User-Defined Class Types

Defining a mapper class

Without mapped bases ...

We have seen this sort of thing several times:

namespace cinemascope {
    struct point {
        float x;
        float y;
    QUINCE_MAP_CLASS(point, (x)(y))

The second-last line defines a mapper class, which maps a point by mapping its x and y members.

The type being mapped (point in the example) can be a class or struct. I use the term class for both.

The type being mapped must have a public no-arguments constructor and a public destructor, either handwritten or (as here) compiler-generated.

The type being mapped must be defined in some named namespace (nested is okay). It may or may not also be in an unnamed namespace.

QUINCE_MAP_CLASS must be invoked from a location that meets two criteria:

The sure and easy way to meet both criteria, regardless of whether the class is defined in a .h or a .cpp file, is to invoke the macro immediately after the class's definition.

QUINCE_MAP_CLASS's second argument gives a sequence of mapped members, i.e. non-static, non-const data members that should be mapped, in order to map the class. They must be of mapped types, and they must be direct members of the class, not members by inheritance. (Inherited members can be mapped by other means that we shall see very soon).

The members in this sequence must be individually parenthesized (it's a BOOST_PP_SEQ).

The ordering of members in the sequence determines their significance for comparisons and sorting, and it is also important if the class is used as a collector class for select() or functions in the join() family. It does not need to match the order of member declarations within the class.

The mapped members must be accessable to quince. This can be achieved in either of two ways:

The class may have other members, not mentioned to QUINCE_MAP_CLASS and hence not mapped. Quince will not touch those, except insofar as it implicitly constructs and destructs them via the class's no-arguments constructor and destructor.

The class may also have bases, which cannot be mentioned to QUINCE_MAP_CLASS, and are not mapped. Quince will not touch those either (with the same exception for construction and destruction).

... or with mapped bases

The macro QUINCE_MAP_CLASS_WITH_BASES generalizes QUINCE_MAP_CLASS, to allow for classes with mapped bases as well as mapped members.

namespace cinemascope {
    struct colored_point : point {
        int r;
        int g;
        int b;
    QUINCE_MAP_CLASS_WITH_BASES(colored_point, (point), (r)(g)(b))

The first and third arguments to QUINCE_MAP_CLASS_WITH_BASES are the same as the first and second arguments to QUINCE_MAP_CLASS: viz. the name of the class and a list of its mapped (direct) members. The second argument is a list of the mapped bases (just one in this case), individually parenthesized.

So: our use of QUINCE_MAP_CLASS_WITH_BASES defines a mapper class, which maps a colored_point by mapping its point base and its r, g, b members.

Everything said above about QUINCE_MAP_CLASS applies to QUINCE_MAP_CLASS_WITH_BASES also, with these additions:

The mapped bases must be of mapped types.

The ordering of bases in the second argument determines their significance for comparisons and sorting. It does not need to match the order in the C++ base class list.

The mapped bases must be accessable to quince.

The class may have other bases, not mentioned to QUINCE_MAP_CLASS_WITH_BASES and hence not mapped. Quince will not touch those (with the exceptions for construction and destruction).

Diamond-pattern class hierarchies, where some class B is a base of some class D via more than one path, are not permitted when B is (directly or indirectly) a mapped base of D.

Classes' mapper classes: class_mapper<...>

Following an invocation of QUINCE_MAP_CLASS(C, ...) or QUINCE_MAP_CLASS_WITH_BASES(C, ...), the class type C becomes a mapped type. Furthermore, C is statically mapped: its mapper class is always class_mapper<C>.

(Detail: class_mapper<C> is not the name that QUINCE_MAP_CLASS(C, ...) or QUINCE_MAP_CLASS_WITH_BASES(C, ...) defines. Those macros give the mapper class an obscure name, in the same namespace as C. class_mapper<C> is an alias of that obscure name. class_mapper<...>, like optional_mapper<...> and tuple_mapper<...>, is defined in namespace quince.)

In addition to mapping C to and from column formats, class_mapper<C> provides two features that act as analogs, within quince expressions, of features that C provides for normal C++ expressions:

Or in trochaic heptameter:

It follows that, if C inherits a mapped member bm from a mapped base B, then class_mapper<C> inherits a member called bm, which is a const reference to the mapper for B::bm. Or if you insist[7]:


// Include the two code fragments above

extern table<point> points;

const query<float> xs =>x);  // member of mapper

extern table<colored_point> colored_points;

const query<float> colored_xs =>x);  // inherited member of mapper


When classes are compared on the server side with the relational operators, or sorted on the server side with order(), then the order is lexicographic, with the first mapped base most significant, followed by the second mapped base, and so on; then the first mapped member, the second mapped member, and so on.


class_mapper represents data by delegating to each of the base mappers and member mappers.

When a class mapper with name N delegates to member mapper m, it prepends N and a period to all the column names that m produces. This principle is applied recursively, so that classes can be nested to any depth, and the column names are always fully qualified, to avoid clashes.

[7] dactylic hexameter this time.