By using the #ifndef directive, you can include a block of text only if a particular expression is undefined; then, within the header file, you can define the expression. These Conditional Compilation Directives allow us to include certain portion of the code depending upon the output of constant expression Block is Called as Conditional Group Examples of some preprocessor directives are: #include, #define, #ifndef etc.

As DATA macro is not defined, it will be defined now.If we do not end with endif, we will get a compile time error as:We can define a global variable in the ifndef block as well like:In case, the macro DATA is defined previously, then the two code statements of defining DATA macro and global_i variable will not be defined.This will throw a compilation error as global_i will not be defined.As both sodium.h and salt.h is included and both are using ifndef, there is no problem for the duplicate structure. They delimit blocks of program text that are compiled only if a specified condition is true. The preprocessor determines if the provided macro does not exist before including the subsequent code in the compilation process.

Remember that # symbol only provides a path that it will go to the preprocessor, and command such as include is processed by preprocessor program. This pattern (`ifndef `define) is commonly used in OVM. Consider the following code where we will define the DATA macro twice.MACROs are platform specific as specific MACROs are defined only if certain conditions are satisfied like:Hence, in such scenarios, a code may compile on a certain environment but it will fail in other environment. So if you want your code to be portable use the first method. These logical operations determine the fate of information processed the preprocessor. It's equivalent to #if 0 when identifier hasn't been defined, or has been undefined by the #undef directive. The #ifdef identifier statement is equivalent to #if 1 when identifier has been defined. Grey Wolf.

I've used the first way ("#ifndef") in the past... but "#pragma once" seems simpler and less lines to type and whatnot so I'm wondering if it's legit.

#ifndef: We know that the in #ifdef directive if the macroname is defined, then the block of statements following the #ifdef directive will execute normally but if it is not defined, the compiler will simply skip this block of statements. The #ifndef directive is simply opposite to that of the #ifdef directive. The #if directive, with the #elif, #else, and #endif directives, controls compilation of portions of a source file. The preprocessor determines if the provided macro exists before including the subsequent code in the compilation process. It is referred to as the approximatorWe will start by implementing a basic class design which initialize various parameters of SDL with a clear black window. This ensures that the code in the #ifndef is included only the first time the file is loaded.

If we had not used ifndef, there this situation will give an error.ifndef is widely used as in large codebases, it may not be possible to track.If we redefine a macro, it will give a compilation error. this is a snippet code taken from ovm_object_defines file. In this example, we see a simple use of ifndef for defining a macro named DATA in a C code. But I wonder the purpose to do this. This C tutorial explains how to use the #ifndef preprocessor directive in the C language.In the C Programming Language, the #ifndef directive allows for conditional compilation. In the C Programming Language, the #ifdef directive allows for conditional compilation. 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.

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