Understanding header files in C++

[psandip]

Hello everybody,

While reading C++, I have been looking into iostream header file to understand a bit more about header file organization. I am not able to understand much of the code in there.
For example the header starts with #ifndef _GLIBCXX_IOSTREAM. Does this check whether _GLIBCXX_IOSTREAM has been defined or not? If so then who is supposed to use this variable _GLIBCXX_IOSTREAM? I know it is not a good question but I am kind of curious.

[Fbody]

Any line that starts with '#' is a "preprocessor directive" . This particular one sounds like what is called a "header guard" or an "inclusion guard". It should be immediately followed by something along the lines of

#define _GLIBCXX_IOSTREAM 1

. At the end of the file, there should also be a statement similar to this:

#endif /* _GLIBCXX_IOSTREAM */

This statement ends the statement block created by the #ifndef.

The #ifndef statement checks to see "if _GLIBCXX_IOSTREAM is NOT defined". If it finds that it is NOT defined, it returns true and executes the statements contained within its statement block. The next statement, #defines _GLIBCXX_IOSTREAM. As a result, _GLIBCXX_IOSTREAM now is defined. This causes #ifndef _GLIBCXX_IOSTREAM to come false if is #included again so that it doesn't double up the #include. Here is more C++ - Specific preprocessor information.

This combination of statements is used to prevent the header being included in the code more than once which can cause compilation and linking issues.

[psandip]

Hello Fbody,

Thanks for the reply. Now I think I can understand what this means. _GLIBCXX_IOSTREAM is just a flag name for use in the header guard and the name has no other significance. PLease correct me if I am wrong.

[Fbody]

That's essentially correct. It's actually what's called a "macro". It checks to see if the macro already exists. If it doesn't, the #define creates the macro.

These types of macros simply exist, and make compiling easier. They really are of no other use within your program.

[psandip]

Many thanks :)

[jaswanth424]

what is 1(one) at the end of #define _GLIBCXX_IOSTREAM 1.????

what is the 1(one) use of this????

[Fbody]

what is 1(one) at the end of #define _GLIBCXX_IOSTREAM 1.????

what is the 1(one) use of this????

Did you even read the thread? It's all laid out for you right in the thread. The "constant" _GLIBCXX_IOSTREAM has no use in the program other than to act as a header guard. You would not use it any where else.