Pragma once what is

That one might be unable to use pragma once. In general I prefer pragma once because when you duplicate a header file in order to do incremental refactoring by duplication or extending a class, you cannot forget to change the name of the include guard macro.

Therefore I do not know of any hard problem you have with pragma once , except for certain compilers. In using pragma once you are giving up portability. Given that the name and location of a file is unique, I use that as my include guard. Furthermore because I have in the past targetted very old preprocessors, I use as a habit. Specifies that the compiler includes the header file only once, when compiling a source code file.

The use of pragma once can reduce build times, as the compiler won't open and read the file again after the first include of the file in the translation unit.

It's called the multiple-include optimization. It has an effect similar to the include guard idiom, which uses preprocessor macro definitions to prevent multiple inclusions of the contents of the file.

It also helps to prevent violations of the one definition rule : the requirement that all templates, types, functions, and objects have no more than one definition in your code. We recommend the pragma once directive for new code because it doesn't pollute the global namespace with a preprocessor symbol.

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