This question already has answers here:
Should one use forward declarations instead of includes wherever possible?
(9 answers)
Closed 6 years ago.
I am reading book about using Qt5 (author Max Schlee), and I noticed that some examples have declared existing Qt class in the header file, for example like this:
class QProgressBar;
So, my question – why don't we just include the header file QProgressBar without declaring class QProgressBar; in our header file?
It's not about Qt, it's c++.
It's called forward declaration.
Basically in the .h you just say that QProgressbar is a class and do not complain about the fact that it's not defined. Then in the .cpp file you put the header, so that at compile time, the compiler will have everything well defined.
This can save compile time, as #include force the compiler to open more files and process more input.
This can also save on unnecessary recompilation. #include can force your code to be recompiled more often, due to unrelated changes in the header.
Of course on large projects you can have drawbacks.
You can find more HERE.
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This question already has answers here:
What should go into an .h file?
(12 answers)
Closed 2 years ago.
I have searched almost every website I could, but nobody has been able to answer my question. Basically what I ask, is what exactly should I place in a header file? I know of function declarations, but are variable declarations in a .h file also a thing? I need to know what the "tradition" or the "usual method" is of usage of header files.
EDIT : If this helps, I just recently started C++ and my objective with learning this language is an opportunity to get into the industry, mainly gamedev, and I need to know what the proper way of doing stuff is. Hence the question.
Header files conventionally contain declarations of entities that you wish to share between translation units.
That usually means class definitions, type aliases, and function declarations.
A translation unit, very roughly speaking, is "a .cpp file".
So, if you use a function in a.cpp, and you also use it in b.cpp, a declaration for it needs to be available in both translation units, and the best way to do that is to write it once in a header file, then include the header file in both .cpp files.
That's it.
While it's possible to declare variables in a header file, it's rare to do so. For starters, you'd want to put extern on it to make it not a definition (you don't want multiple definitions!). At that point, you have to ask yourself why you're not using a class.
This question already has answers here:
What should go into an .h file?
(12 answers)
Closed 5 years ago.
I'm attempting to use classes in Arduino, so far its working successfully however I cannot reference a .H file more than once.
Error compiling for board... "multiple definition of `variable'"
I am basically looking to create a 'variables' class that multiple .cpp files can have access to. Any ideas on how to approach this?
This is different from "What should go into an .h file? " As its not the structure of the .H or .cpp file i necessarily need help with.
In c# I would create a class called for example Variables and then access them from any other class using Variables.Time... Any file can access this class and update the variables. However in Arduino it seems I am only able to access it from 1 file, whereas I need to access it from a library created in tabbed form and from the main .ino
Try Adding a #pragma once to the top of your .h file.
This question already has answers here:
How to make closed source for the library written with template in C++
(3 answers)
Closed 6 years ago.
C++ Compiler needs to have access to the implementation of the methods in order to instantiate a template class. Hence we need to either include the definitions of a template in the header file that declares that template or define them in header files. But I want to hide the implementation (definitions) of my functions from my users due to many reasons and want to ensure that they have only access to the function declarations. Is it possible to do so while using templates in C++?
This is usually done (for eg) in boost by creating a directory hierarchy impl and detail.
impl will usually have the implementatios for the exposed API member function while inside the detail as you might have guessed, has the gritty details which you do not want to expose. This system is fairly well understood by the C++ developers.
As an example:
my_service.hpp
impl/my_service.hpp
impl/my_service.ipp ( OR )
detail/my_service.ipp
So, the idea is to split the header files and provide the users to only include your top level header file which will internally include the other files.
But nobody is stopping the users to include the impl header file as well directly. If you want to avoid that too, then you can do something on the lines of Prevent header from being included in some files, in compilation time?
This question already has answers here:
Why have header files and .cpp files? [closed]
(9 answers)
Closed 6 years ago.
Comming from a mostly Java background I recently started learning a bit of C++.
I'm trying to create a class that describes a LinkedList. Since I want this class to be something like an external library, so I can use it in other applications, I need it to be declared as a header file.
I understand that from a design point of view C++ needs both a header and a cpp file, where the header will mostly contain declarations and the implementation will be left for the cpp file.
What I don't understand though is what is the actual purpose of the above design technique. The point of being able to quickly go through the header file and read the comments about the functionality of each method seems moot to me, since C++ (or at least my Codeblocks editor) supports the
/** comments like Java so it is much easier to create an object of the class and simply hit the . button to open the methods description menu, and read any description I like in a more orderly and structured manner than going through a header file.
Also I noticed that the program runs fine if put everything inside the header file and completly ignore the cpp file.
So my question boils down to whether I should just implement everything in the header file, or whether this is considered unacceptable C++ coding practice and if so, why.
Headers contain the bare minimum required to be able to use the functionality, since they get to some intermediate preprocessed file. The smaller that file, the easier to compile that unit.
They have to contain structure definitions and function declaration in this sense. This is unlike Java which doesn't require prior declaration in order to use and is a historical artifact which cannot be fixed now. The #include lines really paste the text in the header within the actual C/CPP file (sort of -- the preprocessing, conditional compilation etc have to happen in the headers too)
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Why can templates only be implemented in the header file??
Hi,
I have a c++ project which consists of several header and source files. In order to modify the class with a template, I found that all the method definitions must be included in the header file itself, else resulting in a link-time error. What is the reason for this and how can I get over this ??
Thank You!!
This is intentional. Templates go in header files, end of story. There is no way around it. In order for the compiler to instantiate the template it must be able to see it, which means it must be in a header file. Templates are not like C# generics, where the model allows them to be hidden an essentially instantiated by the linker - templates are much more complicated than that, so they have to live in header files.
In your travels you will find mention of the export keyword, which was supposed to solve this. To cut a long story short, it didn't, no one implemented it (apart from one very brave compiler vendor) and it will soon not be part of C++ any more.