C++ Boost

Tuple Library : design decisions rationale

About namespaces

There was a discussion about whether tuples should be in a separate namespace or directly at the boost namespace. The common principle is that domain libraries (like graph, python) should be on a separate sub-namespace, while utility like libraries directly in the boost namespace. Tuples are somewhere in between, as the tuple template is clearly a general utility, but the library introduces quite a lot of names in addition to just the tuple template. As a result of the discussion, tuple definitions are now directly under the boost namespace.

For those who are really interested in namespaces

Note! The following discussion is not relevant for the Tuple library, as the 'no sub-namespace' decision was taken, but it may be useful for other library writers.

In the original tuple library submission, all names were under the namespace tuples. This brought up the issue of naming sub-namespaces. The rationale for not using the most natural name 'tuple' was to avoid having an identical name with the tuple template. Namespace names are, however, not generally in plural form in boost libraries. Further, no real trouble was reported for using the same name for a namespace and a class. But we found some trouble after all. One solution proposed to the dilemma of introducing a sub-namespace or not was as follows: use a sub-namespace but lift the most common names to the boost namespace with using declarations. Both gcc and edg compilers rejected such using declarations if the namespace and class names were identical:

namespace boost {
  namespace tuple {
    class cons;
    class tuple; 
      ...
  }
  using tuple::cons; // ok
  using tuple::tuple; // error
    ...
}
Note, however, that a corresponding using declaration in the global namespace seemed to be ok:
using boost::tuple::tuple; // ok;

The end mark of the cons list (nil, null_type, ...)

Tuples are internally represented as cons lists:

tuple<int, int>
inherits from
cons<int, cons<int, null_type> >
null_type is the end mark of the list. Original proposition was nil, but the name is used in MacOS, and might have caused problems, so null_type was chosen instead. Other names considered were null_t and unit (the empty tuple type in SML).

Note that null_type is the internal representation of an empty tuple: tuple<> inherits from null_type.

Element indexing

Whether to use 0- or 1-based indexing was discussed more than thoroughly, and the following observations were made:

Tuple access with the syntax get<N>(a), or a.get<N>() (where a is a tuple and N an index), was considered to be of the first category, hence, the index of the first element in a tuple is 0.

A suggestion to provide 1-based 'name like' indexing with constants like _1st, _2nd, _3rd, ... was made. By suitably chosen constant types, this would allow alternative syntaxes:

a.get<0>() == a.get(_1st) == a[_1st] == a(_1st);
We chose not to provide more than one indexing method for the following reasons:

Tuple comparison

The comparison operator implements lexicographical order. Other orderings were considered, mainly dominance (a < b iff for each i a(i) < b(i)). Our belief is, that lexicographical ordering, though not mathematically the most natural one, is the most frequently needed ordering in everyday programming.

Streaming

The characters specified with tuple stream manipulators are stored within the space allocated by ios_base::xalloc, which allocates storage for long type objects. static_cast is used in casting between long and the stream's character type. Streams that have character types not convertible back and forth to long thus fail to compile. This may be revisited at some point. The two possible solutions are:

Back to the user's guide

© Copyright Jaakko Järvi 2001.