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In computer programming, a directive or pragma (from "pragmatic") is a language construct that specifies how a compiler (or other translator) should process its input. Depending on the programming language , directives may or may not be part of the grammar of the language and may vary from compiler to compiler.
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 preprocessor (CPP) is a text file processor that is used with C, C++ and other programming tools.The preprocessor provides for file inclusion (often header files), macro expansion, conditional compilation, and line control.
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.
Weak symbols are not mentioned by the C or C++ language standards; as such, inserting them into code is not very portable. Even if two platforms support the same or similar syntax for marking symbols as weak, the semantics may differ in subtle points, e.g. whether weak symbols during dynamic linking at runtime lose their semantics or not.
While there is no standard way of defining the alignment of structure members (while C and C++ allow using the alignas specifier for this purpose it can be used only for specifying a stricter alignment), some compilers use #pragma directives to specify packing inside source files. Here is an example:
In computer programming, a precompiled header (PCH) is a (C or C++) header file that is compiled into an intermediate form that is faster to process for the compiler.Usage of precompiled headers may significantly reduce compilation time, especially when applied to large header files, header files that include many other header files, or header files that are included in many translation units.
C++11 provides a standardized syntax for compiler/tool extensions to the language. Such extensions were traditionally specified using #pragma directive or vendor-specific keywords (like __attribute__ for GNU and __declspec for Microsoft). With the new syntax, added information can be specified in a form of an attribute enclosed in double square ...