Which is better for short functions (e.g. getters and setters)
In the class definition header file
At the end of the header file
In the source file (in this case should I use the inline keyword or extern inline?)
You can't put inline functions in the source file (and have them used as inline) because their definition won't be available at the points the compiler needs them to inline the code.
Between the other two options, I tend to put one liners into the class definition and any others at the end of the header.
In the class definition header file
typically, unless your build times are more important (assuming that's not the case, based on the question). an exception follows
At the end of the header file
rarely. i have when dealing with nasty templates, just to clean things up. the body is usually so small that it is not distracting, and this can expose implementation details (as a substitute for documentation, but i figure some people will rail me for that). example:
void compute() {
assert(this->hasEnoughEnergyToCompute());
...computing
}
this approach can be good hygiene in some cases. i've actually used secondary files for this (a fourth option).
In the source file
this option is ideal - if it's visible in every translation where you call it. otherwise, send it back to the header.
in this case should I use the inline keyword or extern inline?
just 'inline'. in fact, your compiler probably declares methods inline implicitly. omit it altogether if all your required compilers support this.
My personal preference is to place it within the header file:
class A
{
private:
int a;
public:
const int getA() const { return a; }
void setA(int val) { a = val; }
};
Just because the getX, setX functions are so tiny, they can easily fit in one line.
In general, my get and set methods typically are one line (Only on a few occasions did they require actual non-trivial code).
If you had some non-trivial code in there, like bounds checking or some extra calculations, I would advise against sticking it in the header file.
Where your coding guidelines says to put them. There's no
absolute rule, but in general:
-- it's best to avoid inline functions until the profiler tells
you you need them, and a lot of programming guidelines
forbid them, and
-- it's usually best to avoid as much as possible in the class
definition that isn't relevant to the actual interface; most
coding guidelines suggest putting the implementation at the
end of the header, or even in a separate file included from
the header.
Having said that, if the function is so very simple that it fits
on the end of the line with the function declaration, I don't
think it hurts readability that much.
Related
Based on the answer here, It is only needed to define the method inside header file in order to make it inline. So my question is, why there is inline keyword?
For example, if you want to define a free function inside a header (not a member function of a class), you will need to declare it inline, otherwise including it in multiple translation units will cause an ODR violation.
The usual approach is to declare the function in the header and define it in a separately compiled .cpp file. However, defining it in the header as an inline function allows every translation unit that includes it to have its definition available, which makes it easier for that function to actually be inlined; this is important if the function is heavily used.
The historical reason for the inline keyword is that older C compilers weren't as capable of optimising code as good quality modern C and C++ compilers are. It was therefore, originally, introduced to allow the programmer to indicate a preference to inline a function (effectively insert the function body directly into the caller, which avoids overheads associated with function calls). Since there was no guarantee that a compiler could inline the function (e.g. some compilers could only inline certain types of functions) the inline keyword was made a hint rather than a directive.
The other typical use of inline is not as discretionary - if a function is defined in multiple compilation units (e.g. because the containing header is #included more than once) then leaving off inline causes a violation of the one definition rule (and therefore in undefined behaviour). inline is an instruction to the compiler (which in turn probably emits instructions to the linker) to resolve such incidental violations of the one-definition rule, and produce a working program (e.g. without causing the linker to complain about multiple definitions).
As to why it is still needed, functionally, the preprocessor only does text substitution ... such as replacing an #include directive with contents of the included header.
Let's consider two scenarios. The first has two source files (compilation units) that do;
#include "some_header"
//some other functions needed by the program, not pertinent here
and
#include "some_header"
int main()
{
foo();
}
where some_header contains the definition
void foo() { /* whatever */ }
The second scenario, however, omits the header file (yes, people do that) and has the first source file as
void foo() { /* whatever */ }
//some other functions needed by the program, not pertinent here
and the second as;
void foo() { /* whatever */ }
int main()
{
foo();
}
In both scenarios, assume the two compilation units are compiled and linked together. The result is a violation of the one definition rule (and, practically, typically results in a linker error due to multiply defined symbols).
Under current rules of the language (specifically, the preprocessor only doing text substitution), the two scenarios are required to be EXACTLY functionally equivalent. If one scenario was to print the value 42, the so should the other. If one has undefined behaviour (as is the case here) so does the other.
But, let's say for sake of discussion, that there was a requirement that a function be magically inlined if it is defined in a header. The code in the two scenarios would no longer be equivalent. The header file version would have defined behaviour (no violation of the one definition rule) and the second would have undefined behaviour.
Oops. We've just broken equivalence of the two scenarios. That may not seem much, but programmers would practically have trouble understanding why one version links and the other doesn't. And they would have have no way of fixing that ... other than moving code into a header file.
That means, we need some way to make the two scenarios equivalent. This means there needs to be something in the code which makes them equivalent. Enter the inline keyword, and prefix it to the definitions of foo().
Now, okay, one might argue that the preprocessor should do something a bit more intelligent i.e. do more than simple text substitution. But, now you are on a slippery slope. The C and C++ standards do (at length) specify that the preprocessor does that. And changing that would introduce a cascade of other changes. Whether that is a good idea or not (and, certainly, there is some advocacy for eliminating the preprocessor from C++ entirely), that is a much bigger change, with numerous ramifications on the language, and on programmers (who, whether it is good or bad, can rely on the preprocessor behaving as it does).
Short answer. It's not required. Compilers generally ignore the inline keyword.
More comprehensive answers are already given in other questions, not to mention the second answer in the question you linked.
Why is it a bad practice to define the functions of the class in the header files?
Lets say I have a header file and I define the functions of the class in the class definition itself like,
headerfile.hpp
#ifndef _HEADER_FILE_
#define _HEADER_FILE_
class node{
int i;
public:
int nextn(){
......
return i;
}
}
#endif //_HEADER_FILE_
So defining the function in the class like this makes the function "Inline".So if we include this header file in say two .cpp files, will it cause "Multiple definition error" ??Is it a bad practice to define the functions like this in the class definition?
It is a bad practice for the following reasons: If you need to change the code, let's say to add a trace in a simple setter (they are commonly in the .h); then you will need to recompile all CPP files that #includes the change (and any dependency of). In my current project that could reach up to 1 hour lost. If you later need to add another trace, then another and so on you quickly loose 1-2 days or work waiting for the compiler.
If you place your code in the CPP, then you only need to re-link, and that takes only a few minutes. Your project may be small today, but who knows in a few years. It's just a good habit to take.
Another (not so good) reason is that if you search your code base for the string "::MyFonction" you will not find it in the declaration since there is no "::" (we only want implementations). But a good IDE should find it anyway using a context search instead of a string search.
It's not bad practise (in fact it's commonplace) and it will not cause multiple definition errors. Inline functions never cause multiple definition errors, that's one of the meanings of inline.
The convention to separate prototypes (that is, the declaration of the class, its functions, their types) from implementation comes from both a design and a performance point of view.
Type checking and compiling your dependants is cheaper. Something that uses your class can be safely compiled without knowing your implementation.
Your compiler won't need to parse and recompile the same information lots of times each time you do compile those dependants.
The thing is to remember what it really means with you write #include at the top of a file in C++: it means "take all the contents of some other file, and put them here." So if you're using a class in lots of places all over your code base, then it's getting parsed every single time, and re-compiled in the context of that compilation unit.
This is precisely the reason why you have to put implementations of template classes in-line in the header file; the compiler needs to re-parse and compile the class for every different template instantiation (because that's what templates are about).
To answer your question directly:
* No, you will not get a multiple definition error.
* Maybe, some people would consider it back practice from a design points of view (others wouldn't)
* You might see a difference in performance (though not necessarily a degredation, as I believe - though I could be wrong), that despite the above, it can still be faster to compile header-only libraries.
Probably avoid doing this if your implementations are long, the class is used often in the codebase, and will be subject to frequent change.
For further reading, it might be worth checking up on "precompiled headers."
It is legal to define (inline) functions in your hpp file. Note that some people prefer to gather then under a dedicated extension like "inl.hpp", but this is just a style preference.
I know what it means when static function is declared in source file. I am reading some code, found that static function in header files could be invoke in other files.
Is the function defined in the header file? So that the actual code is given directly in the function, like this:
static int addTwo(int x)
{
return x + 2;
}
Then that's just a way of providing a useful function to many different C files. Each C file that includes the header will get its own definition that it can call. This of course wastes memory, and is (in my opinion) a quite ugly thing to be doing, since having executable code in a header is generally not a good idea.
Remember that #include:ing a header basically just pastes the contents of the header (and any other headers included by it) into the C file as seen by the compiler. The compiler never knows that the one particular function definition came from a header file.
UPDATE: In many cases, it's actually a good idea to do something like the above, and I realize my answer sounds very black-and-white about this which is kind of oversimplifying things a bit. For instance, code that models (or just uses) intrinsic functions can be expressed like the above, and with an explicit inline keyword even:
static inline int addTwo(int *x)
{
__add_two_superquickly(x);
}
Here, the __add_two_superquickly() function is a fictional intrinsic, and since we want the entire function to basically compile down to a single instruction, we really want it to be inlined. Still, the above is cleaner than using a macro.
The advantage over just using the intrinsic directly is of course that wrapping it in another layer of abstraction makes it possible to build the code on compilers lacking that particular intrinsic, by providing an alternate implementation and picking the right one depending on which compiler is being used.
It will effectively create a separate static function with the same name inside every cpp file it is included into. The same applies to global variables.
As others are saying, it has exactly the same meaning as a static function in the .c file itself. This is because there is no semantic difference between .c and .h files; there is only the compilation unit made up of the file actually passed to the compiler (usually named .c) with the contents of any and all files named in #include lines (usually named .h) inserted into the stream as they are seen by the preprocessor.
The convention that the C source is in a file named .c and public declarations are in files named .h is only a convention. But it is generally a good one. Under that convention, the only things that should appear in .h files are declarations so that you generally avoid having the same symbol defined more than once in a single program.
In this particular case, the static keyword makes the symbol be private to the module, so there isn't a multiple-definition conflict waiting to cause trouble. So in that one sense, it is safe to do. But in the absence of a guarantee that the function would be inlined, you take the risk that the function would be instantiated in every module that happened to #include that header file which at best is a waste of memory in the code segment.
I am not certain of what use cases would justify doing this at all in a generally available public header.
If the .h file is generated code and only included in a single .c file, then I would personally name the file something other than .h to emphasize that it isn't actually a public header at all. For example, a utility that converts a binary file into an initialized variable definition might write a file that is intended to be used via #include and could very well contain a static declaration of the variable, and possibly even static definitions of accessor or other related utility functions.
If you define the function in a header file (not simply declare it), a copy of the function will be generated in each translation unit (basically in each cpp file which includes this header).
This may increase the size of your executable, but this may be negligible if the function is small. The advantage is that the most compilers may inline the function, which may increase the code performance.
But there may be a big difference in doing this which wasn't mentioned in any answer. If your function uses a static local variable such as:
static int counter()
{
static int ctr = 0;
return ctr++;
}
Rather than:
//header
int counter();
//source
int counter()
{
static int ctr = 0;
return ctr++;
}
Then each source file including this header will have its own counter. If the function is declared inside the header, and defined in a source file, then the counter will be shared across your whole program.
So saying that the only difference will be performance and code size is wrong.
There is not semantic difference in defining in source file or header file, basically both means the same in plain C when using static keyword that, you are limiting the scope.
However, there is a problem in writing this in header file, this is because every time you include the header in a source file you'll have a copy of the function with same implementation which is much similar to have a normal function defined in header file. By adding the definition in header you are not achieving the what the static function is meant for.
Therefore, I suggest you should have your implementation only in your source file and not in header.
It is usefull in some "header-only" libraries with small inline functions. In a such case you always want to make a copy of the function so this is not a bad pattern. However, this gives you an easy way to insert separate interface and implementation parts in the single header file:
// header.h
// interface part (for user?!)
static inline float av(float a, float b);
// implementation part (for developer)
static inline float av(float a, float b)
{
return (a+b)/2.f;
}
Apple vector math library in GLK framework uses such constuction (e.g. GLKMatrix4.h).
I tend to put the body of smaller functions right into the header file foo.h, not into the separate implementation file foo.cc, because then the compiler can always see those functions completely and decide to inline them.
Example:
// File "foo.h":
struct Foo {
// ...
int GetCount() const { return count_; }
};
Now any code that uses Foo sees the whole GetCount implementation and can decide to inline it.
Is that good practice? If so, do you put slightly larger function definitions into the header file as well?
Modern compilers have link time optimization, so it is not necessary anymore to have the code available in the header for a function to get inlined. Hence I would say it would be a better practice to just mark the declaration as inline, but put the definition in the cpp.
It's good practice, yes. I think, practically speaking, for this scenario, as long as you understand the behaviors, then you can feel free to inline your functions as you wish. Note that the compiler will make the ultimate decision about inlining though.
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.