What are the advantages and disadvantages of separating declaration and definition as in C++?

Question

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?

Answer 1

Advantage

Classes can be referenced from other files by just including the declaration. Definitions can then be linked later on in the compilation process.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

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. ;-)

Answer 8

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.

Answer 9

1. Separation gives clean, uncluttered view of program elements.
2. Possibility to create and link to binary modules/libraries without disclosing sources.
3. Link binaries without recompiling sources.

Answer 10

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.

Answer 11

When done correctly, this separation reduces compile times when only the implementation has changed.