Note that header files may be included in the source file multiple times, leading to compiler errors. Usually, this is prevented with the ifndef preprocessor directive, which is called wrapper ifndef. In case the user includes the same header for the second time, the ifndef directive will evaluate false and ignore the code before the endif directive.
As a result, the compiler will get only one copy of the code from this header file and translate it successfully. Another method to accomplish the same results is to include pragma once directive in the header file.
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LiraNuna LiraNuna Also it prevents recursive inclusions Imagine "alice. Kevin: that is what I mean. I wanted to manipulate a form which was opened by the form to manipulate. It gaveme lots of errors and I didn't know what to do. CIsForCookies Punch "one definition rule" into your favorite search engine. QuantumFool 3 3 gold badges 9 9 silver badges 29 29 bronze badges. Roy Roy 1, 1 1 gold badge 13 13 silver badges 11 11 bronze badges.
Identifiers starting with an underscore are reserved; you shouldn't define them yourself. I know this is an old comment, but actually the underscore restriction only applies to "external identifiers" - identifiers that could end up in the compiled object's symbol table, i. It does not apply to macro names. Is Stu's comment true?
I just read stackoverflow. In practice such symbols may be used to define macros in the standard libraries, which could collide with user-defined symbols of the same form I look forward to a response in ; greetings, from the age of the rat plague! Exception: user-defined literals. Add a comment. This prevent from the multiple inclusion of same header file multiple time. They are called ifdef or include guards.
Mohit Jain Mohit Jain 6 6 bronze badges. The Overflow Blog. Does ES6 make JavaScript frameworks obsolete?
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