By reference vs value

This is a discussion on By reference vs value within the C++ Programming forums, part of the General Programming Boards category; Hi all, Im just working my way through my latest book, and the book states most c++ programmers when working ...

  1. #1
    Registered User RocketMan's Avatar
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    By reference vs value

    Hi all,

    Im just working my way through my latest book, and the book states most c++ programmers when working in classes, pass objects via reference and not by value (pointers) because value can cause numerous error (lost pointers)? Is this true, or do most advanced programmer work with purely value?

    As i understand it, working with value (pointers), save vast amounts of memory, and therefore are a benifit over reference?

    Hope i got reference and value round the right way?

    Look forward to all the guru replies

    Cheers

    RocketMan

  2. #2
    C++ Witch laserlight's Avatar
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    Passing by value does not necessarily mean passing pointers. For example:
    Code:
    #include <iostream>
    #include <string>
    
    void print(std::string str)
    {
        std::cout << str << std::endl;
    }
    
    int main()
    {
        std::string str("Hello world!");
        print(str);
    }
    In the code above, str is passed by value.

    the book states most c++ programmers when working in classes, pass objects via reference and not by value (pointers) because value can cause numerous error (lost pointers)?
    I am not sure what the book is talking about, but passing by reference is typically used to avoid the copying that comes from passing the object by value, and/or to have changes to the object passed be reflected in the caller. Pointers can also be used for this purpose, but the reference syntax tends to have less clutter.

    What book is that, by the way?

    As i understand it, working with value (pointers), save vast amounts of memory, and therefore are a benifit over reference?
    That is just not true.

    Hope i got reference and value round the right way?
    I think you are confusing pass by value with the concept of pointers.

    I am not sure if this is too much for you to handle at the moment, but you could try reading an article on Pointers, References and Values.
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    You have a misunderstanding. Passing by pointer is a form of "pass-by-reference" and is not pass by value.

    To pass-by-reference you can use references or pointers. To pass by value you use neither and a copy of the object is created.

    Pass-by-reference (with references or pointers) can save memory since it avoids a copy. This generally applies to classes rather than built-in types.

    C++ programmers often prefer passing by reference (or reference to const) over pointers because they have a more natural syntax and do not allow nulls to be passed. I'm not sure exactly what you mean by "lost pointers" but that might be what is being referred to.

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    Registered User RocketMan's Avatar
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    Hi,

    Thanks for your reply.

    The book is: C++ in 21 days 4th edition, by Jesse Liberty. Have you read this book?

    Ok thanks, i will have a read of that link.

    Thanks

    RocketMan

  5. #5
    C++ Witch laserlight's Avatar
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    The book is: C++ in 21 days 4th edition, by Jesse Liberty. Have you read this book?
    No, but DougDbug recommended it in our book recommendation thread, so perhaps it is good with just a little awkward wording concerning this topic.
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    Registered User RocketMan's Avatar
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    Hi,

    Thanks, it might be my understanding. I will read that chapter again.

    Cheers.

    Do you ever sleep laserlight? You are on this forum alot - its a good think!

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by Daved View Post
    I'm not sure exactly what you mean by "lost pointers" but that might be what is being referred to.
    I think it may refer to that if you lose a pointer to an object allocated on the heap, it's a memory leak (lost?).
    Getting a pointer with NULL is typically used when the variable is optional or to say it isn't used, so there's not much that can go wrong here, I think.
    That's what I believe anyway. I might be wrong
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Registered User RocketMan's Avatar
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    Hi,

    Code:
    I think it may refer to that if you lose a pointer to an object allocated on the heap, it's a memory leak (lost?).
    Correct, i just explained it in slightly the wrong term - sorry.

    thanks again

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    C++まいる!Cをこわせ! Elysia's Avatar
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    Bingo! Spot on!
    Anyway, this point is kind of moot in today's C++, since you can use smart pointers to get rid of the memory when no longer in use. Therefore, you never have to worry about deleting pointers.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Quote Originally Posted by Daved View Post
    Pass-by-reference (with references or pointers) can save memory since it avoids a copy. This generally applies to classes rather than built-in types.
    It applies to all types. Built-in types, when passed by value, have to be copied and that consumes memory.

    Passing any argument to a function logically involves copying something (eg passing a pointer means copying the value of the pointer; an analogous argument can be made for C++ references, except that the details of what is actually passed are up to the compiler). That copying implies some expense (eg machine instructions, execution time, memory consumption, use of registers).

    The advantage of passing by pointer or reference is that the expense has a fixed, and relatively small, upper bound. Whereas, when passing other types, that expense is influenced by the physical size of the object (sizeof(whatever)) which has no logical upper bound. The expense is also affected by any associated need for other operations (eg invoking a non-trivial copy constructor for class types) because of a need to create a temporary copy of whatever is being passed.

    The argument gets slightly blurrier with optimising compilers, as the standard specifically allows a compiler to avoid creating temporary copies in some circumstances.
    Last edited by grumpy; 01-13-2008 at 02:27 PM.

  11. #11
    C++まいる!Cをこわせ! Elysia's Avatar
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    As you know, even when passing through reference (or pointers), a dword is actually copied - the address of the argument, so it's moot to pass built-in types by reference unless it's an array.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Quote Originally Posted by Elysia View Post
    As you know, even when passing through reference (or pointers), a dword is actually copied - the address of the argument, so it's moot to pass built-in types by reference unless it's an array.
    Not true in general.

    Firstly, your comment about dword's is specific to some compilers/systems, it is not generally true.

    Second, I can offer a real counter-example. On quite a few 32 bit operating systems, a pointer is 32 bit (4 8-bit bytes) and a double is 8 bytes in size. Passing a double by value, on those systems, can therefore be more expensive than passing it by reference -- if we assume that copying four bytes is less expensive than copying eight.

    If the mechanism of passing arguments to a function becomes a critical concern affecting program performance or memory usage, then the only correct approach is to profile, analyse, and test the application behaviour for all target platforms.

  13. #13
    C++まいる!Cをこわせ! Elysia's Avatar
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    I forget sometimes that there are non-32-bit operating systems out there. Anyway, on a 32-bit system, passing anything other double or INT64 as reference is pretty moot since it's typically converted to a 32-bit value and passed anyway.
    On 64-bit systems, a 64-bit value is passed.
    But 4 bytes vs. 8 bytes isn't a big difference. You probably won't notice any difference at all since it's lightning fast via exchange of one or two registers.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Quote Originally Posted by Elysia View Post
    I forget sometimes that there are non-32-bit operating systems out there. Anyway, on a 32-bit system, passing anything other double or INT64 as reference is pretty moot since it's typically converted to a 32-bit value and passed anyway.

    Quote Originally Posted by Elysia View Post
    But 4 bytes vs. 8 bytes isn't a big difference. You probably won't notice any difference at all since it's lightning fast via exchange of one or two registers.
    Last I checked, 8 was twice the value of 4. "Lightning fast" is not the same as "zero impact". If I perform that set of instructions a few billion times in a loop, I can be reasonably confident of detecting a performance difference because of that "twice" factor on many real world machines -- particularly those where two operations on registers cost more than one operation. And if I have an application that requires that loop .....

  15. #15
    C++まいる!Cをこわせ! Elysia's Avatar
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    Get your point.
    In time critical code, it's a big issue. Otherwise you can get away with it.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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