I have read the Why can templates only be implemented in the header file? and Why can’t I separate the definition of my templates class from its declaration and put it inside a .cpp file?
If I create the templates then I am to provide access also to their cpp-files additionally to their h-files, or write the definitions directly in the header file.
Therefore, if I want to allow to use fully my templates in other applications, then I can't hide their implementation from outside eyes (for protection of intellectual property). Am I right?
In general you're right... the implementation must be exposed.
If your client only needs to instantiate them for a finite set of specific types that they can list for you, you can provide them with a pre-compiled object/library containing the implementations of the instantiations for just those types: see https://isocpp.org/wiki/faq/templates#separate-template-fn-defn-from-decl
Obfuscation is another possibility - let them see the code, but make it confusing and unmaintainable.
If neither of those options suit, consider whether you can provide a templated adapter that creates a run-time polymorphic interface over their user-provided type, capturing the specific set of functions your algorithms need. Accept those adapters as a front-end to your code. This does have runtime costs.
Intellectual property is mostly protected by legal means, not technical ones.
(e.g. the technical possibility to read some header files do not give me the right to use it, or to copy its code elsewhere)
However, you might consider code obfuscation techniques. You might even customize your recent GCC for that purpose, i.e. write your MELT free software extension. It could mean weeks (or months) of work.
Alternatively, consider publishing your header-only template C++ library as free software... (perhaps with GPL license).
But IANAL. You should ask your lawyer.
We are creating a set of libraries with a public API which is to be used by different third parties. Some of the libraries are pure C so obviously they have a C styled header with functions and struct definitions and the corresponding library. They are ok.
Some of the libraries are written with the usage of a moderately complex C++ (targeting older compilers), so there we have implemented some form of the famous pimpl idiom. This is ok too.
On the other end a significant part of the header files is C++ using heavily templated code. Knowing Why can templates only be implemented in the header file? but also not willing to disclose too much implementation details to eyes who are not supposed to see them we have heavily refactored them to exclude as much internal details as possible and having only the really necessary bits... and there is still a significant amount of code left.
So it puzzles me: Is there a preferred way of distributing header files which largely contain templates? What good practices, best approaches and tips and tricks are there?
Look at your C++ compiler's header files, for an inspiration. The standard C++ library is full of templates, and you will generally find all the template code in the headers.
Having said that, if particular templates are meant to be used with a small number of possible classes (or values) as template parameters, you do have an option of explicitly instantiating templates inside the library itself, leaving just the bare template declarations visible in the header files.
Using a simpler pre-C++11 scenario as an example, a C++ library will typically provide a std::basic_string implementation for only a std::basic_string<char> and std::basic_string<wchar_t>; and leave a bunch of template code inside the library itself, with just a bare std::basic_string template declaration visible in the header files.
I haven't tried D yet, but it seems like a very interesting language that has found some neat solutions to problems in C++. I'm curious, did it also make it possible to separate interface from implementation in templates? If yes, then how?
no any templates used are fully expanded at compile time
this means that the compiler needs to know the full code of the template making it impossible to keep it out of the .di files
At some point in processing the use of a template, D needs all the information about the template. However, there is no reason that this information need be encode as the original source code (OTOH, as an implementation detail, all current D compiler do require that). This is a fundamental issue of any language that has templates stronger than generics. The implications of this depend on what you are trying to do.
If your interest in separation of interface and implementation is to hide the implementation (like shipping binary libraries and header files in C), then this can't be done. The closest you can get is some kind of code obfuscation system.
If, on the other hand, you are interested in avoiding the cost of reprocessing templates for each recompilation, something more general like a binary pre-compiled header format could allow the reuse of the results of the lexical, syntactic and some of the passes while compiling several other modules. In fact, that would be simpler to do with D than in C.
A third option would be link time code generation, but that has little difference from conventional linking with aggressive use of an anolog to pre-compiled headers.
I wish to send some components to my customers. The reasons I want to deliver source code are:
1) My class is templatized. Customer might use any template argument, so I can't pre-compile and send .o file.
2) The customer might use different compiler versions for gcc than mine. So I want him to do compilation at his end.
Now, I can't reveal my source code for obvious reasons. The max I can do is to reveal the .h file. Any ideas how I may achieve this. I am thinking about some hooks in gcc that supports decryption before compilation, etc. Is this possible?
In short, I want him to be able to compile this code without being able to peek inside.
Contract = good, obfuscation = ungood.
That said, you can always do a kind of PIMPL idiom to serve your customer with binaries and just templated wrappers in the header(s). The idea is then to use an "untyped" separately compiled implementation, where the templated wrapper just provides type safety for client code. That's how one often did things before compilers started to understand how to optimize templates, that is, to avoid machine-code level code bloat, but it only provides some measure of protection about trivial copy-and-paste theft, not any protection against someone willing to delve into the machine code.
But perhaps the effort is then greater than just reinventing your functionality?
Just adding some terminology to Alf's answer: The Thin template idiom is what you might look at. It basically simulates the functionality of a generic. Don't get confused by the wikipedia article which pops up in google, you don't have to use void*...
This, of course, does not guarantee binary compatibility. As usual with 'native' c++, you either compile the component for customers platform yourself and deploy the binary, or give them your code... The difference to the pure generic component code is that you can do the former at all.
use some c++ obfuscators may be help?: http://www.semdesigns.com/products/obfuscators/CppObfuscationExample.html or Magle It
First, if you're going to provide the source code, then you have to provide the source code. Sure, you could encrypt it, but even if GCC had a "decrypt before compile" option, it would need to decrypt the code, and if GCC can decrypt the code, so can your customer.
What you're asking is impossible. (If you find a way to do it, I believe the movie industry might have a multi-million contract for you. They currently have to resort to expensive custom hardware to prevent people from ripping content, and that only works to a limited degree)
As for your "obvious reasons" why you don't want to provide the source code, I don't see why they're obvious. What would happen if you provided the source code?
You have two options:
provide the source code in its entirety, or
compile everything that can be precompiled into a (static or dynamic) library, and provide your customer with that, plus the header files.
what about pimpls?
1) My class is templatized. Customer might use any template argument, so I can't pre-compile and send .o file.
2) The customer might use different compiler versions for gcc than mine. So I want him to do compilation at his end.
Now, I can't reveal my source code for obvious reasons. The max I can do is to reveal the .h file. Any ideas how I may achieve this. I am thinking about some hooks in gcc that supports decryption before compilation, etc. Is this possible?
In short, I want him to be able to compile this code without being able to peek inside.
Consideration 2) above encompasses A) ABI differences such that the same code compiled with different compiler versions/vendors on the same platform is incompatible, as well as B) the differences in system libraries, kernel versions etc. that the code might be dependent on. The only general solution is to compile on the specific platforms. Either you do it for all platforms, or you give them all the source code and they do it. That's not just the headers and template implementation, that's your out-of-line functions too. You might mitigate A) a little by building a wall of more interoperable extern "C" functions, but you're basically stuck when it comes to B).
So, can you decrypt during compilation? Only if you ship your own hacked GCC binaries to them, built for their specific system, which is probably more hassle than providing different builds of your own libraries (though it may address the template/header exposure issue).
Alternatively, you could employ source code obfuscation techniques. This is probably - practically - as good as it gets. I don't know what tools are out there, but it's an approach that people have pursued for decades (though I'm yet to hear anyone recommend it), so there's sure to be some mature tools.
Re templated code - other people have suggested a templated front end to a C-style generic implementation shipped as a precompiled object. That may or may not be practical (clearly risks performance degradation, and you have to capture the set of type-specific operations you want - e.g. by instantiating a type-specific class derived from an abstract operations base class) but anyway the precompiled object still runs afoul of B).
One other thought... clients might take your source code, but are unlikely to understand it as well as you. Even if they build more systems dependent on their version of it, in a way they're getting more locked in, and may have more need for your services in future. And, if you see they've not played fair, you charge them for it appropriately when the time comes.
It seems with gcc 4.5 comes the support for plugins. So you can provide your own .so which would be, for instance, called before compilation stage starts. So you can have all kinds of tricks(decryption of source file) in there, neatly hidden. This would also be portable solution as no change is made to g++ per se.
This is exactly what I was looking for. You can read more here:
http://www.codesynthesis.com/~boris/blog/2010/05/03/parsing-cxx-with-gcc-plugin-part-1/
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Many languages, such as Java, C#, do not separate declaration from implementation. C# has a concept of partial class, but implementation and declaration still remain in the same file.
Why doesn't C++ have the same model? Is it more practical to have header files?
I am referring to current and upcoming versions of C++ standard.
Backwards Compatibility - Header files are not eliminated because it would break Backwards Compatibility.
Header files allow for independent compilation. You don't need to access or even have the implementation files to compile a file. This can make for easier distributed builds.
This also allows SDKs to be done a little easier. You can provide just the headers and some libraries. There are, of course, ways around this which other languages use.
Even Bjarne Stroustrup has called header files a kludge.
But without a standard binary format which includes the necessary metadata (like Java class files, or .Net PE files) I don't see any way to implement the feature. A stripped ELF or a.out binary doesn't have much of the information you would need to extract. And I don't think that the information is ever stored in Windows XCOFF files.
I routinely flip between C# and C++, and the lack of header files in C# is one of my biggest pet peeves. I can look at a header file and learn all I need to know about a class - what it's member functions are called, their calling syntax, etc - without having to wade through pages of the code that implements the class.
And yes, I know about partial classes and #regions, but it's not the same. Partial classes actually make the problem worse, because a class definition is spread across several files. As far as #regions go, they never seem to be expanded in the manner I'd like for what I'm doing at the moment, so I have to spend time expanding those little plus's until I get the view right.
Perhaps if Visual Studio's intellisense worked better for C++, I wouldn't have a compelling reason to have to refer to .h files so often, but even in VS2008, C++'s intellisense can't touch C#'s
C was made to make writing a compiler easily. It does a LOT of stuff based on that one principle. Pointers only exist to make writing a compiler easier, as do header files. Many of the things carried over to C++ are based on compatibility with these features implemented to make compiler writing easier.
It's a good idea actually. When C was created, C and Unix were kind of a pair. C ported Unix, Unix ran C. In this way, C and Unix could quickly spread from platform to platform whereas an OS based on assembly had to be completely re-written to be ported.
The concept of specifying an interface in one file and the implementation in another isn't a bad idea at all, but that's not what C header files are. They are simply a way to limit the number of passes a compiler has to make through your source code and allow some limited abstraction of the contract between files so they can communicate.
These items, pointers, header files, etc... don't really offer any advantage over another system. By putting more effort into the compiler, you can compile a reference object as easily as a pointer to the exact same object code. This is what C++ does now.
C is a great, simple language. It had a very limited feature set, and you could write a compiler without much effort. Porting it is generally trivial! I'm not trying to say it's a bad language or anything, it's just that C's primary goals when it was created may leave remnants in the language that are more or less unnecessary now, but are going to be kept around for compatibility.
It seems like some people don't really believe that C was written to port Unix, so here: (from)
The first version of UNIX was written
in assembler language, but Thompson's
intention was that it would be written
in a high-level language.
Thompson first tried in 1971 to use
Fortran on the PDP-7, but gave up
after the first day. Then he wrote a
very simple language he called B,
which he got going on the PDP-7. It
worked, but there were problems.
First, because the implementation was
interpreted, it was always going to be
slow. Second, the basic notions of B,
which was based on the word-oriented
BCPL, just were not right for a
byte-oriented machine like the new
PDP-11.
Ritchie used the PDP-11 to add types
to B, which for a while was called NB
for "New B," and then he started to
write a compiler for it. "So that the
first phase of C was really these two
phases in short succession of, first,
some language changes from B, really,
adding the type structure without too
much change in the syntax; and doing
the compiler," Ritchie said.
"The second phase was slower," he said
of rewriting UNIX in C. Thompson
started in the summer of 1972 but had
two problems: figuring out how to run
the basic co-routines, that is, how to
switch control from one process to
another; and the difficulty in getting
the proper data structure, since the
original version of C did not have
structures.
"The combination of the things caused
Ken to give up over the summer,"
Ritchie said. "Over the year, I added
structures and probably made the
compiler code somewhat better --
better code -- and so over the next
summer, that was when we made the
concerted effort and actually did redo
the whole operating system in C."
Here is a perfect example of what I mean. From the comments:
Pointers only exist to make writing a compiler easier? No. Pointers exist because they're the simplest possible abstraction over the idea of indirection. – Adam Rosenfield (an hour ago)
You are right. In order to implement indirection, pointers are the simplest possible abstraction to implement. In no way are they the simplest possible to comprehend or use. Arrays are much easier.
The problem? To implement arrays as efficiently as pointers you have to pretty much add a HUGE pile of code to your compiler.
There is no reason they couldn't have designed C without pointers, but with code like this:
int i=0;
while(src[++i])
dest[i]=src[i];
it will take a lot of effort (on the compilers part) to factor out the explicit i+src and i+dest additions and make it create the same code that this would make:
while(*(dest++) = *(src++))
;
Factoring out that variable "i" after the fact is HARD. New compilers can do it, but back then it just wasn't possible, and the OS running on that crappy hardware needed little optimizations like that.
Now few systems need that kind of optimization (I work on one of the slowest platforms around--cable set-top boxes, and most of our stuff is in Java) and in the rare case where you might need it, the new C compilers should be smart enough to make that kind of conversion on its own.
In The Design and Evolution of C++, Stroustrup gives out one more reason...
The same header file can have two or more implementation files which can be simultaneously worked-upon by more than one programmer without the need of a source-control system.
This might seem odd these days, but I guess it was an important issue when C++ was invented.
If you want C++ without header files then I have good news for you.
It already exists and is called D (http://www.digitalmars.com/d/index.html)
Technically D seems to be a lot nicer than C++ but it is just not mainstream enough for use in many applications at the moment.
One of C++'s goals is to be a superset of C, and it's difficult for it to do so if it cannot support header files. And, by extension, if you wish to excise header files you may as well consider excising CPP (the pre-processor, not plus-plus) altogether; both C# and Java do not specify macro pre-processors with their standards (but it should be noted in some cases they can be and even are used even with these languages).
As C++ is designed right now, you need prototypes -- just as in C -- to statically check any compiled code that references external functions and classes. Without header files, you would have to type out these class definitions and function declarations prior to using them. For C++ not to use header files, you'd have to add a feature in the language that would support something like Java's import keyword. That'd be a major addition, and change; to answer your question of if it'd be practical: I don't think so--not at all.
Many people are aware of shortcomings of header files and there are ideas to introduce more powerful module system to C++.
You might want to take a look at Modules in C++ (Revision 5) by Daveed Vandevoorde.
Well, C++ per se shouldn't eliminate header files because of backwards compatibility. However, I do think they're a silly idea in general. If you want to distribute a closed-source lib, this information can be extracted automatically. If you want to understand how to use a class w/o looking at the implementation, that's what documentation generators are for, and they do a heck of a lot better a job.
There is value in defining the class interface in a separate component to the implementation file.
It can be done with interfaces, but if you go down that road, then you are implicitly saying that classes are deficient in terms of separating implementation from contract.
Modula 2 had the right idea, definition modules and implementation modules. http://www.modula2.org/reference/modules.php
Java/C#'s answer is an implicit implementation of the same (albeit object-oriented.)
Header files are a kludge, because header files express implementation detail (such as private variables.)
In moving over to Java and C#, I find that if a language requires IDE support for development (such that public class interfaces are navigable in class browsers), then this is maybe a statement that the code doesn't stand on its own merits as being particularly readable.
I find the mix of interface with implementation detail quite horrendous.
Crucially, the lack of ability to document the public class signature in a concise well-commented file independent of implementation indicates to me that the language design is written for convenience of authorship, rather convenience of maintenance. Well I'm rambling about Java and C# now.
One advantage of this separation is that it is easy to view only the interface, without requiring an advanced editor.
No language exists without header files. It's a myth.
Look at any proprietary library distribution for Java (I have no C# experience to speak of, but I'd expect it's the same). They don't give you the complete source file; they just give you a file with every method's implementation blanked ({} or {return null;} or the like) and everything they can get away with hiding hidden. You can't call that anything but a header.
There is no technical reason, however, why a C or C++ compiler could count everything in an appropriately-marked file as extern unless that file is being compiled directly. However, the costs for compilation would be immense because neither C nor C++ is fast to parse, and that's a very important consideration. Any more complex method of melding headers and source would quickly encounter technical issues like the need for the compiler to know an object's layout.
If you want the reason why this will never happen: it would break pretty much all existing C++ software. If you look at some of the C++ committee design documentation, they looked at various alternatives to see how much code it would break.
It would be far easier to change the switch statement into something halfway intelligent. That would break only a little code. It's still not going to happen.
EDITED FOR NEW IDEA:
The difference between C++ and Java that makes C++ header files necessary is that C++ objects are not necessarily pointers. In Java, all class instances are referred to by pointer, although it doesn't look that way. C++ has objects allocated on the heap and the stack. This means C++ needs a way of knowing how big an object will be, and where the data members are in memory.
Header files are an integral part of the language. Without header files, all static libraries, dynamic libraries, pretty much any pre-compiled library becomes useless. Header files also make it easier to document everything, and make it possible to look over a library/file's API without going over every single bit of code.
They also make it easier to organize your program. Yes, you have to be constantly switching from source to header, but they also allow you define internal and private APIs inside the implementations. For example:
MySource.h:
extern int my_library_entry_point(int api_to_use, ...);
MySource.c:
int private_function_that_CANNOT_be_public();
int my_library_entry_point(int api_to_use, ...){
// [...] Do stuff
}
int private_function_that_CANNOT_be_public() {
}
If you #include <MySource.h>, then you get my_library_entry_point.
If you #include <MySource.c>, then you also get private_function_that_CANNOT_be_public.
You see how that could be a very bad thing if you had a function to get a list of passwords, or a function which implemented your encryption algorithm, or a function that would expose the internals of an OS, or a function that overrode privileges, etc.
Oh Yes!
After coding in Java and C# it's really annoying to have 2 files for every classes. So I was thinking how can I merge them without breaking existing code.
In fact, it's really easy. Just put the definition (implementation) inside an #ifdef section and add a define on the compiler command line to compile that file. That's it.
Here is an example:
/* File ClassA.cpp */
#ifndef _ClassA_
#define _ClassA_
#include "ClassB.cpp"
#include "InterfaceC.cpp"
class ClassA : public InterfaceC
{
public:
ClassA(void);
virtual ~ClassA(void);
virtual void methodC();
private:
ClassB b;
};
#endif
#ifdef compiling_ClassA
ClassA::ClassA(void)
{
}
ClassA::~ClassA(void)
{
}
void ClassA::methodC()
{
}
#endif
On the command line, compile that file with
-D compiling_ClassA
The other files that need to include ClassA can just do
#include "ClassA.cpp"
Of course the addition of the define on the command line can easily be added with a macro expansion (Visual Studio compiler) or with an automatic variables (gnu make) and using the same nomenclature for the define name.
Still I don't get the point of some statements. Separation of API and implementation is a very good thing, but header files are not API. There are private fields there. If you add or remove private field you change implementation and not API.