In C++, your classes are often divided into two parts, being the header-file and the actual implementation. In my (unexperienced) opinion, this is awful. It requires me to do all sorts of unnecessary book-keeping, clutters up my project directory and goes against everything I've learned about software development (double implementation). Languages where you only deal with the implementation, such as Java or Python, are much nicer to work with.
I've always learned that the reason to use them was to significantly decrease compilation time. However, wouldn't a modern IDE (CLion in my case) or even the compiler be smart enough to either:
Keep some sort of "shadow"-header file, which would automatically be updated whenever a definition is changed in the implementation?
Automatically split it into the header and implementation during compile time, allowing you to only have to deal with one file? (Something that Lazy C++ seems to do)
Or are there any plugins available that offer this kind of behaviour? C++ modules also seem to offer a solution to this problem, but their current status/support is unclear to me and to make matters worse there seem to be two competing standards (Clang's and Microsoft's).
Unfortunately it is not that simple. Header/source file separation C++ inherited from C due to preprocessor, that both share. Automatic generation of a header file is not possible in general, first of all that separation is not trivial, second header file often has preprocessor code that manually written and generates compilation code. Third almost all templated code goes to a header file due to process of compilation and rules of visibility. Changing all of that would require breaking compatibility with existing code, amount of which is significant and nobody wants to do that. More easy would be to create yet another language (like D) but many people would not want to migrate due to various reasons. We know that committee is working on modules and if they manage to make them work without breaking compatibility, that would be helpful for many of us. But again this is not trivial task at all, the way you describe it would only work in certain environments (when you limit yourself) but cannot be applied to everybody.
I have seen a couple of questions on how to detect unnecessary #include files in a C++ project. This question has often intrigued me, but I have never found a satisfactory answer.
If there are some header files included which, are not being used in a c++ project, is that an overhead? I understand that it means that before compilation the contents of all the header files would be copied into the included source files and that would result in a lot of unnecessary compilation.
How far does this kind of overhead spread to the compiled object files and binaries?
Aren't compilers able to do some optimizations to make sure that this
kind of overhead is not transferred to the resulting object files and
binaries ?
Considering the fact, that I probably know nothing about compiler optimization, I still want to ask this, in case there is an answer.
As a programmer who uses a wide variety of c++ libraries for his work,
what kind of programming practices should I follow to keep avoiding
such overheads ? Is making myself intimately familiar with each
library's working the only way out ?
It does not affect the performance of the binary or even the contents of the binary file, for almost all headers. Declarations generate no code at all, inline/static/anonymous-namespace definitions are optimized away if they aren't used, and no header should include externally visible definitions (that breaks if the header is included by more than one translation unit).
As #T.C. points out, the exception are internally visible static objects with nontrivial constructors. iostream does this, for example. The program must behave as if the constructor is called, and the compiler usually doesn't have enough information to optimize the constructor away.
It does, however, affect how long compilation takes and how many files will be recompiled when a header is changed. For large projects, this is enough incentive to care about unnecessary includes.
Besides the obviously longer compile times, there might be other issues. The most important one IMHO is dependencies to external libraries. You don't want your program to depend on more libraries then necessary.
You also then need to install those libraries in every system you want to the program to build on. This can become a nightmare, especially when the next programmer needs to install some database client library although the program never uses a database.
Also, especially library headers often tend to define macros. Sometimes those macros have very generic names which will break you code or which are incompatible with other library headers you might actually need.
Of course any #include is an overhead. The compiler needs to parse that file.
So avoid them. Use forward declarations where ever possible.
It will speed up compilation. See Scott Myers book on the subject
The simple answer is YES its an overhead as far as the compilation is concerned but for runtime it is merely going to create any difference. Reason being lets say you add #include <iostream> (just for example) and assume that you are not using any of its function then g++ 4.5.2 has some additional 18,560 lines of code to process(compilation). But as far as the runtime overhead is concerned I hardly think that it creates a performance issue.
You can also refer Are unused includes harmful in C/C++? where I really liked this point made by David Young
Any singletons declared as external in a header and defined in a
source file will be included in your program. This obviously increases
memory usage and possibly contributes to a performance overhead by
causing one to access their page file more often (not much of a
problem now, as singletons are usually small-to-medium in size and
because most people I know have 6+ GB of RAM).
A question was asked about whether namespace and folder structure would affect performance of an assembly in C#. The answers were very useful, but where specific to C# and the CLR.
How will the namespace and folder structure affect the performance of an assembly if it is written in C++ with gcc? What's the situation on other OSes, such as Linux or Mac OS?
If there are any significant performance issues, what should I do or avoid doing to maximize performance?
Neither your directory hierarchy nor namespaces will affect your compiled code. The code your compiler will generate will be the same. This goes for all compilers and all OS's.
To expand a bit on what Kyle said:
Namespaces are nothing more than a syntactic way for the user and the compiler to put names in different buckets. They exist to allow you to use more common and appropriate names for things without having to worry about conflicts with someone else. std::vector is a different type from a mathematical vector class. They can share the same name so long as they're in different namespaces.
As far as the compiler is concerned, a function in a namespace is no different from a function anywhere else. It just has a funny name. Indeed, compilers are allowed the freedom to do what's called "name mangling": when the compiler sees std::vector<int>, it can actually convert that into something like __std~~vector~t~~int32_t~~__ or whatever. The mangling algorithm is chosen so that no user-defined name in the global namespace can match the name used by the namespace mangler. Thus all namespace-scoped names are separate from names in other namespaces, even the global one.
Basically, the first step in the compilation process is to effectively eliminate namespaces. Therefore, later compiler steps have no clue what namespace something is even in. So they cannot generate code from it. And thus, namespaces cannot have any effect on the execution speed of compiled code.
Folders... cannot possibly matter. After compilation, you get a single executable, library, or DLL. If a compiler ever did any code generation based on the location of the source files, you would be well advised to avoid that compiler like the plague. The compiler writers would have to be trolling their users to make that happen.
I don't quite understand the point of having a header; it seems to violate the DRY principle! All the information in a header is (can be) contained in the implementation.
It simplifies the compilation process. When you want to compile units independently, you need something to describe the parts that will be linked to without having to import the entirety of all the other files.
It also allows for code hiding. One can distribute a header to allow others to use the functionality without having to distribute the implementation.
Finally, it can encourage the separation of interface from implementation.
They are not the only way to solve these problems, but 30 years ago they were a good one. We probably wouldn't use header files for a language today, but they weren't invented in 2009.
The architects of many modern languages such as Java, Eiffel and C# clearly agree with you -- those languages extract the metadata about a module from the implementation. However, per se, the concept of headers doesn't preclude that -- it would obviously be a simple task for a compiler to extract a .h file while compiling a .c, for example, just like the compilers for those other languages do implicitly. The fact that typical current C compilers do not do it is not a language design issue -- it's an implementation issue; apparently there's no demand by users for such a feature, so no compiler vendor bothers implementing it.
As a language design choice, having separate .h files (in a human-readable and editable text format) gives you the best of both worlds: you can start separately compiling client code based on a module implementation that doesn't yet exist, if you wish, by writing the .h file by hand; or you (assuming by absurd a compiler implementation that supplies it;-) can get the .h file automatically from the implementation as a side effect of compiling it.
If C, C++, &c, keep thriving (apparently they're still doing fine today;-), and demand like yours for not manually writing headers grows, eventually compiler writers will have to supply the "header generation" option, and the "best of both worlds" won't stay theoretical!-)
It helps to think a bit about the capabilities of the computers that were available when, say c, was written. Main memory was measured in kilowords, and not necessarily very many of them. Disks were bigger, but not much. Serrious storage meant reel-to-reel tapes, mounted by hand, by grumpy operators, who really wanted you to go away so they could play hunt the wumpus. A 1 MIPS machine was screaming fast. And with all these limitation you had to share it. Possibly with a score of other users.
Anything that reduced the space or time complexity of compilation was a big win. And headers do both.
Don't forget the documentation a header provides. There is usually anything in it you need to know for using the module. I for my part don't want to scan through a looong sourcecode to learn what there is that I need to use and how to call it... You would extract this information anyway, which effectively results in -- a header file. No longer an issue with modern IDEs, of course, but working with some old C code I really love to have hand-crafted header files that include comments about the usage and about pre- and postconditions.
Keeping source, header and additional documentation in sync still is another can of worms...
The whole idea of inspecting the binary output files of language processors would have been hard to comprehend when C invented .h files. There was a system called JOVIAL that did something like it, but it was exotic and confined more-or-less exclusively to military projects. (I've never seen a JOVIAL program, I've only heard about it.)
So when C came out the usual design pattern for modularity was "no checks whatsoever". There might be a restriction that .text symbols could only link to .text and .data to .data, but that was it. That is, the compilers of the day typically processed one source file at a time and then linkers put them together without the slightest level of error checking other than, if you were lucky, "I'm a function symbol" vs "I'm a data symbol".
So the idea of actually having the compiler understand the thing you were calling was somewhat new.
Even today, if you make a totally bogus header, no one catches you in most AOT compilers. Clever things like CLR languages and Java actually do encode things in the class files.
So yes, in the long run, we probably won't have header files.
No you dont have headers in Java -- but you do have interfaces and I every serious Java guru recommends you define anything used by other projects/systems as an interface and an implementation.
Lets see a java interface definition contains call signatures, type definitions and contants.
MOST C header files contain call signatures, type definitions and constants.
So for all pratical purposes C/C++ header files are just interface definitions and should thus be considered a Good Thing. Now I know its possible to define a myriad other things in header files as well (MARCROs, constants etc. etc. ) but that just part of the whole wonderful world of C:-
int function target () {
// Default for shoot
return FOOT;
}
For Detail Read this
A header file commonly contains forward declarations of classes, subroutines, variables, and other identifiers. Programmers who wish to declare standardized identifiers in more than one source file can place such identifiers in a single header file, which other code can then include whenever the header contents are required.
The C standard library and C++ standard library traditionally declare their standard functions in header files.
And what if you want to give somebody else the declarations to use your library without giving them the implementation?
As another answer points out - the original reason for headers was to make the parse/compile easier on platforms with very simple and limited tools. It was a great step forward to have a machine with 2 floppies so you could have the compiler on one and your code on the other - made things a lot easier.
When you divide code in header and source files you divide declaration and definition. When you look in header files you can see what you have and if you wand to see implementation details you go to source file.
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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.