What Is #Ifndef And #Define?

What is Ifdef C++?

#ifdef: This directive is the simplest conditional directive.

This block is called a conditional group.

The controlled text will get included in the preprocessor output iff the macroname is defined.

The controlled text inside a conditional will embrace preprocessing directives..

Is a preprocessor directive which is used to end the scope of #ifdef?

If the particular identifier is defined, the statements following this preprocessor directive are executed till another preprocessor directive #endif is encountered. #endif is used to end the scope of #ifdef.

Where can I use pragma once?

We recommend the #pragma once directive for new code because it doesn’t pollute the global namespace with a preprocessor symbol. It requires less typing, is less distracting, and can’t cause symbol collisions, errors caused when different header files use the same preprocessor symbol as the guard value.

How does Ifdef work in C++?

#ifdef means if defined. If the symbol following #ifdef is defined, either using #define in prior source code or using a compiler command-line argument, the text up to the enclosing #endif is included by the preprocessor and therefore compiled. #if works similarly, but it evaluates the boolean expression following it.

How does Pragma once work?

In the C and C++ programming languages, #pragma once is a non-standard but widely supported preprocessor directive designed to cause the current source file to be included only once in a single compilation.

What are compiler directives in C?

Before a C program is compiled in a compiler, source code is processed by a program called preprocessor. This process is called preprocessing. Commands used in preprocessor are called preprocessor directives and they begin with “#” symbol.

Should I use Pragma once or Ifndef?

#pragma once is shorter than an include guard, less error prone, supported by most compilers, and some say that it compiles faster (which is not true [any longer]). But I still suggest you go with standard #ifndef include guards.

What is the difference between #ifdef and #ifndef?

The #ifdef identifier statement is equivalent to #if 1 when identifier has been defined. … The defined( identifier ) constant expression used with the #if directive is preferred. The #ifndef directive checks for the opposite of the condition checked by #ifdef.

What is the difference between Ifdef and if defined?

The difference between the two is that #ifdef can only use a single condition, while #if defined(NAME) can do compound conditionals.

Which compilers support pragma once?

Thus, #pragma once serves the same purpose as include guards, but with several advantages, including: less code, avoidance of name clashes, and sometimes improvement in compilation speed….Portability.Compiler#pragma onceIntel C++ CompilerSupportedMicrosoft Visual C++Supported (since 4.2)17 more rows

What directive terminates a if?

#endif Directive6 The #endif Directive. This directive ends the scope of the #if , #ifdef , #ifndef , #else , or #elif directive.

What does Pragma mean?

Pragma, an abbreviation for pragmatic, or from the same root, may refer to: πράγμα, the Greek word; see pragmatism. Directive (programming), also known as a pragma or pragmat in several programming languages. #pragma once.

What is #if defined in C?

defined name and defined ( name ) are both expressions whose value is 1 if name is defined as a macro at the current point in the program, and 0 otherwise. Thus, #if defined MACRO is precisely equivalent to #ifdef MACRO . defined is useful when you wish to test more than one macro for existence at once.

How do I use #define?

In the C Programming Language, the #define directive allows the definition of macros within your source code. These macro definitions allow constant values to be declared for use throughout your code. Macro definitions are not variables and cannot be changed by your program code like variables.

What is #ifndef #define and #endif used for?

#ifndef is often used to make header files idempotent by defining a token once the file has been included and checking that the token was not set at the top of that file. #ifndef _INCL_GUARD #define _INCL_GUARD #endif.

What does #ifndef mean in C?

Description. In the C Programming Language, the #ifndef directive allows for conditional compilation. The preprocessor determines if the provided macro does not exist before including the subsequent code in the compilation process.

What is the use of #ifdef?

In the C Programming Language, the #ifdef directive allows for conditional compilation. The preprocessor determines if the provided macro exists before including the subsequent code in the compilation process.

Can Ifdef be nested?

#ifdef directives can be nested, however, all directives in a given logical level must exist in a single file. The following sample code showing use of the #ifdef directive chain with nested #ifdefs. The expression #ifndef Name evaluates to true in Name has not been #defined. … These directives are interchangeable.