Related
From a class exercise:
Fill in the blanks to print
"it works!" on the screen,
type in the function
prototype before its call.
void some_func();
int main()
{
some_func();
return 0;
}
void some_func() {
cout << "it works!" << endl;
}
If I am using the function right after I declared it, why did this code define it in the end?
I mean, we could just have declared and defined and then call in the main.
I don't get the point of declaring, calling and then defining.
I have just started learning about functions. I am a beginner.
EDIT from perspective of C.S. teacher: This student is wondering about the purpose of function prototypes generally. This is a typical case of teaching a concept with an example that is simple enough to master, yet so simple it doesn't yet serve a purpose. Perhaps it is still not the right question for StackOverflow, but should at least be closed for the right reason. It is a very common and well-defined question.
It is perfectly fine to put the definition before the use in most cases. The only time the C language requires you to put a non-definition declaration before use is when two or more functions use each other, so one or more of them have to be declared before the others can be defined. I prefer building up a module from smaller/lower parts to larger parts, so I generally put function definitions before the routines that use them. However, the teacher may be presenting this exercise to teach concepts about declarations, not to teach you to use a particular style.
If you have not yet learned about them, you will soon learn about header files. As programs grow more complicated, we no longer want to keep all the source code in one file, for several reasons. (It becomes more difficult to manage more code. Grouping functions by some type of service they provide or some sort of common data they work with can help us organize the source code and keep it more sensible to human understanding. It also makes it easier to reuse functions in other programs.) Once we break a program up into separate source files, some definitions will no longer be in the same source file as routine to use them. So, instead of definitions, we put declarations in the source files as necessary. A routine foo defined in one source file, and other files that use it will have declarations of foo.
This brings up a problem that declarations are then repeated in many places. That becomes a burden to maintain—any time the definition of foo is changed, all the declarations of it have to be changed. And it is easy to make mistakes, which can bugs in the program, because declaring a function differently from how it is defined can cause undesired behavior. To deal with this, declarations for one set of things, typically all those defined in one source file (and intended to be used by other source files), are gathered into one file, called a header file and typically given a name ending in .h. Then other source files merely include the header files that contain the declarations they need, using an #include directive that tells the compiler to include contents the header file in the current compilation as if it were a part of the current source file.
Because of this, you must learn about various ways declarations can be used, including putting them before functions that use the declared functions.
(I would include C++, but its declarations can be more complicated.)
Just out of curiosity I wanted to know if is there a way to achieve this.
In C++ we learn that we should avoid using macros. But when we use include guards, we do use at least one macro. So I was wondering if there is a way to write a macro-free program.
It's definitely possible, though it's unimaginably bad practice not to have include guards. It's important to understand what the #include statement actually does: the contents of another file are pasted directly into your source file before it's compiled. An include guard prevents the same code from being pasted again.
Including a file only causes an error if it would be incorrect to type the contents of that file at the position you included it. As an example, you can declare (note: declare, not define) the same function (or class) multiple times in a single compilation unit. If your header file consists only of declarations, you don't need to specify an include guard.
IncludedFile.h
class SomeClassSomewhere;
void SomeExternalFunction(int x, char y);
Main.cpp
#include "IncludedFile.h"
#include "IncludedFile.h"
#include "IncludedFile.h"
int main(int argc, char **argv)
{
return 0;
}
While declaring a function (or class) multiple times is fine, it isn't okay to define the same function (or class) more than once. If there are two or more definitions for a function, the linker doesn't know which one to choose and gives up with a "multiply defined symbols" error.
In C++, it's very common for header files to include class definitions. An include guard prevents the #included file from being pasted into your source file a second time, which means your definitions will only appear once in the compiled code, and the linker won't be confused.
Rather than trying to figure out when you need to use them and when you don't, just always use include guards. Avoiding macros most of the time is a good idea; this is one situation where they aren't evil, and using them here isn't dangerous.
It is definitely doable and I have used some early C++ libraries which followed an already misguided approach from C which essentially required the user of a header to include certain other headers before this. This is based on thoroughly understanding what creates a dependency on what else and to use declarations rather than definitions wherever possible:
Declarations can be repeated multiple times although they are obviously required to be consistent and some entities can't be declared (e.g. enum can only be defined; in C++ 2011 it is possible to also declare enums).
Definitions can't be repeated but are only needed when the definition if really used. For example, using a pointer or a reference to a class doesn't need its definition but only its declaration.
The approach to writing headers would, thus, essentially consist of trying to avoid definitions as much as possible and only use declaration as far as possible: these can be repeated in a header file or corresponding headers can even be included multiple times. The primary need for definitions comes in when you need to derive from a base class: this can't be avoided and essentially means that the user would have to include the header for the base class before using any of the derived classes. The same is true for members defined directly in the class but using the pimpl-idiom the need for member definitions can be pushed to the implementation file.
Although there are a few advantages to this approach it also has a few severe drawbacks. The primary advantage is that it kind of enforces a very thorough separation and dependency management. On the other hand, overly aggressive separation e.g. using the pimpl-idiom for everything also has a negative performance impact. The biggest drawback is that a lot the implementation details are implicitly visible to the user of a header because the respective headers this one depends on need to be included first explicitly. At least, the compiler enforces that you get the order of include files right.
From a usability and dependency point of view I think there is a general consensus that headers are best self-contained and that the use of include guards is the lesser evil.
It is possible to do so if you ensure the same header file is not being included in the same translation unit multiple times.
Also, you could use:
#pragma once
if portability is not your concern.
However, you should avoid using #pragma once over Include Guards because:
It is not standard & hence non portable.
It is less intuitive and not all users might know of it.
It provides no big advantage over the classic and very well known Include Guards.
In short, yes, even without pragmas. Only if you can guarantee that every header file is included only once. However, given how code tends to grow, it becomes increasingly difficult to honour that guarantee as the number of header files increase. This is why not using header guards is considered bad practice.
Pre-processor macros are frowned upon, yes. However, header include guards are a necessary evil because the alternative is so much worse (#pragma once will only work if your compiler supports it, so you lose portability)
With regard to pre-processor macros, use this rule:
If you can come up with an elegant solution that does not involve a macro, then avoid them.
Does the non-portable, non-standard
#pragma once
work sufficiently well for you? Personally, I'd rather use macros for preventing reinclusion, but that's your decision.
I have seen many explanations on when to use forward declarations over including header files, but few of them go into why it is important to do so. Some of the reasons I have seen include the following:
compilation speed
reducing complexity of header file management
removing cyclic dependencies
Coming from a .net background I find header management frustrating. I have this feeling I need to master forward declarations, but I have been scrapping by on includes so far.
Why cannot the compiler work for me and figure out my dependencies using one mechanism (includes)?
How do forward declarations speed up compilations since at some point the object referenced will need to be compiled?
I can buy the argument for reduced complexity, but what would a practical example of this be?
"to master forward declarations" is not a requirement, it's a useful guideline where possible.
When a header is included, and it pulls in more headers, and yet more, the compiler has to do a lot of work processing a single translation module.
You can see how much, for example, with gcc -E:
A single #include <iostream> gives my g++ 4.5.2 additional 18,560 lines of code to process.
A #include <boost/asio.hpp> adds another 74,906 lines.
A #include <boost/spirit/include/qi.hpp> adds 154,024 lines, that's over 5 MB of code.
This adds up, especially if carelessly included in some file that's included in every file of your project.
Sometimes going over old code and pruning unnecessary includes improves the compilation dramatically just because of that. Replacing includes with forward declarations in the translation modules where only references or pointers to some class are used, improves this even further.
Why cannot the compiler work for me and figure out my dependencies using one mechanism (includes)?
It cannot because, unlike some other languages, C++ has an ambiguous grammar:
int f(X);
Is it a function declaration or a variable definition? To answer this question the compiler must know what does X mean, so X must be declared before that line.
Because when you're doing something like this :
bar.h :
class Bar {
int foo(Foo &);
}
Then the compiler does not need to know how the Foo struct / class is defined ; so importing the header that defines Foo is useless. Moreover, importing the header that defines Foo might also need importing the header that defines some other class that Foo uses ; and this might mean importing the header that defines some other class, etc.... turtles all the way.
In the end, the file that the compiler is working against is almost like the result of copy pasting all the headers ; so it will get big for no good reason, and when someone makes a typo in a header file that you don't need (or import , or something like that), then compiling your class starts to take waaay too much time (or fail for no obvious reason).
So it's a good thing to give as little info as needed to the compiler.
How do forward declarations speed up compilations since at some point the object referenced will need to be compiled?
1) reduced disk i/o (fewer files to open, fewer times)
2) reduced memory/cpu usage
most translations need only a name. if you use/allocate the object, you'll need its declaration.
this is probably where it will click for you: each file you compile compiles what is visible in its translation.
a poorly maintained system will end up including a ton of stuff it does not need - then this gets compiled for every file it sees. by using forwards where possible, you can bypass that, and significantly reduce the number of times a public interface (and all of its included dependencies) must be compiled.
that is to say: the content of the header won't be compiled once. it will be compiled over and over. everything in this translation must be parsed, checked that it's a valid program, checked for warnings, optimized, etc. many, many times.
including lazily only adds significant disk/cpu/memory increase, which turns into intolerable build times for you, while introducing significant dependencies (in non-trivial projects).
I can buy the argument for reduced complexity, but what would a practical example of this be?
unnecessary includes introduce dependencies as side effects. when you edit an include (necessary or not), then every file which includes it must be recompiled (not trivial when hundreds of thousands of files must be unnecessarily opened and compiled).
Lakos wrote a good book which covers this in detail:
http://www.amazon.com/Large-Scale-Software-Design-John-Lakos/dp/0201633620/ref=sr_1_1?ie=UTF8&s=books&qid=1304529571&sr=8-1
Header file inclusion rules specified in this article will help reduce the effort in managing header files.
I used forward declarations simply to reduce the amount of navigation between source files done. e.g. if module X calls some glue or interface function F in module Y, then using a forward declaration means the writing the function and the call can be done by only visiting 2 places, X.c and Y.c not so much of an issue when a good IDE helps you navigate, but I tend to prefer coding bottom-up creating working code then figuring out how to wrap it rather than through top down interface specification.. as the interfaces themselves evolve it's handy to not have to write them out in full.
In C (or c++ minus classes) it's possible to truly keep structure details Private by only defining them in the source files that use them, and only exposing forward declarations to the outside world - a level of black boxing that requires performance-destroying virtuals in the c++/classes way of doing things. It's also possible to avoid needing to prototype things (visiting the header) by listing 'bottom-up' within the source files (good old static keyword).
The pain of managing headers can sometimes expose how modular your program is or isn't - if its' truly modular, the number of headers you have to visit and the amount of code & datastructures declared within them should be minimized.
Working on a big project with 'everything included everywhere' through precompiled headers won't encourage this real modularity.
module dependancies can correlate with data-flow relating to performance issues, i.e. both i-cache & d-cache issues. If a program involves many modules that call each other & modify data at many random places, it's likely to have poor cache-coherency - the process of optimizing such a program will often involve breaking up passes and adding intermediate data.. often playing havoc with many'class diagrams'/'frameworks' (or at least requiring the creation of many intermediates datastructures). Heavy template use often means complex pointer-chasing cache-destroying data structures. In its optimized state, dependancies & pointer chasing will be reduced.
I believe forward declarations speed up compilation because the header file is ONLY included where it is actually used. This reduces the need to open and close the file once. You are correct that at some point the object referenced will need to be compiled, but if I am only using a pointer to that object in my other .h file, why actually include it? If I tell the compiler I am using a pointer to a class, that's all it needs (as long as I am not calling any methods on that class.)
This is not the end of it. Those .h files include other .h files... So, for a large project, opening, reading, and closing, all the .h files which are included repetitively can become a significant overhead. Even with #IF checks, you still have to open and close them a lot.
We practice this at my source of employment. My boss explained this in a similar way, but I'm sure his explanation was more clear.
How do forward declarations speed up compilations since at some point the object referenced will need to be compiled?
Because include is a preprocessor thing, which means it is done via brute force when parsing the file. Your object will be compiled once (compiler) then linked (linker) as appropriate later.
In C/C++, when you compile, you've got to remember there is a whole chain of tools involved (preprocessor, compiler, linker plus build management tools like make or Visual Studio, etc...)
Good and evil. The battle continues, but now on the battle field of header files. Header files are a necessity and a feature of the language, but they can create a lot of unnecessary overhead if used in a non optimal way, e.g. not using forward declarations etc.
How do forward declarations speed up
compilations since at some point the
object referenced will need to be
compiled?
I can buy the argument for reduced
complexity, but what would a practical
example of this be?
Forward declarations are bad ass. My experience is that a lot of c++ programmers are not aware of the fact that you don't have to include any header file, unless you actually want to use some type, e.g. you need to have the type defined so the compiler understands what you want to do. It's important to try and refrain from including header files in other header files.
Just passing around a pointer from one function to another, only requires a forward declaration:
// someFile.h
class CSomeClass;
void SomeFunctionUsingSomeClass(CSomeClass* foo);
Including someFile.h does not require you to include the header file of CSomeClass, since you are merely passing a pointer to it, not using the class. This means that the compiler only needs to parse one line (class CSomeClass;) instead of an entire header file (that might be chained to other header files etc etc).
This reduces both compile time and link time, and we are talking big optimizations here if you have many headers and many classes.
In C++, declaration and definition of functions, variables and constants can be separated like so:
function someFunc();
function someFunc()
{
//Implementation.
}
In fact, in the definition of classes, this is often the case. A class is usually declared with it's members in a .h file, and these are then defined in a corresponding .C file.
What are the advantages & disadvantages of this approach?
Historically this was to help the compiler. You had to give it the list of names before it used them - whether this was the actual usage, or a forward declaration (C's default funcion prototype aside).
Modern compilers for modern languages show that this is no longer a necessity, so C & C++'s (as well as Objective-C, and probably others) syntax here is histotical baggage. In fact one this is one of the big problems with C++ that even the addition of a proper module system will not solve.
Disadvantages are: lots of heavily nested include files (I've traced include trees before, they are surprisingly huge) and redundancy between declaration and definition - all leading to longer coding times and longer compile times (ever compared the compile times between comparable C++ and C# projects? This is one of the reasons for the difference). Header files must be provided for users of any components you provide. Chances of ODR violations. Reliance on the pre-processor (many modern languages do not need a pre-processor step), which makes your code more fragile and harder for tools to parse.
Advantages: no much. You could argue that you get a list of function names grouped together in one place for documentation purposes - but most IDEs have some sort of code folding ability these days, and projects of any size should be using doc generators (such as doxygen) anyway. With a cleaner, pre-processor-less, module based syntax it is easier for tools to follow your code and provide this and more, so I think this "advantage" is just about moot.
It's an artefact of how C/C++ compilers work.
As a source file gets compiled, the preprocessor substitutes each #include-statement with the contents of the included file. Only afterwards does the compiler try to interpret the result of this concatenation.
The compiler then goes over that result from beginning to end, trying to validate each statement. If a line of code invokes a function that hasn't been defined previously, it'll give up.
There's a problem with that, though, when it comes to mutually recursive function calls:
void foo()
{
bar();
}
void bar()
{
foo();
}
Here, foo won't compile as bar is unknown. If you switch the two functions around, bar won't compile as foo is unknown.
If you separate declaration and definition, though, you can order the functions as you wish:
void foo();
void bar();
void foo()
{
bar();
}
void bar()
{
foo();
}
Here, when the compiler processes foo it already knows the signature of a function called bar, and is happy.
Of course compilers could work in a different way, but that's how they work in C, C++ and to some degree Objective-C.
Disadvantages:
None directly. If you're using C/C++ anyway, it's the best way to do things. If you've got a choice of language/compiler, then maybe you can pick one where this is not an issue. The only thing to consider with splitting declarations into header files is to avoid mutually recursive #include-statements - but that's what include guards are for.
Advantages:
Compilation speed: As all included files are concatenated and then parsed, reducing the amount and complexity of code in included files will improve compilation time.
Avoid code duplication/inlining: If you fully define a function in a header file, each object file that includes this header and references this function will contain it's own version of that function. As a side-note, if you want inlining, you need to put the full definition into the header file (on most compilers).
Encapsulation/clarity: A well defined class/set of functions plus some documentation should be enough for other developers to use your code. There is (ideally) no need for them to understand how the code works - so why require them to sift through it? (The counter-argument that it's may be useful for them to access the implementation when required still stands, of course).
And of course, if you're not interested in exposing a function at all, you can usually still choose to define it fully in the implementation file rather than the header.
The standard requires that when using a function, a declaration must be in scope. This means, that the compiler should be able to verify against a prototype (the declaration in a header file) what you are passing to it. Except of course, for functions that are variadic - such functions do not validate arguments.
Think of C, when this was not required. At that time, compilers treated no return type specification to be defaulted to int. Now, assume you had a function foo() which returned a pointer to void. However, since you did not have a declaration, the compiler will think that it has to return an integer. On some Motorola systems for example, integeres and pointers would be be returned in different registers. Now, the compiler will no longer use the correct register and instead return your pointer cast to an integer in the other register. The moment you try to work with this pointer -- all hell breaks loose.
Declaring functions within the header is fine. But remember if you declare and define in the header make sure they are inline. One way to achieve this is to put the definition inside the class definition. Otherwise prepend the inline keyword. You will run into ODR violation otherwise when the header is included in multiple implementation files.
There are two main advantages to separating declaration and definition into C++ header and source files. The first is that you avoid problems with the One Definition Rule when your class/functions/whatever are #included in more than one place. Secondly, by doing things this way, you separate interface and implementation. Users of your class or library need only to see your header file in order to write code that uses it. You can also take this one step farther with the Pimpl Idiom and make it so that user code doesn't have to recompile every time the library implementation changes.
You've already mentioned the disadvantage of code repetition between the .h and .cpp files. Maybe I've written C++ code for too long, but I don't think it's that bad. You have to change all user code every time you change a function signature anyway, so what's one more file? It's only annoying when you're first writing a class and you have to copy-and-paste from the header to the new source file.
The other disadvantage in practice is that in order to write (and debug!) good code that uses a third-party library, you usually have to see inside it. That means access to the source code even if you can't change it. If all you have is a header file and a compiled object file, it can be very difficult to decide if the bug is your fault or theirs. Also, looking at the source gives you insight into how to properly use and extend a library that the documentation might not cover. Not everyone ships an MSDN with their library. And great software engineers have a nasty habit of doing things with your code that you never dreamed possible. ;-)
Advantage
Classes can be referenced from other files by just including the declaration. Definitions can then be linked later on in the compilation process.
You basically have 2 views on the class/function/whatever:
The declaration, where you declare the name, the parameters and the members (in the case of a struct/class), and the definition where you define what the functions does.
Amongst the disadvantages are repetition, yet one big advantage is that you can declare your function as int foo(float f) and leave the details in the implementation(=definition), so anyone who wants to use your function foo just includes your header file and links to your library/objectfile, so library users as well as compilers just have to care for the defined interface, which helps understanding the interfaces and speeds up compile times.
One advantage that I haven't seen yet: API
Any library or 3rd party code that is NOT open source (i.e. proprietary) will not have their implementation along with the distribution. Most companies are just plain not comfortable with giving away source code. The easy solution, just distribute the class declarations and function signatures that allow use of the DLL.
Disclaimer: I'm not saying whether it's right, wrong, or justified, I'm just saying I've seen it a lot.
One big advantage of forward declarations is that when used carefully you can cut down the compile time dependencies between modules.
If ClassA.h needs to refer to a data element in ClassB.h, you can often use just a forward references in ClassA.h and include ClassB.h in ClassA.cc rather than in ClassA.h, thus cutting down a compile time dependency.
For big systems this can be a huge time saver on a build.
Disadvantage
This leads to a lot of repetition. Most of the function signature needs to be put in two or more (as Paulious noted) places.
Separation gives clean, uncluttered view of program elements.
Possibility to create and link to binary modules/libraries without disclosing sources.
Link binaries without recompiling sources.
When done correctly, this separation reduces compile times when only the implementation has changed.
My personal style with C++ has always to put class declarations in an include file, and definitions in a .cpp file, very much like stipulated in Loki's answer to C++ Header Files, Code Separation. Admittedly, part of the reason I like this style probably has to do with all the years I spent coding Modula-2 and Ada, both of which have a similar scheme with specification files and body files.
I have a coworker, much more knowledgeable in C++ than I, who is insisting that all C++ declarations should, where possible, include the definitions right there in the header file. He's not saying this is a valid alternate style, or even a slightly better style, but rather this is the new universally-accepted style that everyone is now using for C++.
I'm not as limber as I used to be, so I'm not really anxious to scrabble up onto this bandwagon of his until I see a few more people up there with him. So how common is this idiom really?
Just to give some structure to the answers: Is it now The Way™, very common, somewhat common, uncommon, or bug-out crazy?
Your coworker is wrong, the common way is and always has been to put code in .cpp files (or whatever extension you like) and declarations in headers.
There is occasionally some merit to putting code in the header, this can allow more clever inlining by the compiler. But at the same time, it can destroy your compile times since all code has to be processed every time it is included by the compiler.
Finally, it is often annoying to have circular object relationships (sometimes desired) when all the code is the headers.
Bottom line, you were right, he is wrong.
EDIT: I have been thinking about your question. There is one case where what he says is true. templates. Many newer "modern" libraries such as boost make heavy use of templates and often are "header only." However, this should only be done when dealing with templates as it is the only way to do it when dealing with them.
EDIT: Some people would like a little more clarification, here's some thoughts on the downsides to writing "header only" code:
If you search around, you will see quite a lot of people trying to find a way to reduce compile times when dealing with boost. For example: How to reduce compilation times with Boost Asio, which is seeing a 14s compile of a single 1K file with boost included. 14s may not seem to be "exploding", but it is certainly a lot longer than typical and can add up quite quickly when dealing with a large project. Header only libraries do affect compile times in a quite measurable way. We just tolerate it because boost is so useful.
Additionally, there are many things which cannot be done in headers only (even boost has libraries you need to link to for certain parts such as threads, filesystem, etc). A Primary example is that you cannot have simple global objects in header only libs (unless you resort to the abomination that is a singleton) as you will run into multiple definition errors. NOTE: C++17's inline variables will make this particular example doable in the future.
As a final point, when using boost as an example of header only code, a huge detail often gets missed.
Boost is library, not user level code. so it doesn't change that often. In user code, if you put everything in headers, every little change will cause you to have to recompile the entire project. That's a monumental waste of time (and is not the case for libraries that don't change from compile to compile). When you split things between header/source and better yet, use forward declarations to reduce includes, you can save hours of recompiling when added up across a day.
The day C++ coders agree on The Way, lambs will lie down with lions, Palestinians will embrace Israelis, and cats and dogs will be allowed to marry.
The separation between .h and .cpp files is mostly arbitrary at this point, a vestige of compiler optimizations long past. To my eye, declarations belong in the header and definitions belong in the implementation file. But, that's just habit, not religion.
Code in headers is generally a bad idea since it forces recompilation of all files that includes the header when you change the actual code rather than the declarations. It will also slow down compilation since you'll need to parse the code in every file that includes the header.
A reason to have code in header files is that it's generally needed for the keyword inline to work properly and when using templates that's being instanced in other cpp files.
What might be informing you coworker is a notion that most C++ code should be templated to allow for maximum usability. And if it's templated, then everything will need to be in a header file, so that client code can see it and instantiate it. If it's good enough for Boost and the STL, it's good enough for us.
I don't agree with this point of view, but it may be where it's coming from.
I think your co-worker is smart and you are also correct.
The useful things I found that putting everything into the headers is that:
No need for writing & sync headers and sources.
The structure is plain and no circular dependencies force the coder to make a "better" structure.
Portable, easy to embedded to a new project.
I do agree with the compiling time problem, but I think we should notice that:
The change of source file are very likely to change the header files which leads to the whole project be recompiled again.
Compiling speed is much faster than before. And if you have a project to be built with a long time and high frequency, it may indicates that your project design has flaws. Seperate the tasks into different projects and module can avoid this problem.
Lastly I just wanna support your co-worker, just in my personal view.
Often I'll put trivial member functions into the header file, to allow them to be inlined. But to put the entire body of code there, just to be consistent with templates? That's plain nuts.
Remember: A foolish consistency is the hobgoblin of little minds.
As Tuomas said, your header should be minimal. To be complete I will expand a bit.
I personally use 4 types of files in my C++ projects:
Public:
Forwarding header: in case of templates etc, this file get the forwarding declarations that will appear in the header.
Header: this file includes the forwarding header, if any, and declare everything that I wish to be public (and defines the classes...)
Private:
Private header: this file is a header reserved for implementation, it includes the header and declares the helper functions / structures (for Pimpl for example or predicates). Skip if unnecessary.
Source file: it includes the private header (or header if no private header) and defines everything (non-template...)
Furthermore, I couple this with another rule: Do not define what you can forward declare. Though of course I am reasonable there (using Pimpl everywhere is quite a hassle).
It means that I prefer a forward declaration over an #include directive in my headers whenever I can get away with them.
Finally, I also use a visibility rule: I limit the scopes of my symbols as much as possible so that they do not pollute the outer scopes.
Putting it altogether:
// example_fwd.hpp
// Here necessary to forward declare the template class,
// you don't want people to declare them in case you wish to add
// another template symbol (with a default) later on
class MyClass;
template <class T> class MyClassT;
// example.hpp
#include "project/example_fwd.hpp"
// Those can't really be skipped
#include <string>
#include <vector>
#include "project/pimpl.hpp"
// Those can be forward declared easily
#include "project/foo_fwd.hpp"
namespace project { class Bar; }
namespace project
{
class MyClass
{
public:
struct Color // Limiting scope of enum
{
enum type { Red, Orange, Green };
};
typedef Color::type Color_t;
public:
MyClass(); // because of pimpl, I need to define the constructor
private:
struct Impl;
pimpl<Impl> mImpl; // I won't describe pimpl here :p
};
template <class T> class MyClassT: public MyClass {};
} // namespace project
// example_impl.hpp (not visible to clients)
#include "project/example.hpp"
#include "project/bar.hpp"
template <class T> void check(MyClass<T> const& c) { }
// example.cpp
#include "example_impl.hpp"
// MyClass definition
The lifesaver here is that most of the times the forward header is useless: only necessary in case of typedef or template and so is the implementation header ;)
To add more fun you can add .ipp files which contain the template implementation (that is being included in .hpp), while .hpp contains the interface.
As apart from templatized code (depending on the project this can be majority or minority of files) there is normal code and here it is better to separate the declarations and definitions. Provide also forward-declarations where needed - this may have effect on the compilation time.
Generally, when writing a new class, I will put all the code in the class, so I don't have to look in another file for it.. After everything is working, I break the body of the methods out into the cpp file, leaving the prototypes in the hpp file.
I personally do this in my header files:
// class-declaration
// inline-method-declarations
I don't like mixing the code for the methods in with the class as I find it a pain to look things up quickly.
I would not put ALL of the methods in the header file. The compiler will (normally) not be able to inline virtual methods and will (likely) only inline small methods without loops (totally depends on the compiler).
Doing the methods in the class is valid... but from a readablilty point of view I don't like it. Putting the methods in the header does mean that, when possible, they will get inlined.
I think that it's absolutely absurd to put ALL of your function definitions into the header file. Why? Because the header file is used as the PUBLIC interface to your class. It's the outside of the "black box".
When you need to look at a class to reference how to use it, you should look at the header file. The header file should give a list of what it can do (commented to describe the details of how to use each function), and it should include a list of the member variables. It SHOULD NOT include HOW each individual function is implemented, because that's a boat load of unnecessary information and only clutters the header file.
If this new way is really The Way, we might have been running into different direction in our projects.
Because we try to avoid all unnecessary things in headers. That includes avoiding header cascade. Code in headers will propably need some other header to be included, which will need another header and so on. If we are forced to use templates, we try avoid littering headers with template stuff too much.
Also we use "opaque pointer"-pattern when applicable.
With these practices we can do faster builds than most of our peers. And yes... changing code or class members will not cause huge rebuilds.
I put all the implementation out of the class definition. I want to have the doxygen comments out of the class definition.
IMHO, He has merit ONLY if he's doing templates and/or metaprogramming. There's plenty of reasons already mentioned that you limit header files to just declarations. They're just that... headers. If you want to include code, you compile it as a library and link it up.
Doesn't that really depends on the complexity of the system, and the in-house conventions?
At the moment I am working on a neural network simulator that is incredibly complex, and the accepted style that I am expected to use is:
Class definitions in classname.h
Class code in classnameCode.h
executable code in classname.cpp
This splits up the user-built simulations from the developer-built base classes, and works best in the situation.
However, I'd be surprised to see people do this in, say, a graphics application, or any other application that's purpose is not to provide users with a code base.
Template code should be in headers only. Apart from that all definitions except inlines should be in .cpp. The best argument for this would be the std library implementations which follow the same rule. You would not disagree the std lib developers would be right regarding this.
I think your co-worker is right as long as he does not enter in the process to write executable code in the header.
The right balance, I think, is to follow the path indicated by GNAT Ada where the .ads file gives a perfectly adequate interface definition of the package for its users and for its childs.
By the way Ted, have you had a look on this forum to the recent question on the Ada binding to the CLIPS library you wrote several years ago and which is no more available (relevant Web pages are now closed). Even if made to an old Clips version, this binding could be a good start example for somebody willing to use the CLIPS inference engine within an Ada 2012 program.