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h file is a header, where you only declare your functions, classes, variables (no implementation - just empty declarations). . cpp file is a place, where you define those previously declared symbols - methods, static variables, other functions.
Cpp files don't always have to have a header file associated with it but it usually does as the header file acts like a bridge between cpp files so each cpp file can use code from another cpp file. One thing that should be strongly enforced is the no use of code within a header file!
Files with CPP file extension are source code files for applications written in C++ programming language. A single C++ project may contain more than one CPP files as application source code.
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 . cpp file of the same name as the class.
The C++ build system (compiler) knows no difference, so it's all one of conventions.
The convention is that .h files are declarations, and .cpp files are definitions.
That's why .h files are #included -- we include the declarations.
The .cpp file is the compilation unit : it's the real source code file that will be compiled (in C++).
The .h (header) files are files that will be virtually copy/pasted in the .cpp files where the #include precompiler instruction appears. Once the headers code is inserted in the .cpp code, the compilation of the .cpp can start.
.h files, or header files, are used to list the publicly accessible instance variables and and methods in the class declaration. .cpp files, or implementation files, are used to actually implement those methods and use those instance variables.
The reason they are separate is because .h files aren't compiled into binary code while .cpp files are. Take a library, for example. Say you are the author and you don't want it to be open source. So you distribute the compiled binary library and the header files to your customers. That allows them to easily see all the information about your library's classes they can use without being able to see how you implemented those methods. They are more for the people using your code rather than the compiler. As was said before: it's the convention.
A header (.h, .hpp, ...) file contains
class X { ... }; )inline int get_cpus() { ... } )void help(); )extern int debug_enabled; )A source file (.c, .cpp, .cxx) contains
void help() { ... } or void X::f() { ... } )int debug_enabled = 1; )However, the convention that headers are named with a .h suffix and source files are named with a .cpp suffix is not really required. One can always tell a good compiler how to treat some file, irrespective of its file-name suffix ( -x <file-type> for gcc. Like -x c++ ).
Source files will contain definitions that must be present only once in the whole program. So if you include a source file somewhere and then link the result of compilation of that file and then the one of the source file itself together, then of course you will get linker errors, because you have those definitions now appear twice: Once in the included source file, and then in the file that included it. That's why you had problems with including the .cpp file.
I know the difference between a declaration and a definition.
Whereas:
Therefore any definitions in a header file should be inline or static. Header files also contain declarations which are used by more than one CPP file.
Definitions that are neither static nor inline are placed in CPP files. Also, any declarations that are only needed within one CPP file are often placed within that CPP file itself, nstead of in any (sharable) header file.
A good rule of thumb is ".h files should have declarations [potentially] used by multiple source files, but no code that gets run."
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