Is this good style of C++?

Discussion in 'C++' started by Cheng Mo, Dec 2, 2004.

  1. Cheng Mo

    Cheng Mo Guest

    Recently I happens to read some sourcecode which has different style
    from my previous experience.

    In this style, all functions return a RETURN_TYPE_T which is defined as
    typedef unsigned int RETURN_TYPE_T;
    and the return value can only be enum value
    enum
    {
    SUCCESS = 0,
    FAILURE = 1,
    ABORT = 2
    };
    If a function should return a object, the API is not like
    Foo getFoo();
    or
    Foo* getFoo();
    Instead, it is declared as
    RETURN_TYPE_T getFoo(Foo* foo);
    The caller has to new a Foo or get pointer to Foo object through other
    way first, and then call this method with the pointer as parameter.

    The function fill valuable info into the Foo object pointed by the
    paramter. The result of invocation is judged by inspecting the returned
    RETURN_TYPE_T value.

    This style of coding seems strange to me, but I am told that it is a
    good style, because it applys the rule "Who creates it, who releases
    it". As a result, this style are supposed to reduce risk of memory leak.

    I am still not quite convinced. Anybody with long time C++ exprirence
    can give some comments?
    Thanks & Regards
     
    Cheng Mo, Dec 2, 2004
    #1
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  2. If such a style is used in an application for a particular reason (that
    is, it is not a style), then it is OK.


    Otherwise as a *general style* of programming for C++ it looks primitive
    and ancient.


    If you want to handle errors you had better use exceptions, and not
    return error code checks that this style implies.


    If you want to create bullet proof code without memory leaks or *any
    other resource* leaks, use "Resource Acquisition is Initialisation" (RAII):


    http://groups.google.com/groups?q="...selm=cd01kl$1ti6$&rnum=1
     
    Ioannis Vranos, Dec 2, 2004
    #2
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  3. All in all its horrible.

    Do not use UPPERCASE_ONLY for values. These names should be reserved for
    macroes only.
    Also horrible. The first signature should be preferred unless the call could
    fail in a normal case in which case a smart pointer (see boost) would be
    appropriate.
    In general, using exceptions is far more reliable. There's no forgetting of
    checking the return-type, and you would not have to obfuscate your code with
    all that checking.
    This is good advice, but not so relevant for C++. Use RAII instead.
    Kind regards
    Peter
     
    Peter Koch Larsen, Dec 2, 2004
    #3

  4. Actually one convention is to name all constants with uppercase, be them
    macros, const objects, or members of an enumeration.
     
    Ioannis Vranos, Dec 2, 2004
    #4
  5. Cheng Mo

    Kuba Ober Guest

    If a function should return a object, the API is not like
    Unless errors are frequent and expected, in which case exceptions are not
    such a good choice. Throwing an exception typically has big overheads, so
    unless given error is *exceptional*, it should be handled w/o throwing an
    exception. Network code comes to mind, if transmission errors, timeouts
    etc. are frequent.

    Cheers, Kuba Ober
     
    Kuba Ober, Dec 2, 2004
    #5

  6. Well, my experience is in .NET, where every error is signified as an
    exception and not as a return value, and so far I had no problems with
    it. Also all the C++ standard library with the exception of the C
    subset, uses exceptions to signify run-time errors.
     
    Ioannis Vranos, Dec 2, 2004
    #6
  7. Cheng Mo

    jeffc Guest

    Unexpected behavior is pretty much the definition of an exception.
     
    jeffc, Dec 2, 2004
    #7
  8. Cheng Mo

    Andre Kostur Guest

    I don't agree with that assessment. End-of-file on a stream, for example,
    is _not_ an exception, neither is dynamic_casting a pointer to an unrelated
    type. Offhand I can only think of a couple of places where an exception is
    thrown from the Standard library, and that's because either it really is an
    exceptional case (like running out of memory), or there is no other way to
    indicate an error (like dynamic_casting a reference to an unrelated type,
    or using .at() on a vector past the end of the vector).
     
    Andre Kostur, Dec 2, 2004
    #8

  9. These are not errors. End-of-file is just the end of a file and
    dynamic_cast is actually a run-time test.
     
    Ioannis Vranos, Dec 2, 2004
    #9
  10. Cheng Mo

    Andre Kostur Guest

    Then I don't understand exactly what you would call an "error" then. IMHO:
    If I try to read some data from a stream, and I don't get the data I was
    expecting, then I'd call that an error. If I try to dynamic_cast a pointer
    to an unrelated type, the runtime will return me a NULL to signify that
    error. If I try to dynamic_cast a reference to an unrelated type, the
    runtime will throw an exception to signify that error.
     
    Andre Kostur, Dec 2, 2004
    #10

  11. In general, an unexpected situation.




    Well, when you read the data from a stream, checking if its end has been
    reached is a rational thing to do.




    Actually a 0.



    Dynamic cast is aimed for "Run-time Type Identification" (RTTI), so this
    is not an error. Consider this:



    void someFunc(Base *p)
    {
    Derived1 *dp1= dynamic_cast<Derived1 *>(p);

    if(dp1!= 0)
    {
    // ...

    return;
    }

    Derived2 *dp2= dynamic_cast<Derived2 *>(p);

    if(dp2!= 0)
    {
    // ...

    return;
    }

    // ...
    }
     
    Ioannis Vranos, Dec 2, 2004
    #11
  12. The problem with using the term "error"
    is that it requires a [subjective] value judgment.

    C++ programmers prefer the more neutral term *exception*
    to reference *rare* and *unpredictable* events
    that are *expected* but cannot be *prevented*.

    Programming errors (bugs), on the other hand,
    are unexpected until they are discovered
    at which point they are completely predicable
    and can be prevented by fixing the bug.

    Exceptions should be handled at the point
    where they are detected if possible.
    If the exception cannot be handled where it is detected
    all of the information needed to handle the exception
    in the calling program should be encapsulated in an exception object
    and the exception should be returned or thrown.

    Functions like

    Foo getFoo(void);

    which return a value may be used in expressions
    so they must throw an exception
    if it cannot be completely within getFoo(void)
    and that exception *must* be caught and handled
    by the calling program.
     
    E. Robert Tisdale, Dec 3, 2004
    #12
  13. Cheng Mo

    Morgan Cheng Guest

    I read some doc got from google. It seems that RAII just a axiom that
    requires that acquired resource should be released. However, this is not
    easy to follow in C++ because if object is new-d in one part of code
    and another part release it as design. It is quite possible that new-d
    object is not released for human negligence.

    "Who creates int, who releases it" emphasize that the part of code "new"
    the object should take the responsibility to "delete" the object
    explictly. With this discipline, it is not easy to cause memory leak.
    IMO, this rule is more strict than RAII.
     
    Morgan Cheng, Dec 3, 2004
    #13

  14. Check this:


    http://groups.google.com/groups?q="...selm=cd01kl$1ti6$&rnum=1



    When you manage your memory manually, there can always be a memory leak,
    when an exception is thrown for example.


    So you had better use RAII (= host objects in the stack) for memory and
    other resources.


    What does this mean? When you need to create a particular object in the
    free store, you may use a smart pointer like auto_ptr, or create your
    own host class for a particular resource.


    For many objects use a standard library container like vector, list, etc.


    So consider these examples:


    // 10 Buttons created in the free store.
    // Buttons are destroyed at the end of the scope, or when an exception
    // is thrown.
    vector<Buttons> options(10);


    // p will automatically delete the pointed object
    // at the end of its scope, or when an exception is thrown.
    auto_ptr<Form> p(new Form);


    // Use p as a Form *
    p->Show();



    // There are other resources apart from memory that can leak
    class IPConnection
    {
    // ...

    public:
    IPConnection()
    {
    // Opens the connection ...
    // Perhaps also new something;
    }

    ~IPConnection()
    {
    // Closes the connection...
    // delete something;
    }
    };


    // Destructor is called at the end of the scope and when an
    // exception is thrown, cleaning the resources.
    IPConnection ip1;




    All the above are RAII: Destructor is called at the end of the scope and
    when an exception is thrown, cleaning the resources.
     
    Ioannis Vranos, Dec 3, 2004
    #14
  15. RAII as it is used in C++ is actually a simple thing to do in C++.
    All you need to do, is to encapsulate the resource into a class of its
    own. The constructor manages the allocation, the destructor releases
    the resource. (+copy constructor, + assignment operator).
    Users of that resource never deal with the dynamic allocation aspect. All
    they ever use is an object of that class, and the object does all the
    dynamic work.

    Just think of std::string in contrast to plain-vanilla C-style strings.
    std::string manages the C-style string under the hood. Users of std::string
    never have to deal with that.
    RAII (as it is understood in C++) is exactly that principle encapsulated
    in a class, with constructors and destructors doing all the work.
     
    Karl Heinz Buchegger, Dec 3, 2004
    #15
  16. Cheng Mo

    Morgan Cheng Guest

    If I implements RAII strictly, do you mean that memory leak can be
    avoided totally? even exception is possibly to be thrown?

    So, RAII is based on the fact that: At the end of a scope, through
    normal running or exception thrown, objects in the scope will be
    destructed. If resource is allocated explictly, objects pointed by
    pointer will not be released by releasing the pointer. So, we wrap
    dynamic resource allocation with a local object.
    Am my understanding right?
     
    Morgan Cheng, Dec 6, 2004
    #16

  17. Yes, along with other resource types leaks (IP connections, file
    operations etc).


    All C++ standard library types support RAII (except of the C subset
    types of course).



    Yes.
     
    Ioannis Vranos, Dec 6, 2004
    #17

  18. I want to mention here that using RAII in your applications is fairly easy.


    The most obvious way is to use a vector for your objects (or some other
    standard library container if it is more suitable), instead of an array
    in the free store.


    And you have no space/time cost doing this.
     
    Ioannis Vranos, Dec 6, 2004
    #18
  19. Yes. Just never use raw pointers as holders of a ressource: encapsulate them
    in a smart pointer (e.g. boost) and your (ressource) problems should
    disappear.

    Kind regards
    Peter
     
    Peter Koch Larsen, Dec 6, 2004
    #19
  20. Cheng Mo

    Dave O'Hearn Guest

    Almost! A reference-counted smart pointer, like Boost shared_ptr, does
    not solve the object ownership problem. It just changes its form from
    manual new/delete to something like a reference-counted GC. This
    eliminates dangling pointers, but it does not eliminate all memory
    leaks. If you store a smart pointer away somewhere and forget about it,
    you will leak memory.

    I am mostly aware of this from Java, where I have seen a lot of code
    that leaked memory in this manner. In some ways, dangling pointers are
    easier to deal with than GC or reference counting, because the app will
    (usually...) crash on the use of the dangling pointer. With "safe"
    things like shared_ptr, the app will keep running, with no warning that
    some object you thought you released the last reference to is still
    referenced.
     
    Dave O'Hearn, Dec 6, 2004
    #20
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