Header files in cpp

Here is another example that includes an externally defined constructor with a member initialization list:. In the lesson on header files , you learned that you can put function declarations inside header files in order to use those functions in multiple files or even multiple projects. Classes are no different. Class definitions can be put in header files in order to facilitate reuse in multiple files or multiple projects. Traditionally, the class definition is put in a header file of the same name as the class, and the member functions defined outside of the class are put in a.

Now any other header or code file that wants to use the Date class can simply include "Date. Note that Date. Types which include classes , are exempt from the part of the one-definition rule that says you can only have one definition per program. It depends. Member functions defined inside the class definition are considered implicitly inline. Inline functions are exempt from the one definition per program part of the one-definition rule.

This means there is no problem defining trivial member functions such as access functions inside the class definition itself. Member functions defined outside the class definition are treated like normal functions, and are subject to the one definition per program part of the one-definition rule.

Therefore, those functions should be defined in a code file, not inside the header. So what should I define in the header file vs the cpp file, and what inside the class definition vs outside? In the example above, add. This allows the compiler to catch certain kinds of errors at compile time instead of link time.

For example:. Because something. If something. For another example, see this comment. If you get a compiler error indicating that add. Our use of angled brackets vs double quotes helps give the compiler a clue as to where it should look for header files. The compiler will search for the header only in the directories specified by the include directories. The compiler will first search for the header file in the current directory.

The answer is that iostream. To explain requires a short history lesson. Life was consistent, and it was good. The original version of cout and cin were declared in iostream. When the language was standardized by the ANSI committee, they decided to move all of the functionality in the standard library into the std namespace to help avoid naming conflicts with user-defined identifiers.

However, this presented a problem: if they moved all the functionality into the std namespace, none of the old programs that included iostream. To work around this issue, a new set of header files was introduced that use the same names but lack the. These new header files have all their functionality inside the std namespace.

The functionality from these libraries was also moved into the std namespace to help avoid naming collisions. When including a header file from the standard library, use the version without the. User-defined headers should still use a. Including header files from other directories.

One bad way to do this is to include a relative path to the header file you want to include as part of the include line. While this will compile assuming the files exist in those relative directories , the downside of this approach is that it requires you to reflect your directory structure in your code.

This can generally be done by setting an include path or search directory in your IDE project settings. From here, you will see a line called Include Directories.

The nice thing about this approach is that if you ever change your directory structure, you only have to change a single compiler or IDE setting instead of every code file. Headers may include other headers. Because of this, header files will often include other header files. Looks good when a header file is seen displays the list of functions declared and class variables.

There are times where header files are essential for compilation--not just an organization preference or way to distribute pre-compiled libraries. Say you have a structure where game. If you included. This is what makes most sense to me why header files are important. Hope it helps someone else. I think it has to do with the fact that only alphanumerical characters are allowed in extensions. I dont even know if that is true, just guessing — user Add a comment.

Active Oldest Votes. Where does the HPP fit in all this process? A poor lonesome CPP file HPP here, we decided to declare every symbol defined in B.

CPP include "B. Including a file will, in essence, parse and then copy-paste its content in the CPP file. For example, in the following code, with the A. CPP include "A. HPP in B. Improve this answer. I updated the answer to clarify that. But your point "It won't compile because A. While compiling A. I still don't get the necessity of headers. Seems, they're just a pretty ugly arbitrary design. Bob : While compiling A. No, it doesn't. It does only know the types provided by the user, which will, half the time, won't even bother to read the return value.

Then, implicit conversions happen. And then, when you have the code: foo bar , you can't even be sure foo is a function. So the compiler has to have access to the information in the headers to decide if the source compiles correctly, or not Then, once the code is compiled, the linker will just link together functions calls.

Bob : [continuing] Now, the linker could do the work done by the compiler, I guess, which would then make your option possible. I guess this is the subject of the "modules" proposition for the next standard. Why couldn't you just include B. CPP into A. Show 5 more comments. Not really true. The header still contains a major part of the implementation. Since when were private instance variables part of a class's interface?

Private member functions? Then what the hell are they doing in the publicly visible header? And it falls further apart with templates. That's why I said that it's not perfect, and the Pimpl idiom is needed for more separation.

Templates are a whole different can of worms - even if the "exports" keyword was fully supported in most compilers it would still me syntactic sugar rather than real separation.