Is it a good practice to place C++ definitions in header files?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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:

1. No need for writing & sync headers and sources.

2. The structure is plain and no circular dependencies force the coder to make a "better" structure.

3. Portable, easy to embedded to a new project.

I do agree with the compiling time problem, but I think we should notice that:

1. The change of source file are very likely to change the header files which leads to the whole project be recompiled again.

2. 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.

Answer 6

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.

Answer 7

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 ;)

Answer 8

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.

Answer 9

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.

Answer 10

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.

Answer 11

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.

Answer 12

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.

Answer 13

I put all the implementation out of the class definition. I want to have the doxygen comments out of the class definition.

Answer 14

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.

Answer 15

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.

Answer 16

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.

Answer 17

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.