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3.2.3 Classification of Operations
Static Semantics
1
An operation
operates on
a type T if it yields a value of type
T, if it has
an operand whose expected type (see
8.6) is
T, or if it has an access parameter (see
6.1)
designating
T.
A predefined operator, or other
language-defined operation such as assignment or a membership test, that
operates on a type, is called a
predefined operation of the type.
The
primitive operations of a type are the
predefined operations of the type, plus any user-defined primitive subprograms.
1.a
Glossary entry: The primitive
operations of a type are the operations (such as subprograms) declared
together with the type declaration. They are inherited by other types
in the same class of types. For a tagged type, the primitive subprograms
are dispatching subprograms, providing run-time polymorphism. A dispatching
subprogram may be called with statically tagged operands, in which case
the subprogram body invoked is determined at compile time. Alternatively,
a dispatching subprogram may be called using a dispatching call, in which
case the subprogram body invoked is determined at run time.
1.b
To be honest: Protected
subprograms are not considered to be ``primitive subprograms,'' even
though they are subprograms, and they are inherited by derived types.
1.c
Discussion: We use the
term ``primitive subprogram'' in most of the rest of the manual. The
term ``primitive operation'' is used mostly in conceptual discussions.
2
The
primitive subprograms of a specific type are defined as follows:
3
- The predefined operators of the type (see 4.5);
4
- For a derived type, the inherited (see 3.4)
user-defined subprograms;
5
- For an enumeration type, the enumeration literals (which
are considered parameterless functions -- see 3.5.1);
6
- For a specific type declared immediately within a package_specification,
any subprograms (in addition to the enumeration literals) that are explicitly
declared immediately within the same package_specification
and that operate on the type;
7
- Any subprograms not covered above [that
are explicitly declared immediately within the same declarative region
as the type] and that override (see 8.3) other
implicitly declared primitive subprograms of the type.
7.a
Discussion: In Ada 83,
only subprograms declared in the visible part were ``primitive'' (i.e.
derivable). In Ada 95, mostly because of child library units, we include
all operations declared in the private part as well, and all operations
that override implicit declarations.
7.b
Ramification: It is possible
for a subprogram to be primitive for more than one type, though it is
illegal for a subprogram to be primitive for more than one tagged type.
See 3.9.
7.c
Discussion: The order of
the implicit declarations when there are both predefined operators and
inherited subprograms is described in 3.4,
``Derived Types and Classes''.
8
A primitive subprogram whose
designator is an
operator_symbol
is called a
primitive operator.
Incompatibilities With Ada 83
8.a
The attribute
S'Base is no longer defined for non-scalar subtypes. Since this was only
permitted as the prefix of another attribute, and there are no interesting
non-scalar attributes defined for an unconstrained composite or access
subtype, this should not affect any existing programs.
Extensions to Ada 83
8.b
The primitive
subprograms (derivable subprograms) include subprograms declared in the
private part of a package specification as well, and those that override
implicitly declared subprograms, even if declared in a body.
Wording Changes from Ada 83
8.c
We have dropped the confusing
term operation of a type in favor of the more useful primitive
operation of a type and the phrase operates on a type.
8.d
The description of S'Base has
been moved to 3.5, ``Scalar
Types'' because it is now defined only for scalar types.
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