error: passing const as 'this' argument of ... discards qualifiers..

This is a discussion on error: passing const as 'this' argument of ... discards qualifiers.. within the C++ Programming forums, part of the General Programming Boards category; I've been looking up how to fix this error, apparently it has to do with functions which should be declared ...

  1. #1
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    error: passing const as 'this' argument of ... discards qualifiers..

    I've been looking up how to fix this error, apparently it has to do with functions which should be declared const since something they take is const.
    I have a base class, graphobj, which has subclasses triangle, quad, etc.
    Each subclass calls a function, fill, which takes a few parameters, one of which is a Vector3f.

    I'm getting this error:
    graphobj.h: In member function ‘void GraphObj::fill(Image&, Vec3f&, Vec2f)’:
    graphobj.h:13: error: passing ‘const Vec3f’ as ‘this’ argument of ‘int Vec3f:perator!=(const Vec3f&)’ discards qualifiers

    This is the graphobj code:
    Code:
    #ifndef _GRAPHOBJ_H_
    #define _GRAPHOBJ_H_
    
    #include "image.h"
    
    // GraphObj
    
    class GraphObj {
     public:
      virtual ~GraphObj();
      virtual void draw(Image& i) = 0;
      void fill(Image& i, Vec3f& color, Vec2f point) {
        if(i.GetPixel((int)point.x(),(int)point.y()) != color){
          i.SetPixel((int)point.x(),(int)point.y(),color);
          fill(i,color,Vec2f(point.x()+1,point.y()+1));
          fill(i,color,Vec2f(point.x()-1,point.y()+1));
          fill(i,color,Vec2f(point.x()+1,point.y()-1));
          fill(i,color,Vec2f(point.x()-1,point.y()-1));
        }
      }
    };
    
    #endif
    There are quite a few supporting files, I don't want to attach all of them and make this unreadable. If I need to include more information to figure this out, just tell me.

    How do I solve this issue?? Thanks!

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
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    Calling a non-const function from a const function will generate this error.
    I'm not sure what function is const and which isn't, however.
    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.

  3. #3
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    Here is one of the subclasses:
    Code:
    #include "quad.h"
    #include "image.h"
    #include <string>
    #include <sstream>
    
    Quad::Quad(std::string str){
      stringstream ss(str);
      ss >> fl >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4 >> r >> g >> b;
    }
    
    Quad::~Quad(){}
    
    void Quad::draw(Image& i){
      Vec3f col(r,g,b);
      int avgx = (x1+x2+x3+x4)/4;
      int avgy = (y1+y2+y3+y4)/4;
      i.DrawLine(x1,y1,x2,y2,col);
      i.DrawLine(x2,y2,x3,y3,col);
      i.DrawLine(x3,y3,x4,y4,col);
      i.DrawLine(x4,y4,x1,y1,col);
      if(fl.compare("fill")==0){
        fill(i,col,Vec2f(avgx,avgy));
        fill(i,col,Vec2f((avgx+x1)/2,(avgy+y1)/2));
        fill(i,col,Vec2f((avgx+x2)/2,(avgy+y2)/2));
        fill(i,col,Vec2f((avgx+x3)/2,(avgy+y3)/2));
        fill(i,col,Vec2f((avgx+x4)/2,(avgy+y4)/2));
      }
    }

  4. #4
    C++まいる!Cをこわせ! Elysia's Avatar
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    Can I assume this line is causing the error?
    Code:
    if(i.GetPixel((int)point.x(),(int)point.y()) != color){
    If so, then how about posting the relevant code to that?
    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.

  5. #5
    Cat without Hat CornedBee's Avatar
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    You should make int Vec3f::operator!=(const Vec3f&) (and probably a few others) const member functions.
    Or even free functions.
    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

  6. #6
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    This is the GetPixel functino.. it's inside of image.h, which is large, so I wouldn't want to post the entire thing.

    Code:
    const Vec3f& GetPixel(int x, int y) const {
        assert(x >= 0 && x < width);
        assert(y >= 0 && y < height);
        return data[y*width + x]; }

  7. #7
    C++まいる!Cをこわせ! Elysia's Avatar
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    Try CornedBee's suggestion by making the operator != const, at least.
    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.

  8. #8
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    There has to be another way - the operator overloads were written for me, I do not want to change them.

  9. #9
    C++まいる!Cをこわせ! Elysia's Avatar
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    When you start messing with const, you have to make sure the entire path is const or you will get errors somewhere along the way.
    Operator != does not modify the class state, so it should be a const member function and take two const references (for free operator) or one const reference argument (for member function).
    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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