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| <h5 class="subsubsection">15.4.7.2 Omissions from Ada</h5> |
| |
| <p><a name="index-Ada_002c-omissions-from-861"></a> |
| Here are the notable omissions from the subset: |
| |
| <ul> |
| <li>Only a subset of the attributes are supported: |
| |
| <ul> |
| <li><tt>'First</tt>, <tt>'Last</tt>, and <tt>'Length</tt> |
| on array objects (not on types and subtypes). |
| |
| <li><tt>'Min</tt> and <tt>'Max</tt>. |
| |
| <li><tt>'Pos</tt> and <tt>'Val</tt>. |
| |
| <li><tt>'Tag</tt>. |
| |
| <li><tt>'Range</tt> on array objects (not subtypes), but only as the right |
| operand of the membership (<code>in</code>) operator. |
| |
| <li><tt>'Access</tt>, <tt>'Unchecked_Access</tt>, and |
| <tt>'Unrestricted_Access</tt> (a GNAT extension). |
| |
| <li><tt>'Address</tt>. |
| </ul> |
| |
| <li>The names in |
| <code>Characters.Latin_1</code> are not available and |
| concatenation is not implemented. Thus, escape characters in strings are |
| not currently available. |
| |
| <li>Equality tests (‘<samp><span class="samp">=</span></samp>’ and ‘<samp><span class="samp">/=</span></samp>’) on arrays test for bitwise |
| equality of representations. They will generally work correctly |
| for strings and arrays whose elements have integer or enumeration types. |
| They may not work correctly for arrays whose element |
| types have user-defined equality, for arrays of real values |
| (in particular, IEEE-conformant floating point, because of negative |
| zeroes and NaNs), and for arrays whose elements contain unused bits with |
| indeterminate values. |
| |
| <li>The other component-by-component array operations (<code>and</code>, <code>or</code>, |
| <code>xor</code>, <code>not</code>, and relational tests other than equality) |
| are not implemented. |
| |
| <li><a name="index-array-aggregates-_0028Ada_0029-862"></a><a name="index-record-aggregates-_0028Ada_0029-863"></a><a name="index-aggregates-_0028Ada_0029-864"></a>There is limited support for array and record aggregates. They are |
| permitted only on the right sides of assignments, as in these examples: |
| |
| <pre class="smallexample"> (gdb) set An_Array := (1, 2, 3, 4, 5, 6) |
| (gdb) set An_Array := (1, others => 0) |
| (gdb) set An_Array := (0|4 => 1, 1..3 => 2, 5 => 6) |
| (gdb) set A_2D_Array := ((1, 2, 3), (4, 5, 6), (7, 8, 9)) |
| (gdb) set A_Record := (1, "Peter", True); |
| (gdb) set A_Record := (Name => "Peter", Id => 1, Alive => True) |
| </pre> |
| <p>Changing a |
| discriminant's value by assigning an aggregate has an |
| undefined effect if that discriminant is used within the record. |
| However, you can first modify discriminants by directly assigning to |
| them (which normally would not be allowed in Ada), and then performing an |
| aggregate assignment. For example, given a variable <code>A_Rec</code> |
| declared to have a type such as: |
| |
| <pre class="smallexample"> type Rec (Len : Small_Integer := 0) is record |
| Id : Integer; |
| Vals : IntArray (1 .. Len); |
| end record; |
| </pre> |
| <p>you can assign a value with a different size of <code>Vals</code> with two |
| assignments: |
| |
| <pre class="smallexample"> (gdb) set A_Rec.Len := 4 |
| (gdb) set A_Rec := (Id => 42, Vals => (1, 2, 3, 4)) |
| </pre> |
| <p>As this example also illustrates, <span class="sc">gdb</span> is very loose about the usual |
| rules concerning aggregates. You may leave out some of the |
| components of an array or record aggregate (such as the <code>Len</code> |
| component in the assignment to <code>A_Rec</code> above); they will retain their |
| original values upon assignment. You may freely use dynamic values as |
| indices in component associations. You may even use overlapping or |
| redundant component associations, although which component values are |
| assigned in such cases is not defined. |
| |
| <li>Calls to dispatching subprograms are not implemented. |
| |
| <li>The overloading algorithm is much more limited (i.e., less selective) |
| than that of real Ada. It makes only limited use of the context in |
| which a subexpression appears to resolve its meaning, and it is much |
| looser in its rules for allowing type matches. As a result, some |
| function calls will be ambiguous, and the user will be asked to choose |
| the proper resolution. |
| |
| <li>The <code>new</code> operator is not implemented. |
| |
| <li>Entry calls are not implemented. |
| |
| <li>Aside from printing, arithmetic operations on the native VAX floating-point |
| formats are not supported. |
| |
| <li>It is not possible to slice a packed array. |
| |
| <li>The names <code>True</code> and <code>False</code>, when not part of a qualified name, |
| are interpreted as if implicitly prefixed by <code>Standard</code>, regardless of |
| context. |
| Should your program |
| redefine these names in a package or procedure (at best a dubious practice), |
| you will have to use fully qualified names to access their new definitions. |
| </ul> |
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