What does #ifndef Mean?

What does #ifndef Mean?

The #ifndef directive checks whether a macro is not defined. If the identifier specified is not defined as a macro, the lines of code immediately follow the condition are passed on to the compiler.

How does include guard work?

Include guards ensures that compiler will process this file only once, no matter how many times it is included. Include guards are just series of preprocessor directives that guarantees file will only be included once. Preprocessors used: #ifndef: if not defined, determines if provided macros does not exists.

What is a guard in a header file?

Header guards are little pieces of code that protect the contents of a header file from being included more than once. Header guards are implemented through the use of preprocessor directives. The C/C++ preprocessor directives all start with the # character. You are already familiar with some ( #include, #define).

Which choice is an include guard for the header file my B Ed?

C Language Preprocessor and Macros Header Include Guards And thus there is no compilation error.

What is Ifdef and Ifndef?

Use the #ifdef statement when you want to compile a section only if a specified expression has been defined with #define. Use #ifndef when you want to compile a section only if a specified expression has not been defined.

Are include guards necessary?

The key advantage of using include guards is that they will work with all standard-compliant compilers and preprocessors. However, include guards also cause some problems for developers, as it is necessary to ensure the macros are unique within all headers used in a project.

How do you include in C++?

You make the declarations in a header file, then use the #include directive in every . cpp file or other header file that requires that declaration. The #include directive inserts a copy of the header file directly into the . cpp file prior to compilation.

What is an include guard in C?

In the C and C++ programming languages, an #include guard, sometimes called a macro guard, header guard or file guard, is a particular construct used to avoid the problem of double inclusion when dealing with the include directive.

Where do I put endif?

The Endif statement (Eif) marks the end of the If statement and any statements based on the If condition. The Endif statement is only valid when used in combination with the If statement.

When protecting a header file Why would you use #pragma once instead of include guard?

#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]).

How do I use Ifndef and endif?

If the condition checked by #if , #ifdef , or #ifndef is true (nonzero), then all lines between the matching #else (or #elif ) and an #endif directive, if present, are ignored. If the condition is false (0), then the lines between the #if , #ifdef , or #ifndef and an #else , #elif , or #endif directive are ignored.