[33] Compiler dependencies
(Part of C++ FAQ Lite, Copyright © 1991-2001, Marshall Cline, cline@parashift.com)

FAQs in section [33]:

[33.1] Where can I download a free C++ compiler? UPDATED!

[Recently changed to a different site since the old URL was dead thanks to Victor R. Volkman (in 4/01). Click here to go to the next FAQ in the "chain" of recent changes.]

Check out www.idiom.com/free-compilers/LANG/C++-1.html.

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.2] Where can I get more information on using MFC and Visual C++?

The MFC/Visual C++ FAQ (mfcfaq.stingray.com/) is maintained by Michael Pickens (formerly maintained by Scot Wingo). Another FAQ is available at www.mvps.org/vcfaq.

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.3] How do I display text in the status bar using MFC?

Use the following code snipped:

 CString s = "Text";
 CStatusBar* p =
  (CStatusBar*)AfxGetApp()->m_pMainWnd->GetDescendantWindow(AFX_IDW_STATUS_BAR);
 p->SetPaneText(1, s);

This works with MFC v.1.00 which hopefully means it will work with other versions as well.

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.4] How can I decompile an executable program back into C++ source code?

You gotta be kidding, right?

Here are a few of the many reasons this is not even remotely feasible:

But the biggest question is not how you can decompile someone's code, but why do you want to do this? If you're trying to reverse-engineer someone else's code, shame on you; go find honest work. If you're trying to recover from losing your own source, the best suggestion I have is to make better backups next time.

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.5] Where can I get information about the C++ compiler from {Borland, IBM, Microsoft, Symantec, Sun, etc.}? UPDATED!

[Recently fixed "Semantic" to "Symantec" and "Gary Comeau" to "Greg Comeau" thanks to Scott Tringali; fixed the URL to SGI's compiler thanks to Toon Knapen (in 4/01) and added URL for Digital Mars C++ thanks to Walter Bright (in 8/01). Click here to go to the next FAQ in the "chain" of recent changes.]

In alphabetical order by vendor name:

[If anyone has other suggestions that should go into this list, please let me know; thanks; (cline@parashift.com)].

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.6] How do compilers use "over-allocation" to remember the number of elements in an allocated array?

Recall that when you delete[] an array, the runtime system magically knows how many destructors to run. This FAQ describes a technique used by some C++ compilers to do this (the other common technique is to use an associative array).

If the compiler uses the "over-allocation" technique, the code for p = new Fred[n] looks something like the following. Note that WORDSIZE is an imaginary machine-dependent constant that is at least sizeof(size_t), possibly rounded up for any alignment constraints. On many machines, this constant will have a value of 4 or 8. It is not a real C++ identifier that will be defined for your compiler.

 // Original code: Fred* p = new Fred[n];
 char* tmp = (char*) operator new[] (WORDSIZE + n * sizeof(Fred));
 Fred* p = (Fred*) (tmp + WORDSIZE);
 *(size_t*)tmp = n;
 size_t i;
 try {
   for (i = 0; i < n; ++i)
     new(p + i) Fred();           // Placement new
 } catch (...) {
   while (i-- != 0)
     (p + i)->~Fred();            // Explicit call to the destructor
   operator delete[] ((char*)p - WORDSIZE);
   throw;
 }

Then the delete[] p statement becomes:

 // Original code: delete[] p;
 size_t n = * (size_t*) ((char*)p - WORDSIZE);
 while (n-- != 0)
   (p + n)->~Fred();
 operator delete[] ((char*)p - WORDSIZE);

Note that the address passed to operator delete[] is not the same as p.

Compared to the associative array technique, this technique is faster, but more sensitive to the problem of programmers saying delete p rather than delete[] p. For example, if you make a programming error by saying delete p where you should have said delete[] p, the address that is passed to operator delete(void*) is not the address of any valid heap allocation. This will probably corrupt the heap. Bang! You're dead!

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.7] How do compilers use an "associative array" to remember the number of elements in an allocated array?

Recall that when you delete[] an array, the runtime system magically knows how many destructors to run. This FAQ describes a technique used by some C++ compilers to do this (the other common technique is to over-allocate).

If the compiler uses the associative array technique, the code for p = new Fred[n] looks something like this (where arrayLengthAssociation is the imaginary name of a hidden, global associative array that maps from void* to "size_t"):

 // Original code: Fred* p = new Fred[n];
 Fred* p = (Fred*) operator new[] (n * sizeof(Fred));
 size_t i;
 try {
   for (i = 0; i < n; ++i)
     new(p + i) Fred();           // Placement new
 } catch (...) {
   while (i-- != 0)
     (p + i)->~Fred();            // Explicit call to the destructor
   operator delete[] (p);
   throw;
 }
 arrayLengthAssociation.insert(p, n);

Then the delete[] p statement becomes:

 // Original code: delete[] p;
 size_t n = arrayLengthAssociation.lookup(p);
 while (n-- != 0)
   (p + n)->~Fred();
 operator delete[] (p);

Cfront uses this technique (it uses an AVL tree to implement the associative array).

Compared to the over-allocation technique, the associative array technique is slower, but less sensitive to the problem of programmers saying delete p rather than delete[] p. For example, if you make a programming error by saying delete p where you should have said delete[] p, only the first Fred in the array gets destructed, but the heap may survive (unless you've replaced operator delete[] with something that doesn't simply call operator delete, or unless the destructors for the other Fred objects were necessary).

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.8] If name mangling was standardized, could I link code compiled with compilers from different compiler vendors?

Short answer: Probably not.

In other words, some people would like to see name mangling standards incorporated into the proposed C++ ANSI standards in an attempt to avoiding having to purchase different versions of class libraries for different compiler vendors. However name mangling differences are one of the smallest differences between implementations, even on the same platform.

Here is a partial list of other differences:

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.9] GNU C++ (g++) produces big executables for tiny programs; Why?

libg++ (the library used by g++) was probably compiled with debug info (-g). On some machines, recompiling libg++ without debugging can save lots of disk space (approximately 1 MB; the down-side: you'll be unable to trace into libg++ calls). Merely strip-ping the executable doesn't reclaim as much as recompiling without -g followed by subsequent strip-ping the resultant a.out's.

Use size a.out to see how big the program code and data segments really are, rather than ls -s a.out which includes the symbol table.

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.10] Is there a yacc-able C++ grammar? UPDATED!

[Recently rewrote; added a new grammar/lexer plus a new URL for the old grammar/lexer thanks to Ed Willink (in 4/01). Click here to go to the next FAQ in the "chain" of recent changes.]

The primary yacc grammar you'll want is from Ed Willink. Ed believes his grammar is fully compliant with the ISO/ANSI C++ standard, however he doesn't warrant it: "the grammar has not," he says, "been used in anger." You can get the grammar without action routines or the grammar with dummy action routines. You can also get the corresponding lexer. For those who are interested in how he achieves a context-free parser (by pushing all the ambiguities plus a small number of repairs to be done later after parsing is complete), you might want to read chapter 4 of his thesis.

There is also a very old yacc grammar that doesn't support templates, exceptions, nor namespaces; plus it deviates from the core language in some subtle ways. You can get that grammar here or here.

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.11] What is C++ 1.2? 2.0? 2.1? 3.0?

These are not versions of the language, but rather versions of Cfront, which was the original C++ translator implemented by AT&T. It has become generally accepted to use these version numbers as if they were versions of the language itself.

Very roughly speaking, these are the major features:

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

[33.12] Is it possible to convert C++ to C? NEW!

[Recently created thanks to J. Stephen Adamczyk and Greg Comeau (in 8/01). Click here to go to the next FAQ in the "chain" of recent changes.]

Depends on what you mean. If you mean, Is it possible to convert C++ to readable and maintainable C-code? then sorry, the answer is No — C++ features don't directly map to C, plus the generated C code is not intended for humans to follow. If instead you mean, Are there compilers which convert C++ to C for the purpose of compiling onto a platform that yet doesn't have a C++ compiler? then you're in luck — keep reading.

A compiler which compiles C++ to C does full syntax and semantic checking on the program, and just happens to use C code as a way of generating object code. Such a compiler is not merely some kind of fancy macro processor. (And please don't email me claiming these are preprocessors — they are not — they are full compilers.) It is possible to implement all of the features of ISO Standard C++ by translation to C, and except for exception handling, it typically results in object code with efficiency comparable to that of the code generated by a conventional C++ compiler.

Here are some products that perform compilation to C (note: if you know of any other products that do this, please email me):

Note that you typically need to specify the target platform's CPU, OS and C compiler so that the generated C code will be specifically targeted for this platform. This means: (a) you probably can't take the C code generated for platform X and compile it on platform Y; and (b) it'll be difficult to do the translation yourself — it'll probably be a lot cheaper/safer with one of these tools.

One more time: do not email me saying these are just preprocessors — they are not — they are compilers.

[ Top | Bottom | Previous section | Next section | Search the FAQ ]

E-Mail E-mail the author
[ C++ FAQ Lite | Table of contents | Subject index | About the author | © | Download your own copy ]
Revised Aug 15, 2001