CPP_PRIMER.md

C++ Primer

Header Files (.hpp/.h) vs Soure Files (.cpp)

A common source of confusion for new C++ users is the distinction between header and source files.

Header Files

• Header files usually have a .hpp or .h extension, or no extension at all (as is the case with all STL headers)
• Header files typically contain what is called the declaration of either classes or functions. A declaration simply lays out the skeleton of a class or the signature of functions, omitting their actual implementation:

  // MyClass.hpp
  #pragma once
  class MyClass {
  public:
    MyClass();
    double a_member_function(double x);
  };

  // Math.hpp
  #pragma once
  double add(double a, double b);
  double subtract(double a, double b);

• Header files conventionally define one class per header file, or a group of related functions per header file, as shown above. However, it's up to the developer how to organize header files -- the compiler generally does not care.
• Header files are incuded in all the places you wish to use the declared class or functions by using a #include "MyClass.hpp statement, etc.
• Header files typically beging with an include gaurd, e.g #pragma once, so that they are not included more than once in a single .cpp file.

Source Files

• Source files will almost always use .cpp for C++ code, and .c for plain C code
• Source files generally contain the definition or implementation of code declared by header files. If the header file is the table of contents, then the source file is the actual book.

  // MyClass.cpp
  #include "MyClass.hpp"
  
  MyClass::MyClass() { std::cout << "Hello, World" << std::endl; }
  double MyClass::a_member_function(double x) { return 2 * x };

  // Math.cpp
  #include "Math.hpp"
  
  double add(double a, double b) { return a + b; }
  double subtract(double a, double b} { return a - b; }

You Should Know ...

• The reason we split the implementation from the declaration is so that we can include classes and functions everywhere we need them, without multiply defining them. For example, if two other source files need functions from Math.hpp, and the defintions were contained in the header file, the compiler would try to compile the functions twice, causing an error. By moving the implementation to Math.cpp, we ensure only one comilation occurs, while also allowing these functions to be use by multiple files.
• However, sometimes header files will contain the definitions, and omit the source file. This generally the case of template and inline classes/functions, or the header is only ever used once later on.
• Functions and classes defined in source files don't necessiarly have to be first declared in header files. You can add utility classes and free functions to source files without declaring them in separate header files.