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I understand how to use pointers and all but why would you use them? Why would you use a pointer to a variable when you can just use the pointer? The only reason I've found is to use variables in different functions (if they're not global).
Thanks in advanced.
what do you know about pointers?
C++ is not language xyz. Other languages don't need pointers because, by design, that language wouldn't need to indirectly refer to an object through a memory address alone. C++ on the other hand is different. A pointer is the only way to know where something is on ocassion.
can you tell me more about pointers, cause im sort of lost with them too
>> I understand how to use pointers and all but why would you use them?
This exact question is asked a lot. Here are two older threads that try to answer it and include some other relevant discussion:
http://cboard.cprogramming.com/showthread.php?t=56304
http://cboard.cprogramming.com/showthread.php?t=57789
the only time i can see a use for a pointer, i could probably do the same with a reference
Thanks a bunch. I still don't 100% understand but I read in one of those links that they are only really important for more "dynamic programs". So long as I know how to use them (not when), that should be sufficient for now.
Good luck iterating through an array with a reference. References cannot change the object they refer to. Pointers can.Quote:
Originally Posted by Hobgoblin
> References cannot change the object they refer to.
Oh no? Then explain the output of this program...
Code:#include <iostream>
void foo(int& bar)
{
bar = 1;
}
int main()
{
int bar = 0;
foo(bar);
std::cout <<bar <<std::endl;
}
Not change the contents of the object, but start referencing another objectQuote:
Originally Posted by UMR_Student
Yeah, I totally read that wrong. Sorry.
Languages such as Java, for example, have no pointer data type that the programmer can explicitly declare. However, behind the scenes, most everything is actually a pointer. Learn some assembly language and you will see that.
In C++, virtual functions can be used to replace function pointers. But, there is really still a function pointer in there. I believe the goal is to reduce complexity and potential errors by the programmer. But there is something to be said for a programmer who learns how everything works behind the scenes.
And there are some instances in C++ where a reference will not do. For example, references (which are really just a const pointer, logically), cannot change what they point to. Even as a amateur programmer, I have many times needed to have a pointer change what it's pointing at.
I came to appreciate pointers more as I learned more. I think once my programming branched out from a sequence of commands and if-statements, I saw the need for pointers.
Depending on the book/tutorial you're using to learn C++, you could be forgiven for thinking that pointers are a novice-beginner subject (Far too many books & tutorials present them this way) - in fact, the language standard goes to great lengths to protect beginners against the need for pointers until much later on (Not so for 'C' where pointers are more of a bread and butter issue) The availability of references and STL greatly reduces their practical use in simple programs, leaving pointers for more advanced topics, such as the following;
* More in-depth OO techniques such as polymorphism and (GoF) design patterns
* Dynamic memory allocation (Something which is best avoided in C++ when possible - although plenty of old-style tutorials insist on teaching this before delving into the STL)
* Manipulating 'raw' arrays (Ideally, a beginner will already have a firm idea about STL Containers & Iterators before doing this - again, old-style tutorials usually do this the other way around)
* Dynamic data structures - eg, linked lists, binary trees, etc
If these concepts fly over your head, then you're probably not in a position yet where pointers are of any significant use to you. IOW, don't worry about them just yet - focus your effort on understanding the basic language features and the STL for now
I don't see where c++ wants to protect the programmer from pointers. If it would do so there would be
So you need to know about pointers for the simplest possible c++ program.Code:int main(vector<string> argv);
// instead of
int main(int argc, char** argv);
And even if the first version of main would exists one wouldn't be able to list all command line arguments without dereferencing the iterator.
Better get pointers right.
http://www.joelonsoftware.com/articl...vaSchools.html
You are looking at part of C++'s C legacy. That said, I do not think that C++'s goal here is to protect the programmer from pointers. Rather, it is to provide abstractions to ease tasks that would otherwise need direct use of pointers, and yet allow the programmer to use such a low level construct as necessary.Quote:
I don't see where c++ wants to protect the programmer from pointers. If it would do so there would be
Command line arguments can be introduced later. The version of main() that has no parameters does exist, after all.Quote:
So you need to know about pointers for the simplest possible c++ program.
One doesn't need to know anything about pointers to understand iterators.
And the point isn't to completely get rid of pointers from the teaching. Rather, its to limit there use in cases where better alternatives exist. One doesn't necessarily need to understand a function in order to start learning with int main(). Besides, the signature of main that you mention in your example would not be used by most beginners.
I would liken the supposition that pointers aren't useful, to suggesting that the division operator isn't useful.
Sure if every program you can currently think of can be solved easily by only adding addition and multiplication, then division would seem pretty useless.
All it means is that you haven't ventured very far into computer programming yet. Don't worry about it, if you continue programming the question will answer itself soon enough.:)
I don't believe I made such a claim that C++ protects programmers from pointers - indeed it doesn't, as you point out (Although I would add that a perfectly valid standards conforming declaration of main permits those parameters to be omitted).Quote:
Originally Posted by pheres
The issue I was addressing was that of complete newcomers to programming, rather than those who have already passed the basics - That it is possible in C++ to learn a large, useful, portion of the language without having to deal directly with pointers - This is largely thanks to the availability of strings, references, STL containers and iterators.
Of course, once a beginner gains confidence with the easier parts of the language, then the floodgates are open for all sorts of low-level techniques which helps them better understand the why's and how's of things working the way they do. (Taking the "top down" approach in C++ doesn't mean that primitive topics are in any way bypassed, as I believe joelonsoftware is suggesting in his JavaSchools article)
ok you are right, int main() can be used at the beginning.
but what is about reading code?
C++ has A LOT of stuff build in it. And even a lot of that stuff isn't required to write a lot of programs (maybe pointers belongs to that stuff).
Reading code makes good programmers, gets new ideas, let's one see how problems can solved better, cleaner, faster. To have fun with nearly all c++ books or tuts one has to read code.
But people who write interesting code to read about do not bother using int main(), or to resign from using all other deep c++ stuff.
I think you are not reading what we are saying: You have to learn how to use pointers, so you would be more insulated against the Law of Leaky Abstractions (since you want to quote Joel Spolsky :p). But you should not start by learning how to use pointers. Read Koenig and Moo's book Accelerated C++ for an example of how to introduce C++ using the standard library, leaving pointers for as late as chapter 10 (and that includes a chapter 0).Quote:
C++ has A LOT of stuff build in it. And even a lot of that stuff isn't required to write a lot of programs (maybe pointers belongs to that stuff).
IMO "WHEN" to use pointers depends on how you are designing your program. You usually run into pointers when you need interchangability and flexibility. For example, you have a list of functions that all have the same prototype and you want to be able to assign any of those functions to be executed when you press the Z Key on your keyboard. You would use a function pointer. Now what happens when you press the Z Key depends entirely on which of the functions you have assigned to the pointer. Another example would be when you do not want to create a duplicate copy of the data. Lets say you have several integer arrays that have 10 values each and you need to modify them all by adding 1 to each of their values. You can store a list of all the integer arrays by creating an array of 10 pointers to them. Now you can loop through them to process all 10 arrays.Quote:
Thanks a bunch. I still don't 100% understand but I read in one of those links that they are only really important for more "dynamic programs". So long as I know how to use them (not when), that should be sufficient for now.
Thanks for the tip, I will get that book as soon I get money :)Quote:
Originally Posted by laserlight
I guess we are just talking from different situations. You are right, it would be nice if everyone could learn everything step by step from ground to top. But not everyone learns c++ for pleasure. Sometimes you just have to get the thing done asap. for example loading code from a dll you get at run time. this is no big deal so far, but I bet chapter 0 to 9 doesn't explain it. They can't, because you need a silly pointer. Now.
I remember hearing from a friend about a math major where he worked and that when the math major discovered pointers he wrote a program with 13+ levels of indirection.
He kept complaining to his workmates that the compiler kept crashing.
The response was "just because you can think it, doesn't mean the compiler can do it"
Dave
I once wrote a program that would automatically generate a pointer with very large numbers of indirection. I can't remember the exact results, but I think that at least 3000 levels of indirection were supported (this was GCC 2.95). It depended on the surrounding code, so I think that when the compiler eventually crashed it was because it ran out of stack space.
Anyway, I think the C89 standard specifies that a compiler must support at least 8 levels of pointer indirection. I'm pretty sure that C99 says 64, but I'm not sure about C++.
With that many levels of indirection, it's VERY difficult to manage memory. One has to make sure that enough memory is allocated at each level so each access at each level is in bounds. His program probably was just buggy. And if the compiler can't handle more than a certain number of levels, it should give an error when the limit is exceeded.Quote:
Originally Posted by Dave++
I shudder to think of the use cases that prompted the standard writers to change this requirement. I mean ...Quote:
Originally Posted by dwks
Code:char ****************************************************************wtf;
You'd be surprised what generated code can look like. I'm sure that sort of thing might be generated by bison or something.
[edit] Actually, this page says (with much elaboration) that C89 supports 12 levels of pointer indirection. At least as far as I read. http://archives.devshed.com/forums/c...e-2322818.html
Strange. I thought I read somewhere that C89 supported 8 levels. I guess not. [/edit]
The final draft of C99 also speaks of 12 levels of indirection.