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This next example is a C/C++ programming language boilerplate, #include guard. #ifndef MYINTERFACE_H #define MYINTERFACE_H ... #endif This checks, and sets up, a global flag to tell the compiler whether the file myinterface.h has already been included.
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.
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 long or complex preprocessor conditionals, every #else and #endif should have a comment explaining the condition for the code below (for #else) or above (for #endif). Files [ edit ]
Comparison of ALGOL 68 and C++; ALGOL 68: Comparisons with other languages; Compatibility of C and C++; Comparison of Pascal and Borland Delphi; Comparison of Object Pascal and C; Comparison of Pascal and C; Comparison of Java and C++; Comparison of C# and Java; Comparison of C# and Visual Basic .NET; Comparison of Visual Basic and Visual Basic ...
The Compaq C++ compiler on OpenVMS VAX and Alpha (but not IA-64) and Tru64 UNIX has two name mangling schemes. The original, pre-standard scheme is known as the ARM model, and is based on the name mangling described in the C++ Annotated Reference Manual (ARM).
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.