.14.7/1 in the standard:
The pointer literal is the keyword nullptr. It is a prvalue of type std::nullptr_t.
And this is from 18.2/9:
nullptr_t is defined as follows:
namespace std {
  typedef decltype(nullptr) nullptr_t;
}
So nullptr is of type nullptr_t, and nullptr_t is defined to be the type of nullptr.

Um, okay.

Scott

5 comments:

Tianyu Zhu said...

Probably so that it doesn't get cast to int, right?

Tianyu Zhu said...
This comment has been removed by the author.
Scott Meyers said...

@Tianyu Zhu: I don't know why they defined things the way they did, but there are other ways to define things such that casting to int is not permitted (without a reinterpret_cast).

Anonymous said...

That's a somewhat consecrated design, also present in Scala and D. "nullptr" is a bottom type in the type lattice just above the "none" type that cannot be instantiated. As such, it has its own interesting properties - most notably it can be instantiated but only with null.

Defining nullptr_t as "the type of nullptr" is a simple solution to avoiding introducing a new keyword for that type.

Anonymous said...

For me really surprising thing about nullptr is that it is not a pointer, i.e. std::is_pointer<decltype(nullptr)>::value == false. For me it's yet another defect in the C++11.