These are my notes for compile time proxies generated from C++.  I’m not sure I will be able to understand them in the future, so good luck to you if you feel the need to read them.

Java Dynamic proxies are a well established means of reducing code by extracting a cross cutting concern. The C++ philosophy is more “Why put off to runtime that which can be performed at compile time.” How would we get the same kind of flexibility from C++ as we get from Java Dynamic proxies?

First, we would need a handful of helper classes that mimic the introspection API of Java. If we have the simple classes of Method, Field, Parameter, and Class, we can perform much of the logic we need. Refer to the Java reflexion API to see roughly what these classes should contain and what they do.

Code generation is the obvious approach, and the lack of introspection of the C++ makes abstract syntax tree analysis  it the only viable approach currently available. We can get all the information we require from g++ if we just ask nicely. FOr example, if we add the flag -fdump-translation-unit to g++ we get the file with the AST in an ultra-normalized form. For example, I want to find all of the classes defined in the file generated when I compile ExampleTestCase.cpp. The file ExampleTestCase.cpp.t00.tu on line 414 has:

@1086 identifier_node strg: ExampleTestCase lngt: 15

If we then search for what @1086 means:

adyoung@adyoung-devd$ grep -n “@1086 ” ExampleTestCase.cpp.t00.tu

1749:@783 type_decl name: @1086 type: @554 srcp: ExampleTestCase.h:14
1762:@787 function_decl name: @1086 type: @1093 scpe: @554
2414:@1086 identifier_node strg: ExampleTestCase lngt: 15
4237:@1932 type_decl name: @1086 type: @554 scpe: @554
4242:@1935 function_decl name: @1086 mngl: @2450 type: @2451
28445:@13185 function_decl name: @1086 mngl: @14801 type: @14802
We see that this identifier is used several places, but the two interesting ones are the type_decl lines, and they both refer to entry @554. Most likely the function definitions are something like the constructors. This is the data on that record:

@554    record_type      name: @783     size: @43      algn: 64
vfld: @784     base: @785     accs: priv
tag : struct   flds: @786     fncs: @787
binf: @788

It needs some prettying up, to get it all on one line, but other than that, it looks right. The big thing is the tag: struct that tells us this is a c struct. C++ must be forced to conform to c at some point, so classes become structs.

Let’s take it even simpler.  If we make an empty C++ file, called empty.cpp and compile it with:

g++   -fdump-translation-unit   -c -o empty.o empty.cpp

we get a file with a lot of standard symbols defined:

grep identifier empty.cpp.001t.tu | wc -l
1215

If we add a single static variablle, the venerable xyzzy, we can easily find it in the file:

adam@frenzy:~/devel/cpp/proxy$ echo “static int xyzzy;” >> xyzzy.cpp
adam@frenzy:~/devel/cpp/proxy$ g++   -fdump-translation-unit   -c -o xyzzy.o xyzzy.cpp
adam@frenzy:~/devel/cpp/proxy$ grep identifier  xyzzy.cpp.001t.tu | wc -l
1216

We’ve only added a single line, that looks like this:

@4      identifier_node  strg: xyzzy    lngt: 5

If we now add a Noop struct to that, we get a little bit more info:

adam@frenzy:~/devel/cpp/proxy$ echo “struct Noop{}; static int xyzzy;” >> Noop.cpp
adam@frenzy:~/devel/cpp/proxy$ make Noop.o
g++  -fdump-translation-unit    -c -o Noop.o Noop.cpp
adam@frenzy:~/devel/cpp/proxy$ grep identifier  Noop.cpp.001t.tu | wc -l
1217

Note that I’ve added -fdump-translation-unit  to the CPPFLAGS in a Makefile.

Each change has a significant effect on the resultant file:

adam@frenzy:~/devel/cpp/proxy$ wc -l Noop.cpp.001t.tu
6853 Noop.cpp.001t.tu
adam@frenzy:~/devel/cpp/proxy$ wc -l xyzzy.cpp.001t.tu
6845 xyzzy.cpp.001t.tu
adam@frenzy:~/devel/cpp/proxy$ wc -l empty.cpp.001t.tu
6841 empty.cpp.001t.tu

Because the symbol gets added early (@4) it bumps all of the other symbols in the file up one, so a diff would take a little parsing.  A visual inspection quickly shows that the following section has been added to xyzzy.cpp.001t.tu

@3      var_decl         name: @4       type: @5       srcp: xyzzy.cpp:1
chan: @6       link: static   size: @7
algn: 32       used: 0
@4      identifier_node  strg: xyzzy    lngt: 5
@5      integer_type     name: @8       size: @7       algn: 32
prec: 32       sign: signed   min : @9
max : @10

If we compare the two files based on the @ signs:

adam@frenzy:~/devel/cpp/proxy$ grep — @ xyzzy.cpp.001t.tu | wc -l
4427
adam@frenzy:~/devel/cpp/proxy$ grep — @ empty.cpp.001t.tu | wc -l
4424

We can see we have added three, which corresponds with what we have above.

Just adding the emptyr struct adds 10 lines:

adam@frenzy:~/devel/cpp/proxy$ grep — @ Noop.cpp.001t.tu | wc -l
4434.

To make iut a little easier, I went in and put a carriage return after struct Noop{};  Now I can look for Noop.cpp:1 or Noop.cpp:2

This eems to be the set of lines added for struct Noop:

@6      type_decl        name: @11      type: @12      srcp: Noop.cpp:1
note: artificial              chan: @13
@7      integer_cst      type: @14      low : 32
@8      type_decl        name: @15      type: @5       srcp: <built-in>:0
note: artificial
@9      integer_cst      type: @5       high: -1       low : -2147483648
@10     integer_cst      type: @5       low : 2147483647
@11     identifier_node  strg: Noop     lngt: 4
@12     record_type      name: @6       size: @16      algn: 8
tag : struct   flds: @17      binf: @18

Let’s see what happens if we add field.

Here’s OneOp.cpp

struct OneOp{
int aaa;
};
static int xyzzy;

adam@frenzy:~/devel/cpp/proxy$ grep — @ Noop.cpp.001t.tu | wc -l
4434
adam@frenzy:~/devel/cpp/proxy$ grep — @ OneOp.cpp.001t.tu | wc -l
4439

We get another five lines.  Let’s see if this is linear.

adam@frenzy:~/devel/cpp/proxy$ grep — @ TwoOp.cpp.001t.tu | wc -l
4444

adam@frenzy:~/devel/cpp/proxy$ grep — @ ThreeOp.cpp.001t.tu | wc -l
4449

Let’s try a function now.

adam@frenzy:~/devel/cpp/proxy$ cat OneFunc.cpp
struct OneFunc{
int narf();
};
static int xyzzy;

adam@frenzy:~/devel/cpp/proxy$ grep — @ OneOp.cpp.001t.tu | wc -l
4439
adam@frenzy:~/devel/cpp/proxy$ grep — @ OneFunc.cpp.001t.tu | wc -l
4448

About double the info.

My next goal will be to diagram out the data structures we have here using UML.

Things look fairly straight forward in the decifering until we get to function_type.  There, we have a reference to retn which in this case happens to be a void, but could concievably be any of the data types.

I have long since abandonded this approach, but may pick it back up again some day, so I will publish this and let the great crawlers out there make it avaialble to some poor sap that wants to continue it.  If you do so, please let me know.