The motivation for these containers was use in ColdFrame, an open-source code generator backend for use with UML tools in a restricted environment (for example, a BBC micro:bit).

In such an environment, it's normal to strip out unused code and data at link time (-gc-sections with GNU ld, -dead_strip with Apple ld).

Unfortunately, it turns out that no primitive subprograms of tagged types can be stripped (they are all referenced by the dispatch table).

These containers are still tagged, because (without compiler extensions) ColdFrame expects to use prefixed notation (object.primitive subprogram (...), as would users. However, the number of subprograms has been much reduced, and support packages aren't needed (except for sorting vectors).

The containers provided are Bounded_Hashed_Maps and Bounded_Vectors (the ones required by ColdFrame in the Ravenscar version). Both support forward and reverse generalised read-only iteration.

Bounded_Vectors don't support constant or variable indexing. Bounded_Hashed_Maps don't have a Clear operation.

Minimal_Containers are compatible with Alire.

Tampering checks

In their unchecked form, both containers would misbehave if an element was deleted during a forward iteration over the container, in that the "next" element would be skipped.

This would be undesirable, so a generation check has been introduced, raising Program_Error if this is detected.

Unlike Maps, Vectors support both forward and reverse iteration; if it's necessary to delete some elements from a Vector, e.g. those that have become invalid, do it in a reverse loop over the index,

for Index in reverse Vector.First_Index .. Vector.Last_Index loop
   if Is_Invalid (Vector.Element (Index)) then
      Vector.Delete (Index);
   end if;
end loop;

Experimental results

Microbit

The "standard" containers are those provided as part of Cortex GNAT RTS; the "minimal" containers are these.

The builds used GCC 13.0.1 (20230129). Using the "minimal" containers resulted in a reduction of 29% (40788 bytes) in the text size.

Containers	text	data	bss
standard	139380	744	1684
minimal	98592	824	1700

STM32F4

With gnat_arm_elf

The "embedded" containers are those provided in Alire by the gnat_arm_elf 2.2.1 compiler as embedded-stm32f4.

The builds used the gnat_arm_elf 2.2.1 compiler, with binder switches -minimal, -d512 (default primary stack size), -D128 (default secondary stack size) and -Q0 (meant to say don't generate any secondary stacks, though I don't see how they'd be used, but in fact appears to generate one). Using the "minimal" containers resulted in a reduction of 27% (83764 bytes) in the text size.

Containers	text	data	bss
embedded	305408	4244	32384
minimal	221644	4124	32408

With Cortex GNAT RTS

The builds used GCC 13.0.1 (20230129), with binder switch -minimal. The "standard" containers are those from Cortex GNAT RTS, based on the GCC 4.9.1 version. Using the "minimal" containers resulted in a reduction of 22% (37156 bytes) in the text size.

Containers	text	data	bss
standard	166956	1352	3768
minimal	129800	1432	3800