C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  can some one explain.....standard library (std) & <iostream>          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Michael Auton Guest Posted: Fri Jun 27, 2003 4:49 pm    Post subject: can some one explain.....standard library (std) & <iostream> I can understand the simple code below, but I cant get my head around why I need #include <iostream> and also std:: to call the cout function. #include <iostream> int main() { std::cout << "Hello world!n"; ... } Ive read that cout is in the std library, so thats why you need std::cout. But ive also read that cout is part of <iostream>. Although when I compile I actually need both! Would I be correct in saying that iostream contains the std library which contains the cout function?? Or am I looking at this from the wrong angle? Anyone please explain this, as I can see that im going to need to understand this for further functions. many thanks [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top MiniDisc_2k2 Guest Posted: Sat Jun 28, 2003 12:32 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr You're really confusing yourself here... "Michael Auton" <antonhs (AT) blueyonder (DOT) co.uk> wrote Quote: I can understand the simple code below, but I cant get my head around why I need #include <iostream> and also std:: to call the cout function. That's just how the standard was written. Quote: #include <iostream int main() { std::cout << "Hello world!n"; ... } Ive read that cout is in the std library, so thats why you need std::cout. Yes it is. Quote: But ive also read that cout is part of iostream>. Yes it is. You're in school aren't you. I wish they would stop teaching the header files to be called "libraries." (at least my teacher did) It can make things get really confusing. Quote: Although when I compile I actually need both! Would I be correct in saying that iostream contains the std library which contains the cout function?? Yes but there's something called "scope." Take this example: class A { class B { public: int a; }; }; B b; // this is illegal That is illegal because it is "out of scope." The braces define the scope, if you will. If you are outside of that set of braces you are not allowed to access anything within the braces. This applies the same to variables declared within do-while loops, and anything else which uses braces. In fact, braces are so important that they actually can let you get away with things. You're not allowed to initialize variables within a switch/case statement, but with braces you can "close the scope" and initialize them in there all you want. Anyways what I meant to get at is for the same reason you're not allowed to access B from "global scope," you're not allowed to access cout from "global scope." You must access it _through_ std. Quote: Or am I looking at this from the wrong angle? Anyone please explain this, as I can see that im going to need to understand this for further functions. Yes you will definately need to understand this for later. It's the basic concept behind object oriented programming and minimal visibility. You'll probably want to do a google search or something to get information on scope, as I could just scrape the surface here. Quote: many thanks No problem -- MiniDisc_2k2 P.S. If you really must know (I'm not quite sure if this is standard, though), you could #include <iostream.h> which enables you to just use cout. It's a little different from <iostream>. Also, read up on the "using" declaration, as that will also allow you to circumvent std (although I don't know how, I just use <iostream.h>). To reply, replace nospam.com with cox dot net. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Natalie Guest Posted: Sat Jun 28, 2003 12:33 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr "Michael Auton" <antonhs (AT) blueyonder (DOT) co.uk> wrote Quote: Ive read that cout is in the std library, so thats why you need std::cout. But ive also read that cout is part of <iostream> so thats why I need iostream>. More properly, cout is defined in the iostream header. This is why you need it. It is declared in that header to be in the std namespace. Look up namespaces in your C++ book. You have to either qualify it with std:: or use using to bring it into the scope of where you use it. Quote: Although when I compile I actually need both! Would I be correct in saying that iostream contains the std library which contains the cout function?? iostream is NOT a library. This isn't Java. iostream provides the necessary definitions so that you can use the standard library functions. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Drew Hall Guest Posted: Sat Jun 28, 2003 12:47 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr "Michael Auton" <antonhs (AT) blueyonder (DOT) co.uk> wrote Quote: Ive read that cout is in the std library, so thats why you need std::cout. But ive also read that cout is part of <iostream> so thats why I need iostream>. Although when I compile I actually need both! Would I be correct in saying that iostream contains the std library which contains the cout function?? The "standard library" is the set of classes & functions defined in the language standard, all of which are made accessible by #including various headers. <iostream> is one of these headers, and is thus part of the standard library (along with <string>, <vector>, etc). The parts of the standard library that are not directly inherited from C all live in a namespace that is specially reserved for them, "std". This includes all of the headers that don't end in ".h". This prevents the names of the standard library components from interfering with names in code that you develop. So, you need to say #include <iostream> to make the std. library iostream components accessible, then use "std::" to make it clear that when you say "cout", you mean the cout object that lives in the std namespace (and whose declaration lives in <iostream>. Sounds like you need a good tutorial or book to help you get comfortable with the lay of the land. I recommend Koenig & Moo's "Accelerated C++" or Lippman & Lajoie's "C++ Primer". Good luck! Drew [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Artie Gold Guest Posted: Sat Jun 28, 2003 12:54 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr Michael Auton wrote: Quote: I can understand the simple code below, but I cant get my head around why I need #include <iostream> and also std:: to call the cout function. The header <iostream> declares the stream "cout", which is in the _namespace_ "std". Quote: #include <iostream int main() { std::cout << "Hello world!n"; ... } Ive read that cout is in the std library, so thats why you need std::cout. But ive also read that cout is part of iostream>. Yes, the header <iostream> declares the stream "cout", which is in the _namespace_ "std". Quote: Although when I compile I actually need both! Would I be correct in saying that iostream contains the std library which contains the cout function?? Yes, the header <iostream> declares the stream "cout", which is in the _namespace_ "std". Quote: Or am I looking at this from the wrong angle? Anyone please explain this, as I can see that im going to need to understand this for further functions. You were. You will. ;-) HTH, --ag -- Artie Gold -- Austin, Texas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sebastian Moleski Guest Posted: Sat Jun 28, 2003 12:55 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr "Michael Auton" <antonhs (AT) blueyonder (DOT) co.uk> wrote Quote: I can understand the simple code below, but I cant get my head around why I need #include <iostream> and also std:: to call the cout function. The standard library is distributed over a number of header files. When you include <iostream>, you really only get access to the I/O streaming part of the standard library. Now, despite the fact that the standard library is split up like that over different files, all of its contents reside in one common namespace: std. The standard committee could have decided to host every part of the library in its own namespace (and thus you would have iostreams::, vector::, list::, etc.), but they decided that one namespace should be sufficient. HTH, sm [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top gs_sarge@earthlink.net Guest Posted: Sat Jun 28, 2003 11:05 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr I had this explained just the other day in class when using the <iostream> without the .h extension, you are using the new naming method (forgive me if my terminology is off) where all standard library functions must use the std namespace. However, to keep backwards compatible, if you include iostream.h, with the .h appended to the end, it will keep the global variable cout, which allows you to access without the namespace std. Hope that helps, On 27 Jun 2003 12:49:35 -0400, [email]antonhs (AT) blueyonder (DOT) co.uk[/email] (Michael Auton) wrote: Quote: I can understand the simple code below, but I cant get my head around why I need #include <iostream> and also std:: to call the cout function. #include <iostream int main() { std::cout << "Hello world!n"; ... } Ive read that cout is in the std library, so thats why you need std::cout. But ive also read that cout is part of iostream>. Although when I compile I actually need both! Would I be correct in saying that iostream contains the std library which contains the cout function?? Or am I looking at this from the wrong angle? Anyone please explain this, as I can see that im going to need to understand this for further functions. many thanks [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top llewelly Guest Posted: Sat Jun 28, 2003 11:32 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr "MiniDisc_2k2" <MattDelB (AT) nospam (DOT) com> writes: [snip] Quote: P.S. If you really must know (I'm not quite sure if this is standard, though), This is not standard. Quote: you could #include <iostream.h> which enables you to just use cout. In the long run, I think this is a bad idea. While implementations of <iostream> are converging towards the standard (and therefor becoming more and more similar with each compiler release), <iostream.h> implementations are either stagnant or diverging. Which is currently more portable is an open question, but soon using <iostream> will be more portable. Worse, <iostream.h> has no future; it is not part of the standard, and most implementors do not wish to maintain it indefinitely. Quote: It's a little different from <iostream>. Also, read up on the "using" declaration, as that will also allow you to circumvent std (although I don't know how, I just use <iostream.h>). [snip] I advise you to change now before the transition period is over. (Since there is no official transition period, there's no telling when that will be.) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Kurt Krueckeberg Guest Posted: Sat Jun 28, 2003 11:34 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr Quote: I can understand the simple code below, but I cant get my head around why I need #include <iostream> and also std:: to call the cout function. #include int main() { std::cout << "Hello world!n"; ... } The standard library is defined in a namespace called "std". That's why "std::cout" is needed. To get around having to write "std::cout", add a using declaration for cout using std::cout; or insert the using directive using namespace std; The later will make all std names part of the global namespace. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Corey Murtagh Guest Posted: Sun Jun 29, 2003 11:34 am    Post subject: Re: can some one explain.....standard library (std) & <iostr Kurt Krueckeberg wrote: <snip> Quote: The standard library is defined in a namespace called "std". That's why "std::cout" is needed. To get around having to write "std::cout", add a using declaration for cout using std::cout; or insert the using directive using namespace std; The later will make all std names part of the global namespace. This is generally considered to be a bad idea. There are a lot of things in the 'std' namespace, and one trusts that there is a good reason *why* they are there rather than in the global namespace. I've gotten comfortable over the past few months (since I finally started to actually use the STL, after years of using C++) with typing namespace specifiers before the STL objects (not to mention boost, etc). If it becomes too cumbersome I will occasionaly throw in a 'using' directive at function scope. I always consider the effects of doing so very carefully however. -- Corey Murtagh The Electric Monk "Quidquid latine dictum sit, altum viditur!" [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sun Jun 29, 2003 12:34 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr In message <3efcd4e8.266470954 (AT) news (DOT) earthlink.net>, [email]gs_sarge (AT) earthlink (DOT) net[/email] writes Quote: I had this explained just the other day in class when using the <iostream> without the .h extension, you are using the new naming method (forgive me if my terminology is off) where all standard library functions must use the std namespace. However, to keep backwards compatible, if you include iostream.h, with the .h appended to the end, it will keep the global variable cout, which allows you to access without the namespace std. Hope that helps, In so far as any wrong explanation can help:-) While it is probably true that a cout provided by iostream.h will in fact be bound to the same object as std::cout provided by iostream there is absolutely no requirement that it be so. There are strict rules about the lifetime of std::cout that are different from the pre-standard rules of iostream.h. Imagine mixing an old (pre-namespace) iostream.h with code that uses std::cout. It is all very well in student code, but the potential for problems in real code is just too high and getting higher by the year. -- ACCU Spring Conference 2003 April 2-5 The Conference you should not have missed ACCU Spring Conference 2004 Late April Francis Glassborow ACCU [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Mon Jun 30, 2003 6:34 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr [email]antonhs (AT) blueyonder (DOT) co.uk[/email] (Michael Auton) wrote in message news:<def74e4c.0306270213.57d0d1fc (AT) posting (DOT) google.com>... Quote: I can understand the simple code below, but I cant get my head around why I need #include <iostream> and also std:: to call the cout function. #include <iostream int main() { std::cout << "Hello world!n"; ... } Ive read that cout is in the std library, so thats why you need std::cout. Correctly speaking, the standard defines an entity whose name is std::cout. This entity is part of the library, which is another way of saying that its presence isn't automatic -- you have to say that you want it. Quote: But ive also read that cout is part of why I need <iostream>. #include <iostream> is the way of activating a certain part of the library. The standard is a bit wishy-washy here -- when you activate any part of the library, the implementation may activate other parts as well. Or it may not. In the case of your program, you actually have to activate two parts: #include <iostream> // for std::cout, etc. #include <ostream> // for the << operators Some implementations (but not all) also activate the << operators when you specify "#include Quote: Although when I compile I actually need both! Would I be correct in saying that iostream contains the std library which contains the cout function?? No. First, technically speaking, <iostream> isn't necessarily a file or anything like that. All the language guarantees is that when you include <iostream>, certain parts of the language are activated. In the case of <iostream>, all that is required to be activated are the external declarations for the standard input and output objects; as external declarations, the types are not necessarily complete, and the operators are not necessarily declared. Including <ostream> activates the definition of std::istream (which is the type of std::cout), and most of the << operators. Practically, of course, the technique that most (all?) implementations use for this is to put the declarations in files, and read the files. Exactly as with your header files. -- James Kanze GABI Software mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ http://www.gabi-soft.fr Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France, Tél. : +33 (0)1 30 23 45 16 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top James Kanze Guest Posted: Sun Jul 06, 2003 10:44 pm    Post subject: Re: can some one explain.....standard library (std) & <iostr Francis Glassborow <francis.glassborow (AT) ntlworld (DOT) com> writes: Quote: In message <3efcd4e8.266470954 (AT) news (DOT) earthlink.net>, [email]gs_sarge (AT) earthlink (DOT) net[/email] writes I had this explained just the other day in class when using the <iostream> without the .h extension, you are using the new naming method (forgive me if my terminology is off) where all standard library functions must use the std namespace. However, to keep backwards compatible, if you include iostream.h, with the .h appended to the end, it will keep the global variable cout, which allows you to access without the namespace std. Hope that helps, In so far as any wrong explanation can help:-) While it is probably true that a cout provided by iostream.h will in fact be bound to the same object as std::cout provided by iostream there is absolutely no requirement that it be so. Actually, it will only be the case in implementations which don't have too much respect for their users. The interface of the classical cout is simply not the same as that of std::cout, and while you won't notice the difference in most uses, there are one or two which will bite you; in order to avoid breaking working programs which include <iostream.h>, the cout which it reveals must have a different type than std::cout. Several compiler providers have ignored backwards compatibility in this regard. For this reason, where I am currently working, we are stuck having to compile in mode compat=4 with Sun compilers, and we can't upgrade from g++ 2.95.2. (An additional effect, of course, is that in future projects, I will warn my customers against using these compilers.) Quote: There are strict rules about the lifetime of std::cout that are different from the pre-standard rules of iostream.h. That's one point where the differences are, in practice, almost inexistant. The only one I can think of is that the classical iostream.h contained an instance of ios::Init, whereas the standard doesn't require this of any standard header. In practice, however, all of the more recent libraries I've used do include it in <iostream>. Quote: Imagine mixing an old (pre-namespace) iostream.h with code that uses std::cout. That is, of course, a different problem. Logically, it shouldn't be a problem -- you have two completely separate systems, both synchronized with a third (FILE*), but in practice, it doesn't seem to work. -- James Kanze mailto:kanze (AT) gabi-soft (DOT) fr Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 11 rue de Rambouillet, 78460 Chevreuse, France Tel. +33 1 41 89 80 93 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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