accessing private class data low overhead

This is a discussion on accessing private class data low overhead within the C++ Programming forums, part of the General Programming Boards category; currently I am using pointers and functions to access private class date in ptrClassA->Setval; void setval(double val) {itsval = val;} ...

  1. #1
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    accessing private class data low overhead

    currently I am using pointers and functions to access private class date

    in

    ptrClassA->Setval;
    void setval(double val) {itsval = val;}
    ptrClassA->Getval
    double Getval() {return itsval;}

    is there a faster way to do

    ptrClassA->itsval ?

    many thanks

  2. #2
    (?<!re)tired Mario F.'s Avatar
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    is there a faster way to do

    ptrClassA->itsval ?
    Make it public?

    By implement set and get functions you are theoretically doing the right thing, since you are controlling the access to your class members and providing a common interface that users of your class (you and other programmers declaring objects of your class) can expect.

    Some may argue that on examples like yours where the setter and getter functions limit themselves to assign to and retrieve the private member, there is no need for those functions and you could simply make itsval public. That would allow you to use ptrClassA->itsval and skip a function call.

    I'd be tempted to agree, however there's two disadvantages of the top of my head you should consider:

    . You are limiting your ability to maintain the class. If sometime later you decide that after all you want the class to do other things besides itsval = val; or return itsval; when assigning or retrieving that member, you will then need to create those functions. But the problem is that you have code all around that will stop working as intended and needs to be changed. Anywhere where you are doing something like ptrClassA->itsval = 13.5 or double val = ptrClassA->itsval.

    . The other problem (maybe less important, dunno. I find it important though) is that by not implementing functions of this type, users of this class cannot expect a common interface. That is, if all your members are private and to access them they need to use Getxxx and Setxxx (or any other consistent nomenclature you decide) it makes your class easier to use.
    Last edited by Mario F.; 07-23-2009 at 09:18 PM.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

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    Quote Originally Posted by Mario F. View Post
    Make it public?


    Some may argue that on examples like yours where the setter and getter functions limit themselves to assign to and retrieve the private member, there is no need for those functions and you could simply make itsval public. That would allow you to use ptrClassA->itsval and skip a function call.
    how much time will this impact my code if i may be doing multi millions of loops?

  4. #4
    C++ Witch laserlight's Avatar
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    You may want to consider implementing those functions inline with the class definition first, to see if avoiding the function call overhead has any measurable improvement on performance.
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  5. #5
    Captain Crash brewbuck's Avatar
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    Make the set/access functions inline and forget it. If the compiler refuses to inline them, throw your compiler out the window.

    Altering the visibility of a member as a speed optimization is unacceptably wrong
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  6. #6
    (?<!re)tired Mario F.'s Avatar
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    Yes. I was trying to make a case exactly for that. Should have worded it better. Didn't want the OP to think I was suggesting making it public.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  7. #7
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Mario F. View Post
    Yes. I was trying to make a case exactly for that. Should have worded it better. Didn't want the OP to think I was suggesting making it public.
    It's not really as bad as I make it sound, because making the function inline means you have to expose its internals inside a header file, which means your implementation isn't black-box anymore. One of the fundamental problems with C++, IMHO.

    Of course, I can't actually imagine a standard way of allowing specification of inline functions without revealing their code.

    No wait, I can, but it involves cryptography and that's just goofy
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  8. #8
    (?<!re)tired Mario F.'s Avatar
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    No. It's alright. It was the OP answer to my post that scared me
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  9. #9
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by brewbuck View Post
    It's not really as bad as I make it sound, because making the function inline means you have to expose its internals inside a header file, which means your implementation isn't black-box anymore. One of the fundamental problems with C++, IMHO.

    Of course, I can't actually imagine a standard way of allowing specification of inline functions without revealing their code.
    Ah, but both MSVC and GCC 4.x can inline non-inline functions even if they're in different source files, so it's not all that bad.
    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.

  10. #10
    Cat without Hat CornedBee's Avatar
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    4.x can't do it. 4.5 will be able to, but you'll have to explicitly request it.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  11. #11
    C++まいる!Cをこわせ! Elysia's Avatar
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    Ah, 4.5+ then. But that's alright. You have to explicitly ask MSVC too.
    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.

  12. #12
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by Elysia View Post
    Ah, but both MSVC and GCC 4.x can inline non-inline functions even if they're in different source files, so it's not all that bad.
    I was thinking more along the lines of a user library, where you do not have the source in the first place. What might have been a completely opaque binary implementation now has bits and pieces of the internals leaking out in the form of inline function definitions in the headers.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  13. #13
    (?<!re)tired Mario F.'s Avatar
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    Well, one could save a function call for the getter, retain class opacity and pay with a slight increase in size. Could be surprising at first, but if done consistently I guess class users would get used to it.

    Code:
    class myClass {
    
    public:
    
        myClass(): member_(0), member(member_) {}
        myClass(double val): member_(val), member(member_) {}
        
        void member(double val) { member_ = val; }
    
        const double &member;
    
    private:
    
        double member_;
    
    };
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  14. #14
    Cat without Hat CornedBee's Avatar
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    I don't see how that is any advantage over exposing the variable directly.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  15. #15
    (?<!re)tired Mario F.'s Avatar
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    Not much, no. It's just me exercising.

    Guess the only difference over going all the way and exposing the member is that you still retain some functional abstraction. The member is still private and any attempts at changing it have to go through the setter function.

    Of course, it's much better to just define the class as everyone expects it and during its use, if there is a need for faster read access to the data member, the class user just copies it to the stack.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

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