C file header

The declaration tells the compiler whether the element is an int , a double , a function , a class or some other thing. Furthermore, each name must be declared directly or indirectly in every.

When you compile a program, each. The compiler has no knowledge of what names are declared in other compilation units. That means that if you define a class or function or global variable, you must provide a declaration of that thing in each additional.

Each declaration of that thing must be exactly identical in all files. A slight inconsistency will cause errors, or unintended behavior, when the linker attempts to merge all the compilation units into a single program.

You make the declarations in a header file, then use the include directive in every. The include directive inserts a copy of the header file directly into the. The following example shows a common way to declare a class and then use it in a different source file.

Next, create an implementation file typically with a. If a header file happens to be included twice, the compiler will process its contents twice and it will result in an error. This construct is commonly known as a wrapper ifndef. The preprocessor will skip over the entire contents of the file, and the compiler will not see it twice.

Sometimes it is necessary to select one of the several different header files to be included into your program. For instance, they might specify configuration parameters to be used on different sorts of operating systems.

But as it grows, it becomes tedious, instead the preprocessor offers the ability to use a macro for the header name. This is called a computed include. C - Header Files Advertisements. Here's the syntax:. Again, sometimes it's essential for selecting several diverse header files based on some requirement to be incorporated into your program. For this also multiple conditional preprocessors can be used like this:.

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What are the Header Files.