how to use reference for lvalue

This is a discussion on how to use reference for lvalue within the C++ Programming forums, part of the General Programming Boards category; Hello.. i'd like to make class func that i could use it with lvalue Code: class foo { int x; ...

  1. #1
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    how to use reference for lvalue

    Hello..
    i'd like to make class func that i could use it with lvalue

    Code:
    class foo
    {
      int x;
     
      public:
      
        foo();
        void set_x();
    }
    
    foo::foo()
    {
      x=0;
    }
    
    void foo::set_x()
    {
      ...
      ... //have no idea what to do 
    }
    
    
    int main(){
     foo.set_x()=1;
    
    return 0;
    }
    i know that lvalue has a connection with putting the a value inside an adrs:
    int b=0;
    int *a=&b
    a=1 -> b=1.
    but how could i do it with class

    p.s.
    thnx in advance
    why Gaos didn't had a wife ?
    http://bsh83.blogspot.com

  2. #2
    C++ Witch laserlight's Avatar
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    Why not just pass an argument?
    Code:
    void foo::set_x(int new_x)
    {
        x = new_x;
    }
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
    ZuK
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    guess you want something like this ( I think set_x() is not a good name for a getter function )
    Code:
    class foo {
      int x;
    public:
        foo();
        int & get_x();
    };
    
    foo::foo(){
      x=0;
    }
    
    int & foo::get_x(){
        return x;
    }
    
    int main(){
         foo f;
         f.get_x()=1;
        return 0;
    }
    Kurt

  4. #4
    C++ Witch laserlight's Avatar
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    That may be a little problematic though, since the getter then cannot be const. I think a straightforward setter is better, if you need getter/setters at all.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  5. #5
    Officially An Architect brewbuck's Avatar
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    Quote Originally Posted by laserlight View Post
    That may be a little problematic though, since the getter then cannot be const. I think a straightforward setter is better, if you need getter/setters at all.
    I wouldn't call it get OR set. I'd just call it x(), since it could be used either way. blah.x() = 5.

  6. #6
    C++ Witch laserlight's Avatar
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    I wouldn't call it get OR set. I'd just call it x(), since it could be used either way. blah.x() = 5.
    It would effectively still be a getter that cannot be const, plus "x()" cannot be used as x is already a member variable.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #7
    ZuK
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    Quote Originally Posted by laserlight View Post
    It would effectively still be a getter that cannot be const, plus "x()" cannot be used as x is already a member variable.
    I would agree to any name ( maybe get_x_ref() ? ). I just don't like set_x().
    Kurt

  8. #8
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    wow i didn't thout i get so many replies so fast...

    so if i undrstand cprrectly when i do :
    Code:
    class foo{
        type varibale;
        ..
        ..
        public:
           type& func();
    }
    
    type & func(){
      return varibale;
    }
    what i do is acualy return a refernce (private case of a normal pointer)to the local varibale
    so it is actually like :

    Code:
    #include <iostream>
    
    using namespace std;
    
    class foo
    {
    	int x;
    	
    	public:
    		foo();
    		int* value();
    		void print();
    };
    
    foo::foo()
    {
    x=1;
    }
    int*  foo::value()
    {
     return &x;
    }
    
    void foo::print()
    {
    	cout<<x<<endl;
    }
    
    int main()
    {
    	foo *a=new foo;
    	int *ptr = a->value();
    	a->print();
    	*ptr=2;
    	a->print();
    
    return 0;
    }
    p.s.
    if we used the same attitude in c++ like in our life we would call a refernce a fantic pointer (will never change his opnion)
    XD
    Last edited by jabka; 04-22-2007 at 12:47 PM.
    why Gaos didn't had a wife ?
    http://bsh83.blogspot.com

  9. #9
    C++ Witch laserlight's Avatar
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    Since a reference cannot be changed to refer to something else (though the value of what it refers to can be changed if it is not a const reference), it is probably closer to:
    Code:
    int* const foo::value()
    {
        return &x;
    }
    Nontheless, references are not pointers. You might want to read more on references.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  10. #10
    Cat without Hat CornedBee's Avatar
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    Having a getter return a reference or pointer completely defeats the point in having indirect access in the first place. You can't enforce invariants on setting, you can't hide the data implementation - you might as well make the member public and save yourself the syntactic inconvenience.
    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

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