Penggunaan 'const' di C++

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class CRectangle {

  // private member declaration
  private:
    int *width, *height;
  
  // public member declaration
  // function definition is in its .cpp
  public:
    // constructor declaration
    CRectangle(int, int);
  
    // destructor declaration
    ~CRectangle();
  
    int area() {
      modifyThis = 0;
      return ((*width) * (*height));
    }
  
    int constArea() const {
      /* error. read-only memory: 
       modifyThis = 0;
       */
      
      return ((*width) * (*height));
    }
  
    int modifyThis;
}; 

#include "CRectangle.h"

// public member function definition
// constructor definition
CRectangle::CRectangle(int a, int b) {
  width = new int;
  height = new int;
    
  *width = a;
  *height = b;
}

// destructor definition
CRectangle::~CRectangle(){
  delete width;
  delete height;
}

#include "CRectangle.h"

void test(const CRectangle &);
void test(CRectangle &);
void test(CRectangle *);

int main(int argc, const char * argv[]) {
  // create new CRectangle instance
  // that is stored in stack
  CRectangle rect(3, 4);
  
  // create new CRectangle instance
  // that's stored in heap that's why
  // we need a pointer to keep a reference
  // of it
  CRectangle *pRect = new CRectangle(1,100);
  
  
  // void test(CRectangle &aliasRect)
  test(*pRect);
  
  // void test(CRectangle *aliasRect)
  test(pRect);
  
  // void test(const CRectangle &aliasRect)
  // can be called this way:
  const CRectangle &aRect = *pRect;
  test(aRect);
  
  // or, we can cast existing object
  // pointed by a pointer to conform
  // test() function signature:
  test((const CRectangle &) *pRect);
  
  
  // object made in heap needs
  // to be manually deallocated
  delete pRect;
  
  return 0;
}

void test(const CRectangle &aliasRect) {
  std::cout << "void test(const CRectangle &aliasRect);" << std::endl;
  std::cout << aliasRect.constArea() << std::endl;
  
  // below line will produce compile error
  // because the function we're about to
  // call is not declared itself to be
  // read-only:
  // std::cout << aliasRect.area() << std::endl;
  
  // below line will also produce compile error
  // because we can not modify aliasRect
  // state (it members). we can only read them:
  // aliasRect.modifyThis = 100;
}

void test(CRectangle &aliasRect) {
  std::cout << "void test(CRectangle &aliasRect);" << std::endl;
  std::cout << aliasRect.area() << std::endl;
  std::cout << aliasRect.constArea() << std::endl;
}

void test(CRectangle *aliasRect) {
  std::cout << "void test(CRectangle *aliasRect);" << std::endl;
  std::cout << aliasRect->area() << std::endl;
  std::cout << aliasRect->constArea() << std::endl;
}