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where should “include” be put in C++
Obviously, there are two "schools of thought" as to whether to put #include directives into C++ header files (or, as an alternative, put #include only into cpp files). Some people say it's ok, others say it only causes problems. Does anybody know whether this discussion has reached a conclusion what is to be preferred?
I am not aware of any schools of thoughts concerning this. Put them in the header when they are needed there, otherwise forward declare and put them in the .cpp files that require them. There is no benefit in including headers where they are not needed.
What I found effective is following a few simple rules:
Headers shall be self-sufficient, i.e., they shall declare classes they need names for and include headers for any definition they use.
Headers should minimize dependencies as much as possible without violation the previous point.
Getting the first point rught is fairly easy: Include the header first thing from the source implementing what it declares. Getting the second point exactly right isn't trivial, though, and I think it requires tool support to get it exactly right. However, a few unnecessary dependencies generally aren't that bad.
As a rule of thumb, you don't include the headers in a header as long as full definition of them is necessary there. Most of the time you play around with pointers of classes in a header file so it's just fine to forward declare them there.
I think the issue was settle a long time ago: headers should be self-contained (that is should not depend on the user to have included other headers before -- that aspect is settle for so long that some aren't even aware there was a debate on this, but your put includes only in .cpp seems to hint at this) but minimal (i.e. should not include definitions when a declaration would be enough for self-containment).
The reason for self-containment is maintenance: should an header be modified and now depend on something new, you'd have to track all the place it is used to include the new dependency. BTW, the standard trick to ensure self-containment is to include the header providing the declarations for things defined in a .cpp first in the .cpp.
These are not schools of thought so much as religions. In reality, both approaches have their advantages and disadvantages, and there are certain practices to be followed for either approach to be successful. But only one of these approaches will "scale" to large projects.
The advantage of not including headers inside headers is faster compilation. However, this advantage does not come from headers being read only once, because even if you include headers inside headers, smart compilers can work that out. The speed advantage comes from the fact that you include only those headers which are strictly necessary for a given source file. Another advantage is that if we look at a source file, we can see exactly what its dependencies are: the flat list of header files gives that to us plainly.
However, this practice is hard to maintain, especially in large projects with many programmers. It's quite an inconvenience when you want to use module foo, but you cannot just #include "foo.h": you need to include 35 other headers.
What ends up happening is this: programmers are not going to waste their time discovering the exact, minimal set of headers that they need just to add module foo. To save time, they will go to some example source file similar to the one they are working on, and cut and paste all of the #include directives. Then they will try compiling it, and if it doesn't build, then they will cut and paste more #include directives from yet elsewhere, and repeat that until it works.
The net result is that, little by little, you lose the advantage of faster compiling, because your files are now including unnecessary headers. Moreover, the list of #include directives no longer shows the true dependencies. Moreover, when you do incremental compiles now, you compile more than is necessary due to these false dependencies.
Once every source file includes nearly every header, you might as well have a big everything.h which includes all the headers, and then #include "everything.h" in every source file.
So this practice of including just specific headers is best left to small projects that are carefully maintained by a handful of developers who have plenty of time to maintain the ethic of minimal include dependencies by hand, or write tools to hunt down unnecessary #include directives.
Lately I have started to put more and more functions into header files, mostly for convenience. But I fear I might be overdoing it, my headers are full of includes and I'm not sure if that's a good idea.
What are your rules of thumb for moving functions out of or into header files?
In case you're wondering, I'm talking about developing applications, not libraries.
Edit:
I guess it's helpful if I outline the pros/cons of inline (naturally) header functions versus implementation functions from my point of view:
Pro inline:
More clean/concise.
No need for signature duplication.
No need to change any Makefile to link against new files.
Instant ability to introduce template parameters.
Contra inline:
Increased compile time (I don't care that much)
Many includes in headers (Shouldn't be such a big issue if they use guards)
According to that, it seems like a good idea to put pretty much every function in headers, and I believe that's pretty close to what the STL and Boost are doing (although those are libraries, as opposed to my code).
One of my most inviolable rules: only function bodies which are inline are allowed in header files. Anything else is asking for trouble with multiple definitions in the link phase.
Headers should be left predominantly for declarations rather than definitions. I have exceptions to that rule (being the flexible type) but none of them involve non-inlined function bodies.
My rule of thumb is "Not in the header, unless you have to." And as for convenience, do you find increased compilation times convenient?
There are a few obvious technical aspects
- templates and inline functions must be in headers
- headers included from multiple translation units must be wary of the One Definition Rule
- more bluntly, you'd want a bloody good reason to even consider putting an out-of-line function implementation in a header, and I can't think of any times I've even been tempted.
So, the question boils down to:
inline in header versus out-of-line in implementation file?
Factors:
you say you're designing application level code not libraries, so you don't (currently) have to worry about other teams getting dependent on your code, nor minimise their need to recompile (versus just relink) by keeping implementation out of line
BUT if you're writing good code that has any potential to become useful to other teams, then you might find yourself wishing you'd kept implementation private
inline versus out-of-line typically represents about an order-of-magnitude overhead for trivial data get/set functions... if you have functions that are called repeatedly from performance critical code, then you've reason to prefer inlining
in-header implementation (especially if intermingled with the declarations) can often obfuscate the API, but sometimes actually makes the code more self-documenting
localisation and removed redundancy (of combining declaration/definitions) definitely removes potential for typos/errors and can often improve productivity
Bottom line: if you're finding yourself doing it more and more, then it's obviously working for you and there's no particular reason to think you're about to get burnt. Keep an eye out for the potential issues but don't over-engineer the heck out of stuff based on some hypothetical and unlikely-to-materialise concern.
A good coding standard will tell you to implement methods and functions in the source (cpp) file.
If you prefer it, you can implement templates and inline functions in the header.
Since this has been tagged as C++, why don't you seperate them into logical classes?
Normally I have one class declaration in a header file and it's definition in the corresponding source file.
The two rules I use are
1) If it's an inline functions
2) If it's a template function.
First, template function must be put in headers.
Besides, functions with an empty body, such as a default constructor or default but virtual destructor may be put in headers.
I never use inline because compiler don't guarantee that.
If I am creating a class with small functions that don't do much, is it acceptable to just put them all into the header file? So for a particular class, it's only just the .h with no .cpp to go with it.
Yes, that's acceptable. It will certainly compile. But also, if it makes the code organization cleaner, then that can be good. Most template definitions are already like this out of necessity, so you aren't doing anything unheard of. There can be some drawbacks of that class relies on other classes though. If you end up having to include the whole definition in other files that use the class, it may incur additional compile time compared to only having a brief class declaration.
You can measure your compilation times if this seems to be a real concern.
If you can get a copy, The c++ Programming Language (and many other books) have a detailed section about source code organization and the specific benefits of separating code into .h and .cpp files.
It depends what you are aiming for.
For a pet project, it's acceptable, but then anything is really.
For a real project, you need to ask yourself a number of questions:
do you have few dependencies ? (yes)
is your logic / implementation likely to change often ? (no, or see next question)
how much depends on this class ? (not much or much but it won't change)
If the responses satisfy you, then go ahead.
It is mainly a matter of dependency management. By putting the methods definition within the header, they are likely to get inlined by the compiler, which means that each time they change you'll have to recompile everything that depends on this class.
Templates do so, however templates generally have few dependencies (other includes) and you're relatively forced to proceed like so (though you can externalized non-template dependent code to cut down on dependencies).
For real big projects where dependency management is foremost, then I would advise against. In those projects a stable ABI and the ability to promote binary compatible changes are life-savers in case of issue and they are well worth the "slight" inconvenience for the developer.
In any case, please don't define the methods in the midst of the class. Even inlined you can define them after the class declaration. This makes it easier for people reading it (yourself in a few months) to get a grasp of the interface quickly.
Yes, it is acceptable. Moreover, if you do templates and you don't have a compiler with export support then you don't have a choice.
However, note that it can increase dependencies and thus make compilation slower.
It would be fine if you will use that header file in your future codes or projects, and if you want to share it with the others.
Depends on how you're linking. To avoid having to see messages such as redefinition of blah, previous definition in file blah.h on line 13 just put everything but the declaration in a .cpp.
Is there any way to not have to write function declarations twice (headers) and still retain the same scalability in compiling, clarity in debugging, and flexibility in design when programming in C++?
Use Lzz. It takes a single file and automatically creates a .h and .cpp for you with all the declarations/definitions in the right place.
Lzz is really very powerful, and handles 99% of full C++ syntax, including templates, specializations etc etc etc.
Update 150120:
Newer C++ '11/14 syntax can only be used within Lzz function bodies.
I felt the same way when I started writing C, so I also looked into this. The answer is that yes, it's possible and no, you don't want to.
First with the yes.
In GCC, you can do this:
// foo.cph
void foo();
#if __INCLUDE_LEVEL__ == 0
void foo() {
printf("Hello World!\n");
}
#endif
This has the intended effect: you combine both header and source into one file that can both be included and linked.
Then with the no:
This only works if the compiler has access to the entire source. You can't use this trick when writing a library that you want to distribute but keep closed-source. Either you distribute the full .cph file, or you have to write a separate .h file to go with your .lib. Although maybe you could auto-generate it with the macro preprocessor. It would get hairy though.
And reason #2 why you don't want this, and that's probably the best one: compilation speed. Normally, C sources files only have to be recompiled when the file itself changes, or any of the files it includes changes.
The C file can change frequently, but the change only involves recompiling the one file that changed.
Header files define interfaces, so they shouldn't change as often. When they do however, they trigger a recompile of every source file that includes them.
When all your files are combined header and source files, every change will trigger a recompile of all source files. C++ isn't known for its fast compile times even now, imagine what would happen when the entire project had to be recompiled every time. Then extrapolate that to a project of hundreds of source files with complicated dependencies...
Sorry, but there's no such thing as a "best practice" for eliminating headers in C++: it's a bad idea, period. If you hate them that much, you have three choices:
Become intimately familiar with C++ internals and any compilers you're using; you're going to run into different problems than the average C++ developer, and you'll probably need to solve them without a lot of help.
Pick a language you can use "right" without getting depressed
Get a tool to generate them for you; you'll still have headers, but you save some typing effort
In his article Simple Support for Design by Contract in C++, Pedro Guerreiro stated:
Usually, a C++ class comes in two
files: the header file and the
definition file. Where should we write
the assertions: in the header file,
because assertions are specification?
Or in the definition file, since they
are executable? Or in both, running
the risk of inconsistency (and
duplicating work)? We recommend,
instead, that we forsake the
traditional style, and do away with
the definition file, using only the
header file, as if all functions were
defined inline, very much like Java
and Eiffel do.
This is such a drastic
change from the C++ normality that it
risks killing the endeavor at the
outset. On the other hand, maintaining
two files for each class is so
awkward, that sooner or later a C++
development environment will come up
that hides that from us, allowing us
to concentrate on our classes, without
having to worry about where they are
stored.
That was 2001. I agreed. It is 2009 now and still no "development environment that hides that from us, allowing us to concentrate on our classes" has come up. Instead, long compile times are the norm.
Note: The link above seems to be dead now. This is the full reference to the publication, as it appears in the Publications section of the author's website:
Pedro Guerreiro, Simple Support for Design by Contract in C++, TOOLS USA 2001, Proceedings, pages 24-34, IEEE, 2001.
There is no practical way to get around headers. The only thing you could do is to put all code into one big c++ file. That will end up in an umaintainable mess, so please don't do it.
At the moment C++ header-files are a nessesary evil. I don't like them, but there is no way around them. I'd love to see some improvements and fresh ideas on the problem though.
Btw - once you've got used to it it's not that bad anymore.. C++ (and any other language as well) has more anoying things.
What I have seen some people like you do is write everything in the headers. That gives your desired property of only having to write the method profiles once.
Personally I think there are very good reasons why it is better to separate declaration and definition, but if this distresses you there is a way to do what you want.
There's header file generation software. I've never used it, but it might be worth looking into. For instance, check out mkhdr! It supposedly scans C and C++ files and generates the appropriate header files.
(However, as Richard points out, this seems to limit you from using certain C++ functionality. See Richard's answer instead here right in this thread.)
You have to write function declaration twice, actually (once in header file, once in implementation file). The definition (AKA implementation) of the function will be written once, in the implementation file.
You can write all the code in header files (it is actually a very used practice in generic programming in C++), but this implies that every C/CPP file including that header will imply recompilation of the implementation from those header files.
If you are thinking to a system similar to C# or Java, it is not possible in C++.
Nobody has mentioned Visual-Assist X under Visual Studio 2012 yet.
It has a bunch of menus and hotkeys that you can use to ease the pain of maintaining headers:
"Create Declaration" copies the function declaration from the current function into the .hpp file.
"Refactor..Change signature" allows you to simultaneously update the .cpp and .h file with one command.
Alt-O allows you to instantly flip between .cpp and .h file.
C++ 20 modules solve this problem. There is no need for copy-pasting anymore! Just write your code in a single file and export things using "export".
export module mymodule;
export int myfunc() {
return 1
}
Read more about modules here: https://en.cppreference.com/w/cpp/language/modules
At the time of writing this answer (2022 Feb), these compilers support it:
See here for the supported compilers:
https://en.cppreference.com/w/cpp/compiler_support
See this answer if you want to use modules with CMake:
https://stackoverflow.com/a/71119196/7910299
Actually... You can write the entire implementation in a file. Templated classes are all defined in the header file with no cpp file.
You can also save then with whatever extensions you want. Then in #include statements, you would include your file.
/* mycode.cpp */
#pragma once
#include <iostreams.h>
class myclass {
public:
myclass();
dothing();
};
myclass::myclass() { }
myclass::dothing()
{
// code
}
Then in another file
/* myothercode.cpp */
#pragma once
#include "mycode.cpp"
int main() {
myclass A;
A.dothing();
return 0;
}
You may need to setup some build rules, but it should work.
You can avoid headers. Completely. But I don't recommend it.
You'll be faced with some very specific limitations. One of them is you won't be able to have circular references (you won't be able to have class Parent contain a pointer to an instance of class ChildNode, and class ChildNode also contain a pointer to an instance of class Parent. It'd have to be one or the other.)
There are other limitations which just end up making your code really weird. Stick to headers. You'll learn to actually like them (since they provide a nice quick synopsis of what a class can do).
To offer a variant on the popular answer of rix0rrr:
// foo.cph
#define INCLUDEMODE
#include "foo.cph"
#include "other.cph"
#undef INCLUDEMODE
void foo()
#if !defined(INCLUDEMODE)
{
printf("Hello World!\n");
}
#else
;
#endif
void bar()
#if !defined(INCLUDEMODE)
{
foo();
}
#else
;
#endif
I do not recommend this, bit I think this construction demonstrates the removal of content repetition at the cost of rote repetition. I guess it makes copy-pasta easier? That's not really a virtue.
As with all the other tricks of this nature, a modification to the body of a function will still require recompilation of all files including the file containing that function. Very careful automated tools can partially avoid this, but they would still have to parse the source file to check, and be carefully constructed to not rewrite their output if it's no different.
For other readers: I spent a few minutes trying to figure out include guards in this format, but didn't come up with anything good. Comments?
I understand your problems. I would say that the C++ main problem is the compilation/build method that it inherited from the C. The C/C++ header structure has been designed in times when coding involved less definitions and more implementations. Don't throw bottles on me, but that's how it looks like.
Since then the OOP has conquered the world and the world is more about definitions then implementations. As the result, including headers makes pretty painful to work with a language where the fundamental collections such as the ones in the STL made with templates which are notoriously difficult job for the compiler to deal with. All those magic with the precompiled headers doesn't help so much when it comes to TDD, refactoring tools, the general development environment.
Of course C programmers are not suffering from this too much since they don't have compiler-heavy header files and so they are happy with the pretty straightforward, low-level compilation tool chain. With C++ this is a history of suffering: endless forward declarations, precompiled headers, external parsers, custom preprocessors etc.
Many people, however, does not realize that the C++ is the ONLY language that has strong and modern solutions for high- and low-level problems. It's easy to say that you should go for an other language with proper reflection and build system, but it is non-sense that we have to sacrifice the low-level programming solutions with that and we need to complicate things with low-level language mixed with some virtual-machine/JIT based solution.
I have this idea for some time now, that it would be the most cool thing on earth to have a "unit" based c++ tool-chain, similar to that in D. The problem comes up with the cross-platform part: the object files are able to store any information, no problem with that, but since on windows the object file's structure is different that of the ELF, it would be pain in the ass to implement a cross-platform solution to store and process the half-way-compilation units.
After reading all the other answers, I find it missing that there is ongoing work to add support for modules in the C++ standard. It will not make it to C++0x, but the intention is that it will be tackled in a later Technical Review (rather than waiting for a new standard, that will take ages).
The proposal that was being discussed is N2073.
The bad part of it is that you will not get that, not even with the newest c++0x compilers. You will have to wait. In the mean time, you will have to compromise between the uniqueness of definitions in header-only libraries and the cost of compilation.
As far as I know, no. Headers are an inherent part of C++ as a language. Don't forget that forward declaration allows the compiler to merely include a function pointer to a compiled object/function without having to include the whole function (which you can get around by declaring a function inline (if the compiler feels like it).
If you really, really, really hate making headers, write a perl-script to autogenerate them, instead. I'm not sure I'd recommend it though.
It's completely possible to develop without header files. One can include a source file directly:
#include "MyModule.c"
The major issue with this is one of circular dependencies (ie: in C you must declare a function before calling it). This is not an issue if you design your code completely top-down, but it can take some time to wrap ones head around this sort of design pattern if you're not used to it.
If you absolutely must have circular dependencies, one may want to consider creating a file specifically for declarations and including it before everything else. This is a little inconvenient, but still less pollution than having a header for every C file.
I am currently developing using this method for one of my major projects. Here is a breakdown of advantages I've experienced:
Much less file pollution in your source tree.
Faster build times. (Only one object file is produced by the compiler, main.o)
Simpler make files. (Only one object file is produced by the compiler, main.o)
No need to "make clean". Every build is "clean".
Less boiler plate code. Less code = less potential bugs.
I've discovered that Gish (a game by Cryptic Sea, Edmund McMillen) used a variation on this technique inside its own source code.
You can carefully lay out your functions so that all of the dependent functions are compiled after their dependencies, but as Nils implied, that is not practical.
Catalin (forgive the missing diacritical marks) also suggested a more practical alternative of defining your methods in the header files. This can actually work in most cases.. especially if you have guards in your header files to make sure they are only included once.
I personally think that header files + declaring functions is much more desirable for 'getting your head around' new code, but that is a personal preference I suppose...
You can do without headers. But, why spend effort trying to avoid carefully worked out best practices that have been developed over many years by experts.
When I wrote basic, I quite liked line numbers. But, I wouldn't think of trying to jam them into C++, because that's not the C++ way. The same goes for headers... and I'm sure other answers explain all the reasoning.
For practical purposes no, it's not possible. Technically, yes, you can. But, frankly, it's an abuse of the language, and you should adapt to the language. Or move to something like C#.
It is best practice to use the header files, and after a while it will grow into you.
I agree that having only one file is easier, but It also can leed to bad codeing.
some of these things, althoug feel awkward, allow you to get more then meets the eye.
as an example think about pointers, passing parameters by value/by reference... etc.
for me the header files allow-me to keep my projects properly structured
Learn to recognize that header files are a good thing. They separate how codes appears to another user from the implementation of how it actually performs its operations.
When I use someone's code I do now want to have to wade through all of the implementation to see what the methods are on a class. I care about what the code does, not how it does it.
This has been "revived" thanks to a duplicate...
In any case, the concept of a header is a worthy one, i.e. separate out the interface from the implementation detail. The header outlines how you use a class / method, and not how it does it.
The downside is the detail within headers and all the workarounds necessary. These are the main issues as I see them:
dependency generation. When a header is modified, any source file that includes this header requires recompilation. The issue is of course working out which source files actually use it. When a "clean" build is performed it is often necessary to cache the information in some kind of dependency tree for later.
include guards. Ok, we all know how to write these but in a perfect system it would not be necessary.
private details. Within a class, you must put the private details into the header. Yes, the compiler needs to know the "size" of the class, but in a perfect system it would be able to bind this in a later phase. This leads to all kinds of workaround like pImpl and using abstract base classes even when you only have one implementation just because you want to hide a dependency.
The perfect system would work with
separate class definition and declaration
A clear bind between these two so the compiler would know where a class declaration and its definition are, and would know what the size of a class.
You declare using class rather than pre-processor #include. The compiler knows where to find a class. Once you have done "using class" you can use that class name without qualifying it.
I'd be interested to know how D does it.
With regards to whether you can use C++ without headers, I would say no you need them for abstract base classes and standard library. Aside from that you could get by without them, although you probably would not want to.
Can I write C++ code without headers
Read more about C++, e.g. the Programming using C++ book then the C+11 standard n3337.
Yes, because the preprocessor is (conceptually) generating code without headers.
If your C++ compiler is GCC and you are compiling your translation unit foo.cc consider running g++ -O -Wall -Wextra -C -E foo.cc > foo.ii; the emitted file foo.ii does not contain any preprocessor directive, and could be compiled with g++ -O foo.ii -o foo-bin into a foo-bin executable (at least on Linux). See also Advanced Linux Programming
On Linux, the following C++ file
// file ex.cc
extern "C" long write(int fd, const void *buf, size_t count);
extern "C" long strlen(const char*);
extern "C" void perror(const char*);
int main (int argc, char**argv)
{
if (argc>1)
write(1, argv[1], strlen(argv[1]);
else
write(1, __FILE__ " has no argument",
sizeof(__FILE__ " has no argument"));
if (write(1, "\n", 1) <= 0) {
perror(__FILE__);
return 1;
}
return 0;
}
could be compiled using GCC as g++ ex.cc -O ex-bin into an executable ex-bin which, when executed, would show something.
In some cases, it is worthwhile to generate some C++ code with another program
(perhaps SWIG, ANTLR, Bison, RefPerSys, GPP, or your own C++ code generator) and configure your build automation tool (e.g. ninja-build or GNU make) to handle such a situation. Notice that the source code of GCC 10 has a dozen of C++ code generators.
With GCC, you might sometimes consider writing your own GCC plugin to analyze your (or others) C++ code (e.g. at the GIMPLE level). See also (in fall 2020) CHARIOT and DECODER European projects. You could also consider using the Clang static analyzer or Frama-C++.
Historically hearder files have been used for two reasons.
To provides symbols when compiling a program that wants to used a
library or a additional file.
To hide part of the implementing; keep things private.
For example say you have a function you don't want exposed to other
parts of your program, but want to use in your implementation. In that
case, you would write the function in the CPP file, but leave it out
of the header file. You can do this with variables and anything that
would want to keep private in the impregnation that you don't want
exposed to conumbers of that source code. In other programming
lanugases there is a "public" keyword that allows module parts to be
kept from being exposed to other parts of your program. In C and C++
no such facility exists at afile level, so header files are used
intead.
Header files are not perfect. Useing '#include' just copies the contents
of what ever file you provide. Single quotes for the current working
tree and < and > for system installed headers. In CPP for system
installed std components the '.h' is omitted; just another way C++
likes to do its own thing. If you want to give '#include' any kind of
file, it will be included. It really isn't a module system like Java,
Python, and most other programming lanuages have. Since headers are
not modules some extra steps need to be taken to get similar function
out of them. The Prepossesser (the thing that works with all the
#keywords) will blindly include what every you state is needed to be
consumed in that file, but C or C++ want to have your symbals or
implications defined only one in compilation. If you use a library, no
it main.cpp, but in the two files that main includes, then you only
want that library included once and not twice. Standard Library
components are handled special, so you don't need to worry about using
the same C++ include everywhere. To make it so that the first time the
Prepossesser sees your library it doesn't include it again, you need
to use a heard guard.
A heard guard is the simplest thing. It looks like this:
#ifndef LIBRARY_H
#define LIBRARY_H
// Write your definitions here.
#endif
It is considered good to comment the ifndef like this:
#endif // LIBRARY_H
But if you don't do the comment the compiler wont care and it wont
hurt anthing.
All #ifndef is doing is checking whether LIBRARY_H is equal to 0;
undefined. When LIBRARY_H is 0, it provides what comes before the
#endif.
Then #define LIBRARY_H sets LIBRARY_H to 1, so the next time the
Preprocessor sees #ifndef LIBRARY_H, it wont provide the same contents
again.
(LIBRARY_H should be what ever the file name is and then _ and the
extension. This is not going break anything if you don't write the
same thing, but you should be consistent. At least put the file name
for the #ifndef. Otherwise it might get confusing what guards are for
what.)
Really nothing fancy going on here.
Now you don't want to use header files.
Great, say you don't care about:
Having things private by excluding them from header files
You don't intend to used this code in a library. If you ever do, it
may be easier to go with headers now so you don't have to reorganise
your code into headers later.
You don't want to repeat yourself once in a header file and then in
a C++ file.
The purpose of hearder files can seem ambiguous and if you don't care
about people telling out it's wrong for imaginary reasons, then save
your hands and don't bother repeating yourself.
How to include only hearder files
Do
#ifndef THING_CPP
#define THING_CPP
#include <iostream>
void drink_me() {
std::cout << "Drink me!" << std::endl;
}
#endif // THING_CPP
for thing.cpp.
And for main.cpp do
#include "thing.cpp"
int main() {
drink_me();
return 0;
}
then compile.
Basically just name your included CPP file with the CPP extension and
then treat it like a header file but write out the implementations in
that one file.