1. Using ASIS to Generate C++ Bindings
Gachia -
Generating a C++ Header Interface Automatically
11/15/2018
Ausden & Nyberg

2. Background / Motivation
One component (FLTS) of the FAA ERAM (En Route Automation Modernization)
ERAM primarily developed in Ada
Utilizes components from previous ATC developments, also primarily in Ada
Need to provide access / interface for C++ client programs (primarily display interfaces)
Rapidly changing Ada API during development – C++ API lagging severely behind
Emphasize that this is only ONE component of the overall ERAM project. This effort was therefore somewhat of a prototype / research project, which wasn’t necessarily assured of being any more widely used. It was just that this was a BIG component, still under development and a primary interface to the C++ programs. Other components also had C++ requirements, but these were smaller and / or had already been implemented manually.
11/15/2018
Ausden & Nyberg

3. Approach Use ASIS to generate API headers
Package / File
Supporting Utilities (Data Serialization - incomplete)
Determine language construct mapping
Packaging
Types
Functions / Procedures
Exceptions
Integration with other components
Mention that because of the pre-existing quantity of code, alternative solutions, such as CORBA, could not be injected into the development at this point in the lifecycle.
For data serialization, mention the typing tool and recommend that people see the corresponding presentation tomorrow afternoon.
11/15/2018
Ausden & Nyberg

4. Ada / C++ Language Mapping
Packaging
Types
Intrinsic
Composite / Structured
“Abstract” (includes abstract, tagged, variant, discriminant)
Functions / Procedures
Exceptions
Mostly done on the fly, rather than through a studied comparison…
11/15/2018
Ausden & Nyberg

5. Ada / C++ Packaging Ada packages C++ namespaces
Both have “child” capabilities
Really straightforward…
Ada parent-child.ads, .adb
C++ parent-child.h, .cpp
11/15/2018
Ausden & Nyberg

6. Ada / C++ Types Intrinsic Composite / Structured “Abstract” Constants
Enumeration types
Integer types
Arrays (including strings)
Composite / Structured
Records / structs
“Abstract”
Abstract
Tagged
Variant records
Discriminant types
Most of these were also pretty straightforward.
11/15/2018
Ausden & Nyberg

7. Intrinsic Constants Enumeration types Integer types
C++ requires unique per namespace
Integer types
C++ “expansion” to int
Arrays (including strings)
C++ “0” based
11/15/2018
Ausden & Nyberg

8. Composite / Structured
Records / structs
Ada variant
C++ union (not addressed)
Bit layout issues NOT addressed at this point
11/15/2018
Ausden & Nyberg

9. “Abstract” Types Private Types Mapped to C++ Classes
Interesting issues
Variants (often different constructors)
Tagged (“protected” rather than “private” data members)
11/15/2018
Ausden & Nyberg

10. Ada / C++ Types (contd) Type Issues Approach Dynamic Types
E.g. C++ vectors, lists
Booch Components
Approach
Use the “natural” C++ style in header
Use collections / “type tool” in wrapper implementation (presentation tomorrow by Matt Mark)
11/15/2018
Ausden & Nyberg

11. Ada / C++ Exceptions Defined in base level packages for both languages
Implemented cross-language exception passing in wrappers via an “errno” like approach
Another compiler dependent “pop-stack-and-call option” used elsewhere
11/15/2018
Ausden & Nyberg

12. ASIS Overview
Abstract interface to underlying compiler’s internal representation of code
Tutorial Information on the Web – start at
Requires Ada code (specifications on for this project) be compiled
Requires ASIS implementation by vendor
May require compilation with special flags (e.g., GNAT’s “-gnatt” to retain parse tree)
11/15/2018
Ausden & Nyberg

13. ASIS Components Compilation Unit Declaration Definitions Elements
There are many more components, these are just the primary ones used here
11/15/2018
Ausden & Nyberg

14. ASIS – Compilation Unit
Contains queries about compilation units
What declarations are contained
Whether in specification or body
Private or public (visible)
Whether body is required
Name
11/15/2018
Ausden & Nyberg

15. ASIS – Declarations Queries for declaration objects
Type (variable, constant, number, package, procedure / function, generic / not, etc.)
Name
Parameter profile (for functions / procedures)
Result (return) profile
Visibility
11/15/2018
Ausden & Nyberg

16. ASIS – Definitions Typing Parent type Structure / record components
Enumeration literals
Integer / real range constraints
Mark
Array bounds
Discriminants and variants
11/15/2018
Ausden & Nyberg

17. Elements Context (“with”) clauses attached to packages
Function call parameters
Type kinds
11/15/2018
Ausden & Nyberg

18. Generated outputs Basic Interface: Support interface:
C++ header file (*.h)
Support interface:
Ada interface wrapper files (*-w.ad[bs])
C++ wrapper files (*-w.h)
C++ skeleton “implementation” (*.cpp)
Data Serialization (TBD)
11/15/2018
Ausden & Nyberg

19. Basic Interface Iterate through the package declarations
Recurse through “with’d” packages
Generate corresponding intrinsic types
Collect functions / procedures for C++ classes
Generate C++ class interfaces
11/15/2018
Ausden & Nyberg

20. Support Interface Ada interface wrapper files (*-w.ad[bs])
Pragma export C++ names
Create everything procedurally
Wrap an “errno” around existing interfaces
C++ wrapper files (*-w.h)
Use “externs” to limit C++ compiler name mangling
C++ skeleton “implementation” (*.cpp)
Like a “stubsrc”, to link against
Data Serialization (TBD)
Again – attend tomorrow’s talk by Matt Mark
11/15/2018
Ausden & Nyberg

21. Integration with other Components
Some other components already had C++ interfaces defined
Standard C++ features used in order to be more natural for the C++ programmer
Effectively drew a “line in the sand” and implemented with textual substitution and manual editing
11/15/2018
Ausden & Nyberg

22. Limitations
Development process required inspection of all code (generated or manual)
Some rough edges not completed
Ada API included some components not required by C++ clients
Optimizations possible when clients access was limited to “reading” the data
11/15/2018
Ausden & Nyberg

23. Results Effective for bulk of header files Not fully implemented
Most simple types
Generated classes and methods
Created ~ 500 header files for FLTS
Not fully implemented
Serialization deferred
Inherited types incomplete
C++ “Bodies” unimplemented
11/15/2018
Ausden & Nyberg

24. Use of ASIS Intuitive interface Straightforward implementation
Reasonable performance (5 – 6 minutes execution time for hundreds / thousands of files)
Much nicer to be working in Ada rather than awk, perl, grep, sed, cpp!
11/15/2018
Ausden & Nyberg