C and C++ distinguishes between declarations an definitions.
You can declare a symbol many times, but you are allowed to define it only once. By learning this I have an idea to put declarations outside the guards, while definitions inside the guards:
// declarations
int foo(int x, int y);
int bar(int x, int y);
extern double something;
class X;
#ifndef _MY_HEADER_H_
#define _MY_HEADER_H_
#include "otherheader1.h"
#include "otherheader2.h"
// definitions of structures, classes.
class X
{
// blah blah...
};
#endif
By this way I can include my headers whatever order I want. And probably circular dependencies won't be a problem.
So why to protect the entire header with guard tokens if we can put the declarations outside?
My rationale was the following:
You probably often run into issues when two headers refer to each other somehow. You usually get an undeclared symbol error, and your first reflex is to include the necessary headers. But when two of your headers happen to include each other, you get cryptic errors.
a.h:
#ifndef A_H
#define A_H
#include "b.h"
class A {B *b;}
#endif
b.h
#ifndef B_H
#define B_H
#include "a.h"
class B {A *a;}
#endif
When in b.cpp you include the b.h you get an error in a.h that B is not declared but the header is included. (It's a wtf moment.)
It's because that header guards won't nest:
#ifndef B_H
#define B_H
#ifndef A_H
#define A_H
// B_H already defined no include here.
class A {B *b;}
#endif
class B {A *a;}
#endif
If you put declarations outside the guard, you can prevent this:
class B; // in b.h
#ifndef B_H
#define B_H
class A; // in a.h
#ifndef A_H
#define A_H
class B; // again from b.h
// B_H already defined no include here, no redefinition.
class A {B *b;}
#endif
class B {A *a;}
#endif
No problem here.
UPDATE: put header inclusion into the guards (Sorry it was a mistake).
JavaScript questions and answers, JavaScript questions pdf, JavaScript question bank, JavaScript questions and answers pdf, mcq on JavaScript pdf, JavaScript questions and solutions, JavaScript mcq Test , Interview JavaScript questions, JavaScript Questions for Interview, JavaScript MCQ (Multiple Choice Questions)
