Headers and includes why and how

If a header file is paired with a code file e. In order to use this header file in main. When the preprocessor processes the include "add. Because our add. The end result is a program that is functionally the same as the one where we manually added the forward declaration at the top of main. Source files should include their paired header. 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. The compiler needs to know the size of B to determine the total size of A.

In this case b. Class C Class C is included only as a pointer reference. The size or actual content of C are not important to a.

Thus only a forward declaration has been included in a. Notice that c. Class D Class D is just used as a pointer reference in a. Thus a forward declaration is sufficient. But a. Key Points Header files should be included only when a forward declaration will not do the job. By not including c. By not including c. If this is followed for all classes, i. This makes it tolerant to duplicate inclusions of a.

Cyclic dependency exists between class X and Y in the following example. This dependency is handled by using forward declarations. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Including includes in header file vs source file Ask Question. Asked 11 years, 7 months ago. Active 5 months ago. Viewed 58k times. Improve this question. Duncan Jones Rob from Utah Rob from Utah 1 1 gold badge 7 7 silver badges 3 3 bronze badges.

Add a comment. Active Oldest Votes. Header File Inclusion Example The following example illustrates different types of dependencies. Class ABase: ABase is the base class, so the class declaration is required to complete the class declaration. The compiler needs to know the size of ABase to determine the total size of A. In this case abase. Class B: Class A contains Class B by value , so the class declaration is required to complete the class declaration.

The compiler needs to know the size of B to determine the total size of A. In this case b.