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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.
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.
If-then-else flow diagram A nested if–then–else flow diagram. In computer science, conditionals (that is, conditional statements, conditional expressions and conditional constructs) are programming language constructs that perform different computations or actions or return different values depending on the value of a Boolean expression, called a condition.
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.
C++ changes some C standard library functions to add additional overloaded functions with const type qualifiers, e.g. strchr returns char* in C, while C++ acts as if there were two overloaded functions const char *strchr(const char *) and a char *strchr(char *). In C23 generic selection will be used to make C's behaviour more similar to C++'s. [11]
The condition is evaluated true or false as a Boolean expression. On the basis of the evaluation of the Boolean condition, the entire expression returns value_if_true if condition is true, but value_if_false otherwise. Usually the two sub-expressions value_if_true and value_if_false must have the same type, which determines the type of the ...
As an example, in the Microsoft Visual C++ compiler, it causes omission of certain optimizations such as instruction reordering around calls to library functions, since such calls may cause input/output actions or accesses to memory locations marked volatile, and changes in the order of those change observable behavior.