As long as no two headers define the same guard macro, the code will compile as expected. And if two headers do define the same guard macro, the programmer can go in and change one of them. #pragma once has no such safety net -- if the compiler is wrong about the identity of a header file, either way, the program will fail to compile.

If you decide to put the include guard in the including file, you should define the macro there as well: #ifndef HEADER_H #include "header.h" #define HEADER_H #endif But, as other answers have already said, it's much better to put the guard in the header itself: header.h : #ifndef HEADER_H #define HEADER_H /* contents of header.h */ #endif

2016年4月11日 · Regardless, caching does come into play. How does the preprocessor know to include code outside of the guards. Example... extern int foo; #ifndef INC_GUARD #define INC_GUARD class ClassInHeader{}; #endif In this case the preprocessor will have to include extern int foo; multiple times if you include the same file multiple times.

A web guard is the traditional cookie store - so that web guard instructs Laravel to store and retrieve session information the classic way. The API guard, on the other hand, uses tokens. So you would use the API guard if you want to authenticate users and requests using an API token in the header (bearer) or query parameter.

2015年1月7日 · This header file has an include guard. However, I'm kind of confused as to what #define HEADER_FILE is actually doing. Let's say I were to forget the include guard, it would have been perfectly legal for me to completely ignore adding '#define HEADER_FILE'. What exactly are we doing when we define HEADER_FILE? What are we defining?

2017年8月13日 · Guard, round, and sticky bits of 0, 1, 1 tell you that the “residue bits” (by which I mean the bits starting just below the point where we are going to round) are .01xxx…, where xxx… is not known but contains at least one 1 bit. Thus, the “residue” portion is more than ¼ ULP (is .01 plus something positive) but less than ½ ULP (is ...

2011年1月22日 · The #ifndef/#define directives should be the first two lines of the file, and #endif should be the last. Include guards are only used in headers. Do not define your main function in a header: put it in an implementation file. If you have a header that will define a type and declare a function, but also needs a header itself:

2019年10月4日 · 1) A web guard is the traditional cookie store - so that web guard instructs Laravel to store and retrieve session information. 2) The API guard, on the other hand, uses tokens. So you would use the API guard if you want to authenticate users and requests using an API token in the header (bearer) or query parameter.

If you have #define SOME_SYMBOL with nothing in front of it, it would replace it every it sees it with nothing. So #define A_H means "define a symbol named A_H which replaces with nothing". The part that is important to us is the "define a symbol named A_H", because later we want to have an if on whether the symbol is defined (#ifndef). –

2015年2月18日 · Include guard allows loading rest of your header file. The next time: AnotherHeader.h is included unconditionally again; Include guard prevents from loading rest of your header file. If AnotherHeader.h is include-guarded, nothing bad should happen.