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In computer programming, conditional compilation is a compilation technique which results in differring executable programs depending on parameters specified. This technique is commonly used when these differences in the program are needed to run it on different platforms, or with different versions of required libraries or hardware.
Conditional compilation is supported via the if-else core directives #if, #else, #elif, and #endif and with contraction directives #ifdef and #ifndef which stand for #if defined(...) and #if !defined(...), respectively. In the following example code, the printf() call is only included for compilation if VERBOSE is defined.
An expected minor difference in the first line, otherwise it works. This method is now documented in Advanced_templates on Meta. There are apparently some drawbacks with substituting templates containing many ifdef-constructs, it replaces pointless code for ifndef cases. -- Omniplex 01:58, 22 April 2006 (UTC)
Translation units define a scope, roughly file scope, and functioning similarly to module scope; in C terminology this is referred to as internal linkage, which is one of the two forms of linkage in C. Names (functions and variables) declared outside of a function block may be visible either only within a given translation unit, in which case they are said to have internal linkage – they are ...
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.
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 difference from weak symbols is that weak symbols are interpreted by the linker. The CPP is run during the compilation of each translation unit before the C compiler. The build process (e.g. make) can be implemented in a conditional way such that just different versions of a symbol are created or different (specialized) libraries are used ...
A difference list f is a single-argument function append L, which when given a linked list X as argument, returns a linked list containing L prepended to X. Concatenation of difference lists is implemented as function composition. The contents may be retrieved using f []. [1]