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COBOL defines an include directive indicated by copy in order to include a copybook. Generally, for C/C++ the include directive is used to include a header file, but can include any file. Although relatively uncommon, it is sometimes used to include a body file such as a .c file. The include directive can support encapsulation and reuse ...
For #include guards to work properly, each guard must test and conditionally set a different preprocessor macro. Therefore, a project using #include guards must work out a coherent naming scheme for its include guards, and make sure its scheme doesn't conflict with that of any third-party headers it uses, or with the names of any globally visible macros.
These named modules were added to include all items declared in both global and std namespaces provided by the importable standard headers. Macros are not allowed to be exportable, so users have to manually include or import headers that emit macros for use. The C++ standard has reserved std and std.* as module names. [11]
C standard library functions are exported from the C++ standard library in two ways. For backwards-/cross-compatibility to C and pre-Standard C++, functions can be accessed in the global namespace (::), after #include ing the C standard header name as in C. [40] Thus, the C++98 program
As a result, explicit includes of the above header files in any Objective-C code file do not imply a second inclusion because of the #import directive of Objective-C, or more generally with #include because of the use of include guards; hence, these includes can be forgotten, but it is advocated [1] to have them explicitly written in order to keep the source code autonomous and reusable, and ...
Objective-C preprocessors have #import, which is like #include but only includes the file once. A common vendor pragma with a similar functionality in C is #pragma once. C++ as of C++20 has the import and module directives for modules.
Using #pragma once allows the C preprocessor to include a header file when it is needed and to ignore an #include directive otherwise. This has the effect of altering the behavior of the C preprocessor itself, and allows programmers to express file dependencies in a simple fashion, obviating the need for manual management.
The C POSIX library is a specification of a C standard library for POSIX systems. It was developed at the same time as the ANSI C standard. Some effort was made to make POSIX compatible with standard C; POSIX includes additional functions to those introduced in standard C.